[markdown] Fix 999-character limit in LinkParser.parseLabel by jonasfj · Pull Request #2766 · dart-lang/tools

jonasfj · 2026-10-30T13:15:53Z

Check pos - start > 999 instead of maxLoop-- < 0 in LinkParser.parseLabel
so backslash escapes (\x) count as 2 raw characters rather than 1 loop
iteration.

CommonMark 0.31.2 section 4.7 ("Link reference definitions") specifies that a
link label can have at most 999 raw characters inside the square brackets
[...].

Previously, LinkParser.parseLabel attempted to enforce this limit by counting
loop iterations:

var maxLoop = 999;
while (true) {
  if (maxLoop-- < 0) {
    return false;
  }
  ...
  if (char == $backslash) {
    advance();
    ...
  }
  ...
  advance();
}

Whenever char == $backslash, a single iteration of the while (true) loop
called advance() twice (once for \ and once for the escaped character),
consuming 2 raw characters while only decrementing maxLoop by 1. As a result,
labels containing backslash escapes (such as '[${'a' * 998}\[]: /url') had
1,000 raw characters inside [...] (up to 1,998 raw characters for 999
escapes) and were erroneously accepted as valid link reference definitions.

In LinkParser.parseLabel, start records pos at the first character after
the opening [. At the top of each iteration (before checking
char == $rbracket), pos - start is the exact number of raw characters
consumed inside [...] so far. Checking if (pos - start > 999) return false;
accurately counts both characters of any \ escape:

A 999-character label ending with an escape ('${'a' * 997}\[') reaches ]
at pos - start == 999 and succeeds.
A 1,000-character label ending with an escape ('${'a' * 998}\[') reaches the
next iteration at pos - start == 1000 and immediately returns false.

Behavior change

LinkParser.parseLabel counted loop iterations instead of characters, and a
backslash escape consumes two characters per iteration, so a definition label
of up to 1,998 characters was accepted when it contained escapes. Labels are
now limited to 999 raw characters as CommonMark §4.7 requires. This only
affects definitions whose label is over 999 characters long.

Differential testing

Rendered every input with origin/main and with this branch (GFM extension
set, 30 s per-file timeout) and compared the HTML. Sources:

pub.dev: 367,926 Markdown files from 93,452 packages
Real-world docs: 116,231 files from mdn/content, github/docs,
kubernetes/website, freeCodeCamp, tldr, flutter/website,
dart-lang/site-www, microsoft/vscode-docs, withastro/docs,
rust-lang/book, GitHub READMEs, and Wikipedia
Parser test suites: 6,458 cases from cmark, cmark-gfm, commonmark.js,
pulldown-cmark, md4c, goldmark, markdown-it, comrak, marked,
pandoc, and pathological generators
OSS-Fuzz: 57,716 inputs from the cmark, md4c, and markdown-it-py fuzzers

OSS-Fuzz inputs that sit at the 30 s timeout or the stack limit flip between
runs; every one reported as a new failure was re-run standalone on both trees.
Where the HTML changed, the changed region was also compared with the output of
commonmark.js 0.31 for the same input.

Results:

No HTML differences in any source.