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WalkthroughChangesThe PR adds configurable TLS ClientHello fingerprints for TLS fingerprint customization
Ctrl+C process handling
Suggested reviewers: 🚥 Pre-merge checks | ✅ 4✅ Passed checks (4 passed)
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Updated 11:26 PM PT - Aug 23rd, 2026
✅ @robobun, your commit 717c941416adcf00baaaeae3313940ee30d73198 passed in 🧪 To try this PR locally: bunx bun-pr 40275That installs a local version of the PR into your bun-40275 --bun |
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Inline comments:
In `@src/http/tls_fingerprint.rs`:
- Around line 284-305: Update the TLS 1.2-only branch of the unsent-extension
selection in the JA3 validation logic to include EXT_PRE_SHARED_KEY, matching
BoringSSL behavior. Also add extension 41 to the invalid-JA3 cases in the fetch
TLS tests.
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Inline comments:
In `@docs/runtime/networking/fetch.mdx`:
- Line 279: Update the networking documentation sentence beginning “Bun uses
BoringSSL” to avoid claiming the full Chrome fingerprint is reproduced; describe
it as reproducing Chrome-like TLS ClientHello characteristics within BoringSSL’s
constraints, while leaving the following limitations and HTTP/2 qualification
intact.
In `@packages/bun-types/globals.d.ts`:
- Around line 1936-1937: Update the declaration comment describing resumed TLS
sessions to limit the pre_shared_key (41) note to TLS 1.3, mention that TLS 1.2
resumption uses session IDs or session_ticket (35), and state that TLS 1.3
requires pre_shared_key to be the final ClientHello extension, including the
ordering exception when extension permutation is enabled.
In `@src/runtime/socket/SSLConfig.rs`:
- Around line 292-294: Update the fallback condition in from_generated so
ja3.groups is assigned only when generated.ecdh_curve is absent, preserving the
explicit ecdhCurve: "auto" setting even when result.ecdh_curve remains null.
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CI status. Head 717c941 is the branch rebased onto main (the conflict was in The previous head a647d5b passed every job in build 104574 (181 of 181). Before that, on 4ef98b5 (build 104559), the two red lanes were not related to this change: a The default ClientHello (no fingerprint option) is byte-identical between this branch and the release build. I checked it by capturing the first TLS record with a raw TCP listener, before and after the rebase. |
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Automated review ran clean on this revision — no new findings. All earlier inline findings (JA3 parser strictness gaps, dead FFI decl, test hang-on-failure, ecdhCurve: "auto" precedence, session-cache/pre_shared_key note) have been addressed. Given the scope — new user-facing API surface, TLS handshake wiring, and cert-decompression callbacks that process server-controlled data — a human look is still warranted.
What was reviewed:
Ja3::parseinput-space coverage (version-gated extensions both directions, empty/GREASE-only groups, point-formats/TLS-version tie, TLS 1.3 suite position) — now strict.- Cert-decompression callbacks (brotli/zlib/zstd) bounded by the server-declared
uncompressed_len;compress_certificate(27) in a TLS 1.2-only JA3 examined and ruled out. Fingerprintfolded intoSSLConfighash/equality so context and keep-alive caches keep distinct fingerprints apart; CONNECT-tunnel path applies both ctx- and per-SSL knobs.
Extended reasoning...
Overview
Adds nine tls options to fetch() (ja3, grease, permuteExtensions, certificateCompression, applicationSettings, echGrease, ocspStapling, signedCertificateTimestamps, sessionTickets) that shape the TLS ClientHello for fingerprint control. Core is a new 532-line src/http/tls_fingerprint.rs (JA3 parser + BoringSSL apply steps), plus a Fingerprint struct on SSLConfig (hash/eq/clone updated), FFI surface additions in boringssl_sys, wiring through HTTPContext/ProxyTunnel/configure_http_client_with_alpn, bindgen dictionary + generated.rs plumbing, a small C++ shim for bssl::SSL_set_aes_hw_override_for_testing, docs, types, and a 490-line test block that captures and parses the raw ClientHello. src/spawn/ctrl_c.rs appears in the diff but is not part of this PR (base-commit artifact, confirmed on the thread).
Security risks
The change sits directly in the TLS handshake path. Three cert-decompression callbacks (brotli, zlib, zstd) decode server-controlled bytes; each reserves exactly uncompressed_len and decodes into that spare capacity, so a hostile server cannot inflate past the announced length — a larger frame fails and the callback returns 0. apply_to_ssl runs on every fetch TLS open (with Fingerprint::DEFAULT when no options are set), so the pre-existing SCT/OCSP behavior is preserved. The JA3 parser is strict-decimal and length-bounded, so no untrusted-input arithmetic risk. No auth, permission, or credential-handling code is touched.
Level of scrutiny
High. This is new public API surface (naming, defaults, error shapes are user-visible and hard to change later), it modifies the shared SSL-config interning key, it uses a BoringSSL _for_testing symbol in production, and TLS-fingerprint spoofing is a use case with abuse potential that a maintainer should sign off on. The REVIEW.md "API design" section applies. None of that is a code defect — it is exactly the kind of design surface that should not land on bot approval alone.
