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…t reads fetch and the WebSocket client checked the head size cap only when picohttp returned ShortRead. A read that crossed the cap and also delivered the blank line skipped the check, so the same response was accepted or rejected depending on the peer's write sizes. Check the parsed head length too.
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Updated 10:58 PM PT - Sep 18th, 2026
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Included review availability: Your plan provides up to 10 included reviews per hour; 1 remains after this review. WalkthroughThe change enforces response-header size limits for HTTP fetch and WebSocket handshakes. Oversized headers now produce ChangesResponse header size enforcement
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In `@test/js/web/fetch/fetch.test.ts`:
- Line 3655: Update the repetitive string construction in the affected test
lines to use Buffer.alloc(count, fill).toString() instead of repeat(),
preserving the existing counts and fill characters and the resulting string
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LGTM — thanks for addressing the earlier nits (Buffer.alloc for the filler strings, onclose wired to settle the WebSocket probe).
What was reviewed:
- The two post-parse guards in
src/http/lib.rsandWebSocketUpgradeClient.rs— both strictly tighten the existing cap and reuse the same error path (close_and_fail/HeadParse::Invalid) as the ShortRead arm they mirror. - Confirmed
parsed.bytes_read/head_lencount only the head through the blank line, so pipelined body/frame bytes after it are not counted against the cap. - Boundary tests cover at-cap-succeeds and one-past-fails for both delivery shapes; the
connect()helper now settles on all three terminal WebSocket events.
Extended reasoning...
Overview
Two small Rust hunks and two test additions. In src/http/lib.rs the 1 MiB MAX_RESPONSE_HEADER_BUFFER constant is hoisted out of the ShortRead arm and a post-parse check rejects a successfully parsed head whose parsed.bytes_read exceeds it. In src/http_jsc/websocket_client/WebSocketUpgradeClient.rs a symmetric match-guard maps HeadParse::Done { head_len > max_http_header_size() } to HeadParse::Invalid. Both changes make the existing head-size cap depend only on the parsed head length, not on whether the peer's writes happened to leave picohttp in ShortRead at the moment the accumulated buffer crossed the threshold. Tests in fetch.test.ts and websocket-client-short-read.test.ts exercise exact-cap (accepted) and cap+1 (rejected) heads in both chunked and single-write delivery.
Security risks
This is a resource-limit enforcement on peer-controlled bytes. The change is a strict tightening: it adds a rejection path and moves/compacts a comment; nothing is loosened. bytes_read on a successful picohttp parse is the head length through the terminating CRLF, so body bytes and pipelined WebSocket frames after the blank line are not counted — the cap cannot spuriously reject a small head followed by a large body. Failure routes through the same close_and_fail(ResponseHeadersTooLarge) / HeadParse::Invalid machinery as the pre-existing ShortRead check, so no new error surface. No auth, crypto, or permission logic is touched.
Level of scrutiny
Low-to-moderate. The Rust change is ~10 lines net, mechanical (mirror an existing check on the sibling parse outcome), and covered by boundary tests that follow REVIEW.md's at-limit/one-past pattern. Both sibling clients (fetch and WebSocket upgrade) are fixed together, satisfying the "fix the whole class" rule. No CODEOWNERS entry covers these paths.
Other factors
Since the previous review, three follow-up commits landed: f6e5f290 swapped String.repeat for Buffer.alloc(...).toString() and wired ws.onclose to settle the connect probe (both of my prior optional nits), and 70e09c02/e7cc527c shortened the constant comments per the github-actions style checker. All prior inline threads on this PR were bot-originated (coderabbit, github-actions, and my own optional nits) and each has a corresponding code change in the follow-up commits; there is no outstanding human CHANGES_REQUESTED. The tests use port: 0, 127.0.0.1, using/await using for listener cleanup, and settle on every terminal event, matching test/CLAUDE.md conventions.
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Status: ready for review. One decision is open for a maintainer. Reproduced with a raw TCP server that sends the response head in one write or in two writes.
The new cases in This PR replaces #41569. It has the same change to Decision needed: a complete 101 head over the bound now always fails. Before, it opened when no read ended inside it. Node rejects such a head with |
…onse-head-cap-deterministic
The head arrives in two reads. The first read is an incomplete head under the cap. The completed head is over the cap and must be rejected.
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LGTM — small, self-contained change that makes both head-size caps a function of the parsed head length, and the points raised on earlier pushes (onclose wiring, .repeat → Buffer.alloc) are addressed in the current code.
