Headers: throw instead of aborting when a value passes the string length limit by robobun · Pull Request #42218 · oven-sh/bun

robobun · 2026-09-10T16:27:44Z

Problem

Headers.append() and Headers.get("set-cookie") abort the process when the value they build passes String::MaxLength (2**31 - 1 characters): panic(main thread): abort() called, exit code 134. A Headers method that rejects a name that long aborts too, because the message quotes the name.
get("set-cookie") also aborts on about 1 MB of data. It reserved the first value's length times the count, as a 32-bit product.
The cause: makeString() calls CRASH() on overflow (MakeString.h:105), and so does a default StringBuilder (StringBuilder.cpp:46).

Fix

appendToHeaderMap (FetchHeaders.cpp:110) combines with tryMakeString(), which returns a null String on overflow, and reports Exception { OutOfMemoryError }. JS sees RangeError: Out of memory, the error JSC throws for an over-long string. The stored value stays.
HTTPHeaderMap::tryJoinSetCookieHeaders() sums the real length of every value in 64 bits. It returns nullopt past the limit, and FetchHeaders::get() throws the same error.
The five messages that quote a header name go through exceptionWithMessage(), new in Exception.h. Throw instead of aborting when an ERR_* error message passes the string length limit #42202 merged the same tryMakeString pattern for performance.measure.
Verified: test/js/web/fetch/headers.test.ts, four new cases, all fail on main. Also 12 related suites (notes). Self-reviewed: 3 concerns raised, 3 addressed.

Background

A WTF::String holds at most 2**31 - 1 characters. JS has the same limit.
makeString and StringBuilder default to CrashOnOverflow. tryMakeString and OverflowPolicy::RecordOverflow report the overflow instead.
Headers stores each Set-Cookie value of its own. get("set-cookie") is the one reader that joins them with ", ". getSetCookie() returns the array and is unaffected.
ExceptionCode::OutOfMemoryError becomes createOutOfMemoryError in JSDOMExceptionHandling.cpp.

Notes

Reproductions on main (each exits 134):

// append: combine past the limit
const h = new Headers();
const s = "x".repeat(2 ** 30);
h.append("accept", s);
h.append("accept", s);

// get("set-cookie"): join past the limit
const c = new Headers();
c.append("set-cookie", s);
c.append("set-cookie", s);
c.get("set-cookie");

// get("set-cookie"): only the 32-bit capacity product passes the limit, the join is ~1 MiB
const k = new Headers();
k.append("set-cookie", "x".repeat(2 ** 20));
for (let i = 0; i < 2047; i++) k.append("set-cookie", "a");
k.get("set-cookie");

// a message that quotes a name past the limit
const bad = "\0".repeat(2 ** 31 - 20);
new Headers().get(bad);   // also set(), append(), has() and delete() with `bad` as the name

The new test cases:

has() names the invalid header the way get() and delete() do. It closed the name with " instead of ' (pre-existing typo on a line this change rewrites).
The Set-Cookie capacity case: about 1 MB, 0.3 s, no memory gate.
A value past the limit: append combine and get("set-cookie") join, with one 2**30-character string. About 9 to 16 s on a debug ASAN build, mostly append() checking 1 GiB of value for invalid characters. Skipped below 8 GiB of RAM, 60 s ceiling.
A message past the limit: set, append, get, has and delete with a name of 2**31 - 20 characters. Validation stops at the first character, so each call takes 2 to 5 ms. The whole child takes about 2 s and 2.4 GB on a debug ASAN build. Own child, so that the two big strings never coexist. Skipped below 8 GiB of RAM, 30 s ceiling (the one test/js/bun/util/error-message-string-length-limit.test.ts uses for the same work).

Neither heavy case allocates the over-long result: each length is computed before any character is copied.

Coverage of the subsystem: FetchHeaders.cpp has four value joins reachable from JS (", " for a common header, "; " for Cookie, ", " for an uncommon name, and the Set-Cookie join). All four are in this change. The makeString joins left in HTTPHeaderMap::add / addUncommonHeader are fed by the wire parsers, whose header bytes are bounded, and by a fill into an empty map.