Other factors
This PR has already been through five rounds of automated review on this thread; every finding (TLS 1.3 suite position, version-gated extensions in both directions, empty/GREASE-only groups, point-formats field, TLS 1.2-only 41/42/44, dead FFI decl, dead !ja3.groups.is_empty() guard, .catch(() => {}) hang, ecdhCurve: "auto" precedence, session-cache pre_shared_key documentation) was fixed in a follow-up commit and is covered by a test case. The 37-test block passes on ASAN and release, and fails on the released bun. The one candidate this run (extension 27 in a TLS 1.2-only JA3) was verified to be sent on the wire — BoringSSL gates compress_certificate on min_version, not max_version.
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The automated review has no open findings on 4ef98b5, and all earlier review threads are resolved. The PR is ready for a maintainer review. The CI state is in the comment above: the two red lanes are an agent download timeout and a pre-existing crash on main, neither in the files this PR changes. |
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The "Ctrl+C process handling" section of the walkthrough is no longer accurate, and my earlier explanation for that file was wrong. |
Let fetch shape its TLS ClientHello. A `ja3` string sets the cipher suite order, the supported groups, the version bounds and the extension toggles it lists. Each knob is also an explicit option: grease, permuteExtensions, certificateCompression, applicationSettings, echGrease, ocspStapling, signedCertificateTimestamps, sessionTickets. The parse is strict: a cipher suite, group or extension that BoringSSL cannot send throws a TypeError. The options ride on the existing SSLConfig and force a custom SSL_CTX; the fetch client applies the context-wide settings when it builds that context and the per-connection settings before each handshake, for direct connections and for CONNECT tunnels alike.
…NGSSL_NO_CXX The Windows build compiles Bun's C++ with BORINGSSL_NO_CXX, which hides the extern "C++" block of ssl.h where this function lives. The symbol is still in the library, so declare it locally.
…only BoringSSL writes the three TLS 1.3 suites before the configured cipher list and always writes 0x0303 as the legacy version, so a string that puts a TLS 1.2 suite before them, or names another version, cannot be reproduced on the wire. Reject both instead of sending a different fingerprint. Also: drop the unused SSL_CIPHER_get_kx_nid declaration, wire the CONNECT test's error path to the awaited promise, ignore the padding extension when comparing shuffled extension sets (BoringSSL appends it whenever the shuffle leaves an empty extension last), and shorten comments.
supported_groups, signature_algorithms and ALPN are always on the wire, ec_point_formats, extended_master_secret and renegotiation_info whenever TLS 1.2 is offered, supported_versions, psk_key_exchange_modes and key_share whenever TLS 1.3 is. A string without them would produce a superset of its extension set, so it is a TypeError now.
Also buffer the CONNECT request to its blank line in the tunnel test, document the spare-capacity contract of zstd decompress_append and drop the two TLS version constants nothing reads anymore.
…pty group list A TLS 1.2-only string cannot list 43, 45, 51, 65037 or ALPS; a TLS 1.3-only string cannot list 11, 23, 35 or 65281. A groups field with no real entry would fall back to the default group list.
BoringSSL sends ec_point_formats (always 0) only when TLS 1.2 is offered, so the field must be 0 then and empty for a TLS 1.3-only string. Also route the tunnel test's fetch rejection to the capture promise.
Also soften the docs claim and scope the resumption note to TLS 1.3.
A ClientHello carries each cipher suite, extension and group at most once, so a repeated id is a fingerprint BoringSSL cannot reproduce.
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Rebased onto main and force-pushed (head 717c941). The one conflict was in Also in this push: a JA3 field that lists the same id twice now throws ( |
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I reviewed this PR and the automated bug hunt found no new issues on 717c941; every earlier finding (missing/unsent/version-gated extensions, empty groups, pointFormats, duplicate ids, ecdhCurve: "auto" precedence, the session-cache/pre_shared_key note) has been addressed. Because this is a ~1,650-line feature adding new user-facing tls API surface, hand-written #[repr(C)] bindgen extern layouts, and TLS-handshake / certificate-decompression paths — plus a production dependency on BoringSSL's SSL_set_aes_hw_override_for_testing — a maintainer sign-off is still warranted.
What was reviewed:
Ja3::parseinput space: version, cipher order/TLS1.3 constraints, required/unsent/duplicate extensions, groups, pointFormats — each direction now rejects what BoringSSL cannot reproduce.FingerprintinSSLConfighash/equality so the interned-config and SSL_CTX caches key on it;requires_custom_request_ctxset for any non-default fingerprint.- Decompression callbacks bound output by the server-announced
uncompressed_len(brotli/zlib/zstd all fail closed on size mismatch or decode error). apply_to_ssl_ctx/apply_to_sslwiring covers direct fetch, CONNECT tunnel, and leaves WebSocket atNone(default behavior unchanged).
Extended reasoning...
Overview
This PR adds nine new tls options to fetch (ja3, grease, permuteExtensions, certificateCompression, applicationSettings, echGrease, ocspStapling, signedCertificateTimestamps, sessionTickets) that shape the TLS ClientHello for JA3/JA4 fingerprint control. It introduces a new 543-line module src/http/tls_fingerprint.rs (JA3 parser + BoringSSL apply steps + brotli/zlib/zstd cert-decompression callbacks), ~60 lines of new BoringSSL FFI declarations, a Fingerprint struct threaded through SSLConfig's hash/equality/clone, hand-written #[repr(C)] extern union layouts in src/jsc/generated.rs for two new bindgen unions, a C++ shim in NodeTLS.cpp wrapping bssl::SSL_set_aes_hw_override_for_testing, a signature change to configure_http_client_with_alpn (four call sites), and ~490 lines of tests that parse the raw ClientHello off a TCP listener. 17 files, ~1,650 insertions.