What was reviewed:
src/http/lib.rs: the newbytes_read > MAX_RESPONSE_HEADER_BUFFERcheck sits after every successful parse in the loop, so it also covers 1xx heads and the CONNECT reply;close_and_fail+returnmatches the sibling error exits.src/http_jsc/websocket_client/WebSocketUpgradeClient.rs: the guard arm onHeadParse::Donefeeds all three callers (handle_data, proxy reply, decrypted-tunnel path) which already mapInvalidtoErrorCode::InvalidResponse.- Fail-before reasoning: a head of cap+1 bytes can never trip the old
ShortReadcheck (the incomplete accumulator is at mostcapbytes), so the cap+1 cases reject only with the new check at any segmentation; the exact-cap cases are controls. - The 50 ms
setTimeoutinserve101copies the split-read pattern the two existing tests in the same describe block already use.
Extended reasoning...
Overview
The PR touches two HTTP clients: the fetch client's response-head loop in src/http/lib.rs (HTTPClient::handle_response_headers, ~line 3711) and the WebSocket upgrade client's buffer_and_parse_head in src/http_jsc/websocket_client/WebSocketUpgradeClient.rs. In both, the existing head-size cap was checked only on picohttp::ParseResponseError::ShortRead, meaning a head that crossed the limit and also delivered the blank line in the same read escaped the cap. The fix adds a post-parse check on the parsed head length (parsed.bytes_read / head_len) using the same constant each path already used. Net native diff is roughly eight lines plus hoisting the 1 MiB constant out of the ShortRead arm. Tests were added in the existing files for each module: three fetch cases (exact cap in 4 KiB writes accepted; cap+1 in one write and in 4 KiB writes rejected with error.code === "ResponseHeadersTooLarge") and three WebSocket cases (exact cap opens; cap+1 in one write rejected; 21000-byte head split at 16000 rejected).
Security risks
This is a resource-bound hardening change (DoS surface of untrusted response bytes). It strictly tightens: heads at or below the cap are still accepted at any segmentation; heads above the cap are now rejected regardless of segmentation. No new allocation is introduced before validation — the fetch check runs on the already-parsed length, and in the WebSocket path the pre-existing body.to_vec() copy before the guard is bounded by the ShortRead accumulator cap plus one read. No credentials, TLS flags, or auth paths are touched. The 1 MiB vs 16 KiB split between fetch and WebSocket is pre-existing and unchanged.
Level of scrutiny
Moderate — the code sits on the HTTP client hot path, but the added work is a single integer comparison per parsed head, placed alongside existing error exits that use identical close_and_fail/terminate sequencing. I traced the fetch loop: the check is inside the loop, so 1xx interim heads and the CONNECT proxy reply (which share the loop) get the same cap, consistent with how the ShortRead check already applied to them. For WebSocket, all three callers of buffer_and_parse_head (handle_data, the proxy CONNECT reply path, and handle_decrypted_data) already map HeadParse::Invalid to ErrorCode::InvalidResponse, so the new arm has no unhandled consumer. head_len is destructured only in the guard arm and the Ok(done) arm keeps the full value, so the refactor is behavior-preserving for the under-cap case.
Other factors
I checked the fail-before argument statically rather than by execution (test execution was not available in this run): with a head of exactly cap+1 bytes, the incomplete-head accumulator seen by the ShortRead arm can be at most cap bytes (the final \n is missing), so the old check can never fire, and the unfixed build accepts it at every segmentation; the exact-cap cases pass on both builds as controls. The fetch fixture's buildHead does its expect calls synchronously within the test body (not in a callback that may never fire), and the backpressure pump handles partial socket.write returns. The setTimeout(..., 50) in serve101 mirrors the two existing tests in the same describe block that split the 101 head across reads, and the split test remains valid even if the OS coalesces the two writes. My earlier inline comments (missing onclose handler; .repeat instead of Buffer.alloc) are addressed in the current tree. No CODEOWNERS entry covers the changed files, and the bug-hunt exited on a dry streak with no findings.
…e head with its own error Node applies max_http_header_size to the bytes llhttp counts, which leaves out framing. A bound on the raw head at exactly that value rejects heads that Node accepts. Add the framing of 128 fields as slack on both arms, the same bound bun-uws uses for request heads. An over-size head now fails with "Response headers exceed --max-http-header-size" in place of "Invalid response".
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Problem
fetch()caps the response head at 1 MiB (ResponseHeadersTooLarge), only in the picohttpShortReadarm ofhandle_on_data_headers(src/http/lib.rs:3738). A read that crosses 1 MiB and delivers the blank line skips the check.new WebSocket()has the same gap inbuffer_and_parse_head(src/http_jsc/websocket_client/WebSocketUpgradeClient.rs:723) for its 16 KB cap. The cap is also on raw bytes: Node opens a 16400 byte 101 head that Bun rejects when a read ends past 16384.Fix
fetch: after a successful parse, reject whenparsed.bytes_readexceeds the cap. WebSocket: both arms compare the head withmax_http_header_size() + 128 * 4 + 64, the raw bound bun-uws uses for request heads. An over-size head fails withResponse headers exceed --max-http-header-size.HPE_HEADER_OVERFLOW.test/js/web/fetch/fetch.test.ts(3 cases),test/js/web/websocket/websocket-client-short-read.test.ts(4 new, 1 updated). Stock bun fails 2 and 4 of them.Background
ShortReadmeans no blank line yet, so the client buffers and parses again on the next read.on_datadelivers whatrecv()returned, up to 512 KiB. The peer's writes and timing decide where a read ends.max_http_header_sizeis--max-http-header-size. Node applies it to the bytes llhttp reports (status text, field names, values), not to framing (": ","\r\n").Replaces #41569. Refs #20488.