FetchHeaders::get() is rewritten to look the name up once and branch, instead of calling HTTPHeaderMap::get(StringView) and then inspecting the result. For every input that does not overflow it returns what it returned before: the common header value, the uncommon header value, a null String for an absent header, and the TypeError for a name that is not a valid token. The token check now lives only on the uncommon path, because a name that findHTTPHeaderName resolves is always a valid token.

HTTPHeaderMap::get(HTTPHeaderName::SetCookie) returns a null String when the join does not fit, which reads as an absent header. No C++ or Rust caller reads the joined Set-Cookie value today. The JS path goes through FetchHeaders::get(), which throws.

#42692 removed the value from the invalid header value message while this PR was open (Header 'x-bun' has invalid value). That message is now bounded for a known header name, so that overload keeps main's line. For a name that is not a known header the message still quotes the name, so it goes through exceptionWithMessage(). That one site is not in the test: to reach it the name has to be a valid token of 2 GiB, which isValidHTTPToken scans in full, 11 s on a debug ASAN build. Checked by hand: new Headers().set("a".repeat(2 ** 31 - 20), "\0") throws RangeError: Out of memory with this branch.

Suites run with the debug build, all green: headers.test.ts, headers.undici.test.ts, headers-case.test.ts, fetch_headers.test.js, fetch-header-str-bounds.test.ts, cookies.test.ts, deno/fetch/headers.test.ts, bun-serve-headers.test.ts, bun-serve-cookies.test.ts, cookie-map.test.ts, cookie.test.ts, fetch-header-count-limit.test.ts, error-message-string-length-limit.test.ts.

Same mechanism, other files, other PRs: #42202 (ERR_* messages, merged), #42314 (stack frames, merged), #42216 (WebSocket messages), #42259 (domainToASCII, ReadableStream type), #42237 (Cookie), #42312 (YAML, JSON5, XML stringify), #42534 (URL, URLSearchParams), #42308 (process.execve). #42250 (linear Headers.append) builds on the append hunk here.

[human-review] gate passed · iteration 0 · 5 files touched

fails on main (without fix)

ASAN without fix: 4 FAILED
$ BUN_DEBUG_QUIET_LOGS=1 bun scripts/build.ts --profile=debug --quiet test "--reporter=junit" "--reporter-outfile=/tmp/pr_gate.xml" test/js/web/fetch/headers.test.ts
bun test v1.4.3 (367d939d9)

test/js/web/fetch/headers.test.ts:
(pass) Headers > constructor > can create headers from no arguments [3.93ms]
(pass) Headers > constructor > cannot create headers from null [2.84ms]
(pass) Headers > constructor > can create headers from empty object [2.11ms]
(pass) Headers > constructor > can create headers from object [1.49ms]
(pass) Headers > constructor > deleted key in header constructor is not kept [2.28ms]
(pass) Headers > constructor > constructing headers from an object interleaves Get with value conversion [4.19ms]
(pass) Headers > constructor > constructing headers from an object with a getter interleaves Get with value conversion [5.08ms]
(pass) Headers > constructor > constructing headers from an object observes a getter installed by an earlier value's toString [7.68ms]
(pass) Headers > constructor > constructing headers from an object propagates an exception from a getter installed by an earlier value's toString [5.17ms]
(pass) Headers > constructor > construc
... (truncated)

release without fix: 4 FAILED
bun test v1.4.3-canary.1 (367d939d9)