Security risks
The certificate-decompression callbacks (decompress_brotli/decompress_zlib/decompress_zstd) process server-controlled compressed data during the TLS handshake. Each reserves exactly uncompressed_len bytes (server-announced, capped by BoringSSL at 64KB per RFC 8879), decodes into that spare capacity only, and finish_decompression rejects any output whose length differs from uncompressed_len — so a malicious server cannot inflate more than it declared, and a truncated/oversized frame fails closed (returns 0). Allocation failure returns 0 rather than aborting. The JA3 parser validates every id against BoringSSL's known set before use and rejects duplicates, so no untrusted string reaches SSL_CTX_set_cipher_list/set1_groups_list unfiltered. The fingerprint options only reduce or reorder what the client offers relative to defaults (except GREASE/ALPS/ECH-grease/cert-compression, which are additive but standard); they cannot disable certificate verification. I did not find an injection, bypass, or memory-safety issue here, but this is TLS-handshake code and warrants maintainer eyes.
Level of scrutiny
High. This is new user-facing API (the REVIEW.md "API design" situational section applies), it touches the TLS handshake path for every fingerprinted fetch, it adds hand-written ABI-sensitive extern struct layouts that must match the C++ bindgen output byte-for-byte, and it takes a production dependency on a BoringSSL symbol named _for_testing (used to pin the TLS 1.3 suite order machine-independently). Those are exactly the kinds of design/naming/stability calls a maintainer should ratify. The PR author's own status comment says it is "ready for a maintainer review," and coderabbit's suggested reviewers include Jarred-Sumner.
Other factors
Six prior automated review passes (mine and coderabbit's) iteratively closed every gap in the JA3 parser's "cannot reproduce ⇒ throw" contract; all threads are resolved and each fix has a test row. The default (no fingerprint option) path is unchanged: Fingerprint::DEFAULT keeps ocsp_stapling/signed_cert_timestamps on and everything else off, so apply_to_ssl on the default reproduces exactly the two calls the old code made unconditionally, and is_default() keeps requires_custom_request_ctx false so the shared SSL context is still used. Test coverage is thorough (37 tests including CONNECT tunnel and a real handshake), ASAN passes, and the CI reds on the previous head were unrelated infra/pre-existing failures. None of that changes the fact that a ~1,650-line TLS feature with new public API should have a human maintainer approve it.
…ps, WebSocket, SQL) (#44618) ### What does this PR do? Consolidates the open TLS pull requests into one. Each was reproduced on `main` and, for `node:*` behavior, on Node v26.3.0 first. About a third are ported as written, the rest are rewritten smaller or merged into one fix where several PRs patched the same cause. One commit per fix, so it can be read commit by commit. Fixes #43520, fixes #31396, fixes #43635, fixes #37193, fixes #43846, fixes #17932, fixes #41061, fixes #36887, fixes #31810, fixes #35240, fixes #32234, fixes #44365, fixes #43807, fixes #42280, fixes #44517. Addresses #41856 (SNI and `servername`; not `checkServerIdentity` for SQL), #24845 (the spin is gone, shown with fault injection on Linux; not run on macOS), #19754 (node-fetch forwards the agent's TLS options; the Kubernetes client itself was not run). #### The ones that matter most | | On `main` | PRs | |---|---|---| | Client certificate disclosure | `https.request()` with a client certificate sends it to a server it then refuses (wrong name, `checkServerIdentity`, `destroy()` in `'secureConnect'`, `terminate()` in `handshake`). A server can force it with a junk record behind its Finished | #43946 | | False `authorized` | Over a Duplex, `secureConnect` with `authorized === true` for a peer that failed the key proof; `secureConnect` for a plaintext peer with `rejectUnauthorized: false` | #44422, #32929 | | Cleartext https | `https.createServer()` without a usable key/cert answers plain HTTP | #41672, #33539 | | Revoked client certificates | An https mTLS server never sees `crl`, so a revoked client is `authorized` | #41641 | | Pooled sockets | Requests with different client certificates or CAs share an `https.Agent` socket and session | #42498 | | Silent plaintext | `tls: [...]` given to `Bun.listen` / `Bun.connect` is plain TCP | #41490 | | Server weakened by a client knob | `NODE_TLS_REJECT_UNAUTHORIZED=0` turns off a server's client-certificate enforcement | #35245 | | Pins never checked | `WebSocket` never calls `tls.checkServerIdentity` and ignores `tls.serverName` | #41648 | | `verify-full` dropped | `PGSSLMODE=verify-*` is lost next to a `TLS_*` URL variable; `tls: true` sends no SNI | #44498 | | Crashes | use-after-free from `destroy()` in `ALPNCallback` over a Duplex; `abort()` on a late `setSession()`; SIGABRT in `fetch` with an https proxy from the environment and a `Bun.file()` body | #44462, #41671, #44458 | | Stream corruption | A TLS `write()` can lose 16 KiB it reported as written while another socket on the loop is stalled | #44529 | | Hangs and spins | 100% CPU on a failing `send()`; a fatal `SSL_write` leaves the socket open forever; `idleTimeout` never sheds a TLS client that ignores `close_notify` | #34510, #38176, #42336 | | Wrong certificate (regression since 1.3.14) | Connections accepted before `stop()` / `close()` get