Notes
What changes for real traffic. The
fetch()half only affects heads over 1 MiB. The WebSocket half is the one with reach. On main, a head over the cap opens whenever no read ends inside the incomplete head with more than 16384 bytes buffered. Examples: a head of up to 512 KiB that arrives in one read, and a 21000 byte head whose first read is 16000 bytes. With this PR every head overmax_http_header_size() + 576fails, at any segmentation. A head at or under that bound opens at any segmentation. On main a 16400 byte head fails when a read ends past 16384, so the window between 16384 and the bound is now more lenient than main.buffer_and_parse_headhas three callers, so the bound also applies to the proxy CONNECT reply and to the 101 head inside a TLS tunnel.--max-http-header-sizeraises the bound. With--max-http-header-size=32768a 20000 byte head opens.Node comparison (v26.3.0,
http.requestupgrade, one write from a raw TCP server, same head layout as the tests): a head of 16416 raw bytes or less opens. 16417 and above fail withHPE_HEADER_OVERFLOW. Node'sTrackHeaderadds the lengths of the status text, the field names and the values, and fails at 16384. For this head the framing is 33 bytes, which gives the 16417 threshold. This branch opens up to 16960 raw bytes and rejects 16961 and above. So it is never stricter than Node for up to 128 fields, and at most 576 bytes more lenient.Why a raw bound with slack.
packages/bun-uws/src/HttpParser.hdoes the same for request heads:MAX_HEADER_FRAMING_SLACK = UWS_HTTP_MAX_HEADERS_COUNT * 4 + 64, with the comment that raw bounds get the framing as slack "so we don't reject requests Node accepts". The result stays a function of the raw head length alone, which keeps it independent of where reads end. If the field limit of this client grows (#43243 proposes 2000), the slack must follow it.History. #31175 added the cap to the accumulator, before the parse, once buffering had begun. #32394 moved it into the
ShortReadarm so that frames pipelined after the head are not counted. That left a complete head unchecked. This PR checkshead_len, so pipelined frames are still not counted.Intentionally not changed.
handle_proxy_responserejects a CONNECT reply early when the first bytes are notHTTP/1.1 200orHTTP/1.0 200. That check looks at the status line prefix, not at the head size. It has its own dependence on read boundaries: a 407 reply reportsProxy connection failedin one read andProxy authentication requiredwhen the first read has 13 bytes or fewer. That is a separate bug and this PR leaves it alone. Moving the cap intopicohttp::Response::parse_partsis a possible follow-up.Open question for a maintainer. No maintainer has said that a complete head over the bound must be rejected. Node (through llhttp) and the bun-uws request parser reject it. If the preferred policy for the WebSocket client is a growth bound only, the
Okarm check can be dropped and the rest of the PR still stands.Fail-before. Stock bun (1.4.3-canary.1+b52d51348):
fetch.test.ts: accepts a head of 1 MiB + 1 byte in one write and in 4 KiB writes. A head of cap + 1 bytes can never hit the old short-read check, because no incomplete prefix is longer than the cap. The exact 1 MiB case is a control.websocket-client-short-read.test.ts: opens a 16961 byte head, opens a 21000 byte head whose first read is 16000 bytes, rejects a 16400 byte head whose first read is 16390 bytes, and reportsInvalid responsefor an incomplete 20 KB head. The 16960 byte case is a control.Also ran (debug build with ASAN):
websocket-client.test.ts,websocket-upgrade.test.ts,websocket-handshake-event.test.ts,websocket-custom-headers.test.ts,websocket-accept-header-validation.test.ts,websocket-proxy.test.ts,error-event.test.ts,test/js/bun/http/proxy-stress-protocol.test.ts. All pass.websocket-client-short-read.test.tspassed 5 runs in a row.The full
fetch.test.tsrun under the ASAN debug build also fails a set of tests that need public internet (the container proxy denies it), root-sensitive permission tests, and a few(with gc)tests that time out at 5 s. These fail without this change too.should allow very long redirect URLSneeds 5.6 s on the debug build (200 forced GCs) and passes with a larger timeout.Trial merges of this branch with #43243 and with #43181 are clean.
no test proof · iteration 3 · platform-specific test(s) that do not run on this machine, deferring to CI, which covers all platforms: test/js/web/fetch/fetch.test.ts