test/js/web/fetch/headers.test.ts:
(pass) Headers > constructor > can create headers from no arguments [0.04ms]
(pass) Headers > constructor > cannot create headers from null [0.04ms]
(pass) Headers > constructor > can create headers from empty object [0.03ms]
(pass) Headers > constructor > can create headers from object [0.02ms]
(pass) Headers > constructor > deleted key in header constructor is not kept [0.02ms]
(pass) Headers > constructor > constructing headers from an object interleaves Get with value conversion [0.06ms]
(pass) Headers > constructor > constructing headers from an object with a getter interleaves Get with value conversion [0.06ms]
(pass) Headers > constructor > constructing headers from an object observes a getter installed by an earlier value's toString [0.10ms]
(pass) Headers > constructor > constructing headers from an object propagates an exception from a getter installed by an earlier value's toString [0.05ms]
(pass) Headers > constructor > constructing headers from an object keeps own-property semantics after setPrototypeOf mid-conversion [0.05ms]
(pass) Headers > constructor > can create headers from 
... (truncated)

passes on PR (with fix)

ASAN with fix: all passed
$ BUN_DEBUG_QUIET_LOGS=1 bun scripts/build.ts --profile=debug --quiet test "--reporter=junit" "--reporter-outfile=/tmp/pr_gate.xml" test/js/web/fetch/headers.test.ts
bun test v1.4.3 (367d939d9)

test/js/web/fetch/headers.test.ts:
(pass) Headers > constructor > can create headers from no arguments [3.40ms]
(pass) Headers > constructor > cannot create headers from null [2.86ms]
(pass) Headers > constructor > can create headers from empty object [2.05ms]
(pass) Headers > constructor > can create headers from object [1.76ms]
(pass) Headers > constructor > deleted key in header constructor is not kept [2.24ms]
(pass) Headers > constructor > constructing headers from an object interleaves Get with value conversion [4.06ms]
(pass) Headers > constructor > constructing headers from an object with a getter interleaves Get with value conversion [5.11ms]
(pass) Headers > constructor > constructing headers from an object observes a getter installed by an earlier value's toString [7.94ms]
(pass) Headers > constructor > constructing headers from an object propagates an exception from a getter installed by an earlier value's toString [4.95ms]
(pass) Headers > constructor > construc
... (truncated)

release with fix: all passed
$ bun scripts/build.ts --profile=release
[configured] bun-profile → bun (stripped)
  target       linux-x64-gnu
  build type   Release
  build dir    ./build/release
  revision     f7d1b12bc0
  features     lto, baseline

23 deps, 136 codegen, 1176 objects in 669ms

ninja: Entering directory `/workspace/bun/build/release'
[1/4] fetch lolhtml
[lolhtml] up to date
[2/4] fetch rust-argon2
[rust-argon2] up to date
[2/4] cargo plan → /workspace/bun/build/release/rust-target/plan.json
244 units: 172 lib, 16 proc-macro (host), 19 custom-build (host), 15 run custom-build, 17 lib (host), 4 run custom-build (host), 1 rlib
[3/4] reconfigure
[1/1499] mkdir stamps
[2/1499] mkdir codegen
[3/1499] install /workspace/bun
bun install v1.4.3-canary.1 (367d939d9)

Checked 26 installs across 65 packages (no changes) [14.00ms]
[4/1499] install /workspace/bun/packages/bun-error
bun install v1.4.3-canary.1 (367d939d9)

Checked 1 install across 2 packages (no changes) [1.00ms]
[5/1499] install /workspace/bun/src/node-fallbacks
bun install v1.4.3-canary.1 (367d939d9)

Checked 111 installs across 104 packages (no changes) [5.00ms]
[6/1499] rustc unicode_ident 
[7/1499] gen node-fallbacks
... (truncated)

diff hotspot

src/jsc/bindings/Exception.h               |  10 ++
 src/jsc/bindings/webcore/FetchHeaders.cpp  |  37 +++++--
 src/jsc/bindings/webcore/HTTPHeaderMap.cpp |  51 ++++++----
 src/jsc/bindings/webcore/HTTPHeaderMap.h   |   5 +-
 test/js/web/fetch/headers.test.ts          | 158 +++++++++++++++++++++++++++++
 5 files changed, 229 insertions(+), 32 deletions(-)

gate history · 3 passed · 0 rejected · iteration 0

evidence per changed file

file                                        reads  edits  tests
src/jsc/bindings/Exception.h                    3      3     40
src/jsc/bindings/webcore/FetchHeaders.cpp       4      8     40
src/jsc/bindings/webcore/HTTPHeaderMap.cpp      2      3     40
src/jsc/bindings/webcore/HTTPHeaderMap.h        4      3     40
test/js/web/fetch/headers.test.ts               7      8     40