the default certificate and skip their entry's `requestCert` / `ca` | #42355 | | Quadratic Duplex / proxy tunnel | Reading one chunk over a Duplex, CPU: 8 MB 0.88 s → 0.14 s, 16 MB 3.18 s → 0.23 s, 32 MB 11.75 s → 0.39 s; `fetch` upload through CONNECT: 1.7 s → 0.18 s (debug build) | #44464 | #### By area - **fd engine, write path** (`openssl.c`, `socket.c`): #42352, #34510 + #38176 + #42336 as one change, #44529, #44458, #44192. A rejected `send()` ends the write side only and closes at the next writable event unless the peer's bytes are still queued (a 413 sent before a reset is still read). No new per-socket state. Also, on kqueue, **a FIN no longer ends a socket that waits in the low-priority queue** (`loop.c`): with more than 5 TLS handshakes at once, a client that ended right after its handshake could be reset and its server socket report `socket hang up`, because the eof that the sentinel read knote reports was acted on ahead of the unread Finished. That is on `main` too (the macOS entry for `node-tls-server.test.ts` in `test/flaky-tests.txt`: 7 of 48 recent builds of other branches), and this branch made it likelier (6 of 8 builds), since Finished now leaves in one segment with the close_notify. - **Error reporting, both engines**: #44422, #32929, #44516, #37094, #41272 + #42324 + #44223 as one change, #44021, #37472, #43946, #33630. One channel: a fatal error on an established session is reported, then **the engine closes the connection itself**, whatever the owner does with the report. `test/js/bun/net/tls-fatal-error-closes.test.ts` asserts closed-and-nothing-delivered for every owner (node:tls, `Bun.connect`, `Bun.listen`, `fetch` direct and through CONNECT, `Bun.serve`, `WebSocket` direct and through a proxy, Postgres, MySQL, Valkey, Duplex). - **Duplex engine** (`SSLWrapper`, `UpgradedDuplex`): #44462, #43529, #42332, #44464. #43877 + #44394 were in and are **out again**, see "Worth a look" 5. - **node:tls wrap lifecycle** (`net.ts`, `tls.ts`): #38007, #38058, #38028 + #38122 + #38076 as one change (six copies of the attach code become two helpers), #38311, #39008, #38154, #42340 + #42343 + #42339 + #42453 as one change, #43791, #42425, #44085, #42683, #39088, #39040, #40375, and what was still real of #36534. - **SNI, ALPN, server contexts**: #43080, #42050, #37195 + #43849 as one change (**one** SNI matcher for TCP and HTTP/3), #42355, #42285, #33253, part of #37896, part of #37013. A `tls.Server` has one `SSL_CTX`. - **Verification and options**: #44738, #41490, #37005 + the cwd pin of #40984, #31811, #43982, #33483 + #35245, #41810, #32235, #44441, #38092. - **node:tls API and CA store**: #41671, #38145, #32824, #43594, #39997, #41696, #33534, #34748, #42991, #42996, #42970. - **node:https, Agent, `ws`, node-fetch**: #41672 (https half), #41641, #38261, #42498, #44346, #35609, #31397, #42325. - **WebSocket client**: #41648, #37487 + #43048 as one change. - **SQL, Redis**: #33666, #41711, #44498, part of #42054. - **Tests only**: #41426, #40040, #44395, #44016, #37860, #40591, #44440, #41424. Found on the way and fixed here: an upload that a TLS 1.2 server interrupts with a renegotiation never completes on `main` (0 of 32 runs over `https.request`, `fetch`, `node:tls` and `Bun.connect`: the renegotiation ClientHello lands inside an application record that is still unsent, or the socket gets no `drain` again) and completes here, with two tests from robobun; the fix for #40653 (final flight and first write in one segment) stopped working whenever another TLS socket on the loop was stalled, on `main` too; the `tls.Server` prototype pinned the last server constructed and every `SSL_CTX` it owned; `Object.create(process.env).NODE_TLS_REJECT_UNAUTHORIZED = "0"` turned verification off process-wide once a `SHARE_ENV` worker existed; two debug panics when wrapping a shut-down or still-connecting socket; a `fetch` POST through a proxy sent its headers twice when the origin renegotiated; `BlockList` ignored IPv6 zone ids; a test now ties `root_certs.der` to `certdata.txt`. #### Behavior changes - **A server's `ca` without `requestCert` no longer asks for a client certificate** (`Bun.serve`, `Bun.listen`, HTTP/3, node:tls). It matches the docs and Node. On `main` such a server refused clients with no certificate but served any unrelated self-signed one, so it was never authentication. **Set `requestCert: true` to require a certificate.** A matrix test pins that `requestCert: true` still refuses no certificate and an untrusted one on 8 kinds of server, TLS 1.2 and 1.3, with `NODE_TLS_REJECT_UNAUTHORIZED` unset and `0`. - `NODE_TLS_REJECT_UNAUTHORIZED=0` no longer relaxes a server. - `Bun.connect` / `Bun.listen` hear of a fatal TLS error after the handshake through `error(socket, err)`. With no `error` handler the socket just closes. - HTTP/3 server names match like TCP: `*.` covers exactly one label, case is ignored, a trailing dot is ignored, the last registration of a name wins. - `requestCert` on node:https is `=== true`, as in Node. - An array where a generated options dictionary is expected throws (`tls: []`, `jest.useFakeTimers([])`). - `key` / `cert` arrays serve every identity. A client that can use both gets ECDSA, where `main` served whichever pair came last. - `ecdhCurve` is forwarded by node:https, `ws` and node-fetch now, so a group BoringSSL lacks (`X448`) throws there as it already does in `tls.createServer`. - A wrapped socket's error is re-emitted on the TLS socket as in Node, so `raw.destroy(err)` with a listener on `raw` only is uncaught, as in Node. - `sql.options.tls` is always an object, never `true`. `RedisClient` sends SNI. - `tls: { secureContext }` alone asks for TLS on `Bun.listen` / `Bun.connect` (it was plain TCP), and a value that is not a `SecureContext` throws. The context is served as it is: the `requestCert` / `rejectUnauthorized` it was created with hold whatever the options next to it say, and `requestCert` in the options over a context that does not ask throws at `listen()`. - `tls.DEFAULT_CIPHERS` reaches every client once assigned (`fetch`, `WebSocket`, `Bun.connect`, `RedisClient`, `Bun.SQL`, `S3Client`, proxy tunnels) and servers again. A list that selects no cipher throws `ERR_SSL_NO_CIPHER_MATCH` at the assignment. `fetch.preconnect()` dials nothing after an assignment. - The warning for an unreadable `NODE_EXTRA_CA_CERTS` is Node's one line, without the `warn:` prefix. - `BUN_CONFIG_WS_CLOSE_TIMEOUT` (default 30 s): how long a `WebSocket` client waits for the server to close the connection after the closing handshake. #### Worth a look in review 1. **#44529**: the kernel-refused remainder of a TLS write moves from the loop's one slot onto the connection (in the existing rare struct), so the write BIO never refuses a sealed record. Nothing is allocated on an unstalled path (200 writes: 0 appends, same `send()` count as `main`), memory with 16 stalled writers is lower than on `main` (276 KB vs 340 KB, which `main` holds inside BoringSSL's buffers), `us_socket_t` stays 80 bytes. It needs a bound on how long a deferred close waits, or a peer that stops reading pins the fd past `destroy()`: `US_SSL_CLOSE_AFTER_SPILL_TIMEOUT` is a fixed 10 s, not re-armed on progress. Separate commits, but the fix that keeps the client certificate off the wire beside a stalled socket builds on them. 2. **The default name check of node:tls also runs inside the handshake**, so a wrong-name server gets no client certificate on TLS 1.2 either. JS still runs it after every successful handshake, so a difference between the two matchers can only refuse. Error objects are byte-identical. 3. **#44441** widens trust by design: a self-issued leaf whose `keyUsage` lacks `keyCertSign` (`dotnet dev-certs`) is its own anchor when the store holds a byte-identical copy. No BoringSSL change. Expired pin, same subject with another key, wrong EKU and a pinned intermediate are tested to fail. 4. **#32235** only adds Ed25519 and ECDSA P-521 to the verify list. A captured ClientHello shows `main`'s list with the two inserted; `rsa_pkcs1_sha1` stays. 5. **A stream that a TLS socket wraps, when that TLS socket closes.** An earlier state of this branch lost data here while CI was green (found by #44709's report): with the peer closing first, 4 of 8 MiB arrived with TLS in TLS, 4 of 32 MiB on the http2 `emit("connection")` path, and a `write()` with no `'error'` listener ended the process. Three Node-parity changes only hold together: destroying the wrapped stream at the close (#38028 + #38122 + #38076, #38154) is safe only if every write has really completed (#43877), which in turn needs Node's handling of the peer's close_notify, which needs half-open sockets that the GC can collect. So: - #43877 + #44394 are reverted and reopened. A write over a stream completes once the stream has taken the ciphertext, as on `main`. - Until the verdict on the peer lets the session through, the application cannot have written over it. There the wrapped stream is destroyed as in Node, with the sessions below it. That keeps the release of the connection after a failed handshake, a rejected certificate and an early `destroy()`. The same for an http2 socket the application never got, and for `resetAndDestroy()`. - After that it is `main`'s teardown: a `net.Socket` only gets the engine's `end()`, closes at its peer's FIN, keeps its own timeout and reports its own errors. Any other stream is destroyed with the TLS socket. The regular suites cannot see any of this (999 files were green on every broken variant), so it was steered by eleven seeded differential fuzzers run on this build, `main`, Node v26.3.0 and the earlier state: close, `end()`, `destroy()`, `destroySoon()`, resets, hung and half-open peers, paused writers, timeouts, two and three sessions deep, over TCP and over Duplexes, before, at and after the handshake, and http2 requests. See "How did you verify". #### Known limits - `fetch` with a `checkServerIdentity` function still sends the client certificate (not the request) to a server the function refuses. On TLS 1.2 any verdict a JS callback gives is too late, as in Node. - `addContext()` / `SNICallback` still do not apply to a server-side socket on the stream engine (`emit("connection", duplex)`, TLS in TLS, unflushed writes, named pipes), as on `main`. - A CA bundled in a pfx extends an explicit `ca` only, for `ws` / node-fetch / `WebSocket`: the native `ca` can only replace the default store, and that store keeps `SSL_CERT_FILE` / `SSL_CERT_DIR`. - P-521 leaves work on TLS 1.3 only. TLS 1.2 needs secp521r1 in every ClientHello (`it.todo`). - Once `tls.DEFAULT_CIPHERS` is assigned, `fetch(url, { protocol: "http3" })` is `HTTP3Unsupported`, as with an explicit `ciphers`. - `addCACert()` by hand does not extend the chains of a context with several identities. - A throwing `ALPNCallback` sends `no_application_protocol` on both engines. Node sends nothing and its client sees `ECONNRESET`. - TLS in TLS, peer FIN while the outer handshake runs: the inner socket gets one `write EPIPE`, where Node gives `ECONNRESET` (`main` gives it no error at all). - `@SECLEVEL` in `ciphers` is dropped by the `ws` / node-fetch shims, which used to ignore `ciphers`. node:tls keeps throwing `ERR_SSL_INVALID_COMMAND`. - Beside a stalled TLS socket only the first record (16 KiB) of the first write leaves with the handshake flight. The rest goes record by record, which is what bounds the memory of stalled writers. - After a fatal error on an established session the socket emits `'error'` and then `'close'`. Node emits `'error'` and leaves the socket open. - A paused reader whose own write the kernel rejects loses what it had not read yet, with an `EPIPE`, as on Node. `main` reports no error there and delivers it. - On `main` too: a `Bun.listen` socket without `allowHalfOpen` that has unsent ciphertext when the client's `shutdown()` arrives loses that ciphertext (32 KiB), and over plain TCP `end()` with the peer still sending is a close over unread input, so a reset. - Differences from both `main` and Node that the differential runs below found and that stay, all with a peer that aborts: `ECONNRESET` instead of a clean `'end'` after the socket's own `'finish'` when the peer destroyed with unread data; under TLS 1.2, a zero-length `write()` followed by `destroy()` in `'secureConnection'` leaves the client without `'secureConnect'` (a plain `destroy()` there matches Node); a TLS 1.2 client that destroys in `'secureConnect'` gets no `'session'`; a `ClientRequest` whose handshake fails with an alert emits `'error'` and `'close'` but no `'finish'` (`writableFinished` is true). - Once `tls.DEFAULT_CIPHERS` is assigned, `fetch.preconnect()` opens nothing: `fetch()` then uses a context of its own, and a socket warmed under the default one would never be picked up. - A TLS `send()` that the kernel refuses outside a `write()` call (the drain of unsent ciphertext) is reported with the close, as `read EPIPE` / `read ECONNRESET`. Node says `write EPIPE`. `main` does not report it at all. - Once the application has a session over a `net.Socket` (TLS in TLS, http2 `emit("connection")`), a peer that never sends its FIN holds that socket after the TLS socket closed, as on `main`. Node destroys it. Two tests of #38154 are `todo` for this. Closing it any earlier (at its `'finish'`, say) makes the kernel drop what it has not sent yet as soon as the peer's close_notify arrives. - Plaintext that was queued on a socket before it was wrapped (STARTTLS with a backlog) is dropped when the TLS socket is destroyed, or its handshake fails, before the session is accepted. Node drops it too, except on `destroySoon()`. `main` sends it. - Over a stream that is no `net.Socket`, `end()` can still cut what that stream has buffered, and there is no backpressure, both as on `main` (#43877). - `tls.secureContext` (the undocumented door node:tls uses) is not read by a Windows named pipe listener, which builds its context from the options. On `upgradeTLS({ isServer: true })` the options next to it are the policy, as with Node's `SetVerifyMode`. - `selectServerName()` rebuilds the name tree per ClientHello for injected sockets of a server with `addContext()` entries: 0.4 µs for 1 entry, 3.7 µs for 10, 41 µs for 100, against 631–1111 µs for a handshake. #### Not included Left open, because they need a decision or are not TLS: #43877 + #44394 (see "Worth a look" 5; #43874 stays open with them), #38548, #38591 (both shrink who is trusted), #41589 (`verify-full` vs `NODE_TLS_REJECT_UNAUTHORIZED=0`), #37197, #41706, #43216, #33487, #33545, #36707, #32435, #37255, #28691, #40275, #30314 (features), #38120 (needs the BoringSSL fork, as did #33517, which the stale bot has closed since), #38529 (needs a Windows measurement), #34342, #38232, #43089, #44454, #40451, #42710, #44527, #38088, #38093, #41898. #37896, #42054 and #37013 stay open for the halves not taken. `http.createServer({ key, cert })` keeps serving TLS on purpose. One open question: `tls: {}` (an object that names no TLS option) is plain TCP on `Bun.listen` / `Bun.connect`, here and on `main`. It is the same trap as `tls: []`, but changing it changes a Bun default, so it is left alone. ### How did you verify your code works? - Every new test fails on `main` for the stated reason and passes here, except guards that pin existing behavior, each shown to fail when its clause is removed. `node:*` tests also pass on Node v26.3.0; the few that cannot say which Node version has the behavior. - 212 test files that touch TLS, sockets, http, http2, fetch, WebSocket, SQL, Valkey and workers: 6154 pass, 2 fail. Both are seen on `main` too: `serve.test.ts` "root range port" (the box runs as root), and `worker_threads.test.ts` "terminate(): nothing of the worker's runs after the request", which is flaky there and passed in the run below. - 58 of those files the way the ASAN lane runs them (LeakSanitizer + `BUN_JSC_validateExceptionChecks`): 58 files, 48 of them with leak checking, 4132 pass, 3 fail. All three also fail on `main`: `serve.test.ts` "root range port", `node-net.test.ts` "should not leak when connect({path}) fails synchronously on a reused handle" (times out under this environment), `worker_threads.test.ts` "process.exit() with a shell cp in flight" (a `ShellCpTask` leak). - 647 vendored `test-tls-*`, `test-https-*`, `test-net-*`, `test-http2-*`: the only two failures also fail on `main`. - The SNI matcher was diffed against both old matchers: 3 seeds × 1.23 M lookups × 3 registration flavours, every difference in one of the intended classes, TCP and HTTP/3 identical on every lookup. - The headline rows were also driven by hand with scripts against this build, `main` and Node v26.3.0: cleartext https, `crl`, `tls: []` / `{ secureContext }`, the `ca` / `requestCert` matrix, the client certificate on a wrong-name server, late `setSession()`, `destroy()` in `ALPNCallback`, `[rsa, ec]` identities with an intermediate from `ca`, `WebSocket` `checkServerIdentity`, a corrupted record, the Duplex read above, `tls.DEFAULT_CIPHERS`. - The `setSession()` guard was checked against the real `abort()` at 43 handshake states. - `bun run rust:check-all`: 12 of 12 targets. `tsc`, oxlint, source lints, prettier, rustfmt, mordant clean. - usockets' `_Nonnull` is compiled out of debug builds, so 105 of those files were also run on a local release ASAN build with the CI runner's environment (92 with leak checking): 4595 pass, 1 fail, `child_process.test.ts` "spawn reports EPERM after dropping privileges", which cannot pass as root and fails on `main` too. - The close of a TLS socket over another stream ("Worth a look" 5): eleven seeded differential fuzzers, 8,424 scenarios compared, each run on a release ASAN build of this branch, on `main`, on Node v26.3.0 and on the earlier state of the branch. Against `main`: - Data that `main` delivers in full is cut in 5 scenarios, and about 150 that `main` cuts arrive in full. Of the 5, in 2 `main` never notices the peer's close and keeps the socket for good, 2 call `end()` on the middle one of three sessions over an in-memory Duplex, and 1 does the same on Node. - No dead timeout, no silent reset and no uncaught error that `main` does not have (4 uncaught errors fewer). - A socket stays open where `main` closes it in 109, and closes where `main` keeps it in 295. 92 of the 109 do the same on Node or on the earlier state (a `destroy()` that an in-memory Duplex does not show its peer, half-open peers). 14 wait for a peer that paused reading and so does not read the FIN (#42332's backpressure, as in Node); the socket's own timeout fires there. 3 are left: one on a 5 ms timer, two with three sessions over an in-memory Duplex. - The earlier state of the branch cut data in 173 of the 400 scenarios of one of them, where `main` cuts none and this cuts none. - 23 new tests pin what they found. Each earlier attempt at this fix fails the ones that describe it, the earlier state of the branch fails 7, and all pass on Node. - After that change: 999 test files on the release ASAN build (20,246 pass; the 11 files that fail need a database, Docker, DNS or a non-root user, or share a temp directory with a parallel run and pass alone), 61 on the debug build. - TLS over a file descriptor (`openssl.c`, the path of `fetch`, `Bun.serve`, `tls.connect`, `Bun.connect`) got the same treatment after the rebase: seeded differential fuzzers on CI's release build of this branch, on `main` and, for `node:*`, on Node v26.3.0. Every runtime also against itself for the noise floor, injected faults and known bugs of `main` as positive controls, and a difference counts only if it shows in 5 of 5 fresh processes. - `node:tls` over TCP: 11,500 scenarios (one connection with Node as the oracle line by line; 2 to 60 connections beside stalled neighbours; raw peers that break the handshake). HTTPS: about 136,000 runs over `Bun.serve` + `fetch`, `node:https`, `node:http2` and `wss://`, also with the two ends in different runtimes. `Bun.connect` / `Bun.listen` / `upgradeTLS`: 11,500 scenarios and 720 slow connections, with writers driven by what `write()` returns, beside up to 6 stalled, dripping, closing or resetting neighbours, and plain TCP as a second oracle. No crash, hang, duplication, reordering or silent truncation, and no change in time or in connection reuse. - They found six things that `main` does better, none of which any test showed. All are fixed, each with a test that fails on the build before: what the peer sent lost behind a rejected `send()` (23 scenarios, and an early HTTPS response lost with only `EPIPE`), the same silently for a paused reader, `server.close()` never calling back on a half-open server after a ClientHello and a reset (17), `closeAllConnections()` taking 12 s with a stalled client, `end()` losing up to 1.3 of 4 MiB that `write()` had reported while the peer still uploads, and `end()` a little after a stall never closing beside other stalled TLS sockets. The last two fixes also deliver the 1 to 2 MiB that `main` loses there, and close the socket that `main` keeps for good without such neighbours. - All of them again after every fix, on CI's release build of it. That caught one regression of a fix itself (a reader stopped for backpressure lost 86,385 bytes, 1 of 6,000 scenarios), fixed too. On the last build: scenarios that lose data where `main` does not 23 → 2, and Node loses it in both, with the same `EPIPE`; `server.close()` that never calls back 17 → 0; connections held 4 → 0; requests that end in an error only where `main` has a response 6 → 0. With a Node server in another process, a request ends in an error only in 8 and 10 of 1,500 scenarios here, 5 and 3 on `main`, 10 with Node as the client. - `Bun.connect` / `Bun.listen` on the last build against `main`, in scenarios: hangs 0 against 1,031, sockets and fds never released 0 against 965, corrupted data 0 against 345, `abort()` 0 against 26 (`setSession()` after the handshake), writers that never close 0 against 101 of 720 connections. No kind of failure shows here and not on `main`. About a third of the slow connections close later than on `main`, in 1 to 16 s instead of at once, waiting for unsent ciphertext or for the peer's close_notify, and 79 more of them deliver all that `write()` reported. RSS and time with 16 to 256 stalled writers are the same. - What they found that `main` does worse: a `WebSocket` that calls `close()` with sends pending loses messages in 81 of 999 scenarios (0 here), 37 server sockets left open, 10 `server.close()` that never call back, 20 write callbacks that never run. - The kqueue fix cannot be run on Linux. The `connectionListener` count test now says what became of a missing connection, which is how the cause was found (`'tlsClientError'` "socket hang up", then `read ECONNRESET` at the client of the same port, after its `'secureConnect'`). On macOS x64 it failed every attempt of the three builds before the fix and passed at the first attempt of the build with it. - Windows and macOS were only run by CI. Four new tests asserted what only the Linux kernel does (a FIN read ahead of a reset, unread bytes surviving a reset, loopback buffer sizes, `fstat()` on a socket) and now say so per platform. --------- Co-authored-by: autofix-ci[bot] <114827586+autofix-ci[bot]@users.noreply.github.com> Co-authored-by: robobun <117481402+robobun@users.noreply.github.com>
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Problem
fetch()has no way to control the shape of its TLS ClientHello. Servers fingerprint clients by it (JA3, JA4), and users who need to look like a browser have no option short of another HTTP stack.src/httpor thetlsoption parser (src/runtime/socket/SSLConfig.rs) reaches them.Fix
tlsoptions onfetch:ja3,grease,permuteExtensions,certificateCompression,applicationSettings,echGrease,ocspStapling,signedCertificateTimestamps,sessionTickets.ja3expands aversion,ciphers,extensions,groups,formatsstring into the cipher list, group list, version bounds and the extension toggles; an explicit option wins over what the string implies. The parse is strict in both directions: a cipher suite, group or extension BoringSSL cannot send, or an extension it always sends that the string leaves out, is aTypeError, so a caller never gets a silently different fingerprint.src/http/tls_fingerprint.rsholds the parser and the two apply steps. Context-wide settings (GREASE, certificate decompression callbacks for brotli, zlib and zstd) go on the customSSL_CTXthe fetch client already builds for a non-defaultSSLConfig. Per-connection settings (ALPS forh2, ECH GREASE, OCSP, SCT, session tickets, extension shuffle, TLS 1.3 suite order) go on theSSLinconfigure_http_client_with_alpn, which also covers CONNECT tunnels.SSLConfig's hash and equality, so the interned-config and SSL context caches keep distinct fingerprints apart. Chrome's ClientHello is reproduced in full: cipher order, groupsX25519MLKEM768:X25519:P-256:P-384, the two key shares, brotli certificate compression, ALPS, ECH GREASE, GREASE values and shuffled extensions.fetch TLS fingerprint optionsblock intest/js/web/fetch/fetch.tls.test.ts(40 tests; a raw TCP listener parses the ClientHello, through a CONNECT tunnel too; the option tests fail on the released bun). Alsotest/js/node/tls/,tls-keepalive,fetch-abort-ssl-context-eviction,proxy.test.ts,websocket-client, the bun-types test, and a Windows build.Background
EVP_has_aes_hardware(), so the parser pins it with theSSL_set_aes_hw_override_for_testingsetting (C++ linkage only, hence the small shim inNodeTLS.cpp). This keeps a givenja3string machine-independent.Notes
Out of scope, on purpose: exact extension order, Firefox's
delegated_credentials(34) andrecord_size_limit(28) extensions, FFDHE groups and Safari's CBC-SHA256 suites. All of those need a BoringSSL fork patch (curl-impersonate carries one). Aja3string that lists them throws.The options are parsed by the shared
TLSOptionsdictionary, soBun.connectandWebSocketaccept them too. There,ja3still sets the cipher list and the groups (they travel through the usockets options), but the extension toggles only take effect infetch. The types and docs list them under the fetchtlsoptions only.permuteExtensionsis applied perSSLrather than on theSSL_CTX: BoringSSL copies that flag into the connection atSSL_new, and the CONNECT tunnel creates itsSSLbefore the fetch client can touch the context. GREASE and the compression algorithms are read from the context at handshake time, so they can be set afterSSL_new.bun_zstd::decompress_appendis nowpubso the zstd callback can decode into a bounded buffer in one call.HTTP/2 fingerprinting (SETTINGS values, header order, the Akamai string) is not part of this change.
A repeated id inside one JA3 field (
0-5-5-10) is rejected too, since a ClientHello carries each cipher, extension and group once.Rebase onto main (after #40238, which made
bun_core::Stringown its WTF ref): the conflict was insrc/jsc/generated.rs. TheDropimpls this branch had added to release the new string fields were dropped, since the strings now release themselves, and theGenOpt::get()calls on string fields becameas_ref().[review] gate passed · iteration 2 · 17 files touched
fails on main (without fix)
passes on PR (with fix)
diff hotspot
gate history · 7 passed · 0 rejected · iteration 2
evidence per changed file