…gth limit A header value is one String, so it holds at most String::MaxLength (2**31 - 1) characters. Two paths built a longer one and aborted the process, because WTF's makeString() and the default StringBuilder crash on overflow: - append() combines the existing value with the new one. - get("set-cookie") joins the stored Set-Cookie values. Both now report the failure as RangeError: Out of memory, which is what JSC throws for an over-long string. The six error messages that embed a header name or value get the same treatment, through tryMakeString(). The Set-Cookie join also computes its capacity from the real lengths of all the values, in 64 bits. It read only the first value's length and multiplied it by the count in 32 bits.

robobun · 2026-09-10T16:28:17Z

Status: ready for review at f7d1b12 (merged with main at 46e03a5). CI is green (build #119891). All review threads are resolved.

How I reproduced it: on a release build of main, each of these exits 134 with panic(main thread): abort() called:

// append combines past the limit
const h = new Headers(); const s = "x".repeat(2 ** 30);
h.append("accept", s); h.append("accept", s);

// get("set-cookie") asks for a capacity past the limit on ~1 MB of data
const k = new Headers();
k.append("set-cookie", "x".repeat(2 ** 20));
for (let i = 0; i < 2047; i++) k.append("set-cookie", "a");
k.get("set-cookie");

// a message that quotes a name past the limit
new Headers().get("\0".repeat(2 ** 31 - 20));

Checked the same way the tests are checked, against main at 46e03a5: with src/ at origin/main the debug build fails the four new cases in test/js/web/fetch/headers.test.ts (99 pass, 4 fail). With this branch's src/ the file is 103 pass, 0 fail.

#42692 landed on main while this was open and removed the value from the Header '…' has invalid value message. The known-name overload keeps main's line, because its message is bounded. The unknown-name overload still quotes the name, so it stays on exceptionWithMessage().

This needs a maintainer to review and merge. #42250 (linear Headers.append) rewrites the append hunk from this PR, so whichever lands second needs a rebase.

coderabbitai · 2026-09-10T16:31:39Z

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Walkthrough

Summary

The changes add fallible exception-message construction, protect Fetch header mutations and lookups from allocation failures, add overflow-aware Set-Cookie joining, and test String length boundaries in child processes.

Changes

Fetch header allocation safety

Layer / File(s)	Summary
Fallible header operations `src/jsc/bindings/Exception.h`, `src/jsc/bindings/webcore/FetchHeaders.cpp`	`exceptionWithMessage` uses fallible string construction. Header validation and value merging return `OutOfMemoryError` when allocation fails.
Set-Cookie joining and lookup `src/jsc/bindings/webcore/HTTPHeaderMap.h`, `src/jsc/bindings/webcore/HTTPHeaderMap.cpp`, `src/jsc/bindings/webcore/FetchHeaders.cpp`	`HTTPHeaderMap` checks joined lengths against `String::MaxLength`. `FetchHeaders` uses the new join operation for `Set-Cookie` lookup and distinguishes known from uncommon headers.
Boundary validation `test/js/web/fetch/headers.test.ts`	Tests cover invalid-name error consistency, large and oversized header values, failed appends, `Set-Cookie` lookup, preserved values, stderr output, and process exit status.

Suggested reviewers: jarred-sumner

Priority: ⬇️ Low

Merge Risk: ⚪ Minimal · up to f7d1b

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Actionable comments posted: 2

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Inline comments:
In `@src/jsc/bindings/webcore/FetchHeaders.cpp`:
- Line 299: Update the error message in the relevant FetchHeaders validation
path to close the header name with a single quote, matching the opening quote
and the other error-message sites in the file.

In `@test/js/web/fetch/headers.test.ts`:
- Around line 756-757: Replace the repetitive string construction in the
affected test cases, including both usages near the current header setup and the
later occurrence, with Buffer.alloc(count, fill).toString(). Preserve the
existing counts and fill characters for the large cookie value and repeated
single-character values.

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src/jsc/bindings/Exception.h
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src/jsc/bindings/webcore/HTTPHeaderMap.cpp
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test/js/web/fetch/headers.test.ts

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…pens with The message read `Invalid header name: 'a b"`. get() and delete() already close it with a single quote.

claude

Nothing blocking. The comments below are optional suggestions. There is no need to push a fix for them before merging.

claude

Code review found no issues

No high-confidence issues detected in this change.

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Inline comments:
In `@test/js/web/fetch/headers.test.ts`:
- Line 396: Update the invalid-header assertions for each method in the headers
test to capture the thrown error and compare its message with an exact
toBe("Invalid header name: 'a b'") assertion, rather than using toThrow with a
substring match.

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robobun · 2026-09-10T17:27:17Z

Review round addressed in e70a1db, e783246 and a4fc049:

has() now closes the invalid header name with a single quote like get() and delete(). A new test compares the whole message for all three.
The new code comments are one line each.
One-character repeat() stays in the test. It is the faster and smaller option here (measurements in the thread).
The 60 s timeout stays on the 2^30 case only. The reason is in the thread.

All review threads are resolved. Waiting on CI for a4fc049.

claude

Code review found no issues

No high-confidence issues detected in this change.

…ers-string-length-limit

A name or value of 2**31 - 20 characters makes the message that quotes it pass String::MaxLength. Each of the six messages in FetchHeaders.cpp now throws RangeError: Out of memory there. All six abort without the fix.

robobun · 2026-09-14T23:50:07Z

^{Updated 5:49 PM PT - Sep 14th, 2026}

✅ @robobun, your commit 48d8e9272110e3c41c818ec40fe1afbdb58bf456 passed in Build #115712! 🎉

🧪 To try this PR locally:

bunx bun-pr 42218

That installs a local version of the PR into your bun-42218 executable, so you can run:

bun-42218 --bun

claude

Code review found no issues

No high-confidence issues detected in this change.

…ers-string-length-limit main dropped the value from the invalid header value message (#42692), so that message is bounded for a known header name and keeps main's line. For an unknown name the message still quotes the name, so it stays on exceptionWithMessage(). The test no longer expects an over-long value to reach a message, and covers the five calls that quote an over-long name.

claude

Code review found no issues

No high-confidence issues detected in this change.

robobun · 2026-09-30T07:28:00Z

#42250 now has two of the three changes of this PR. The request there was one PR for all the joins of repeated header values.

The limit for a join in append() and in the constructors. Join repeated header values in linear time, and throw at the string length limit #42250 has it in HTTPHeaderMap::combine for all 7 join sites, and appendToHeaderMap has no join of its own there.
HTTPHeaderMap::tryJoinSetCookieHeaders(), under the same name, with FetchHeaders::get() as its caller.

The change that only this PR has is exceptionWithMessage() for the five messages that quote a header name, with its test and the quote in the has() message.

The two PRs conflict in FetchHeaders.cpp, HTTPHeaderMap.cpp, HTTPHeaderMap.h and headers.test.ts. If #42250 merges first, this PR becomes Exception.h, the five call sites and their two tests.

github-actions Bot added the claude label Sep 10, 2026

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Comment thread src/jsc/bindings/webcore/FetchHeaders.cpp Outdated

Comment thread test/js/web/fetch/headers.test.ts

Headers.has(): close the invalid header name with the same quote it o…

e70a1db

…pens with The message read `Invalid header name: 'a b"`. get() and delete() already close it with a single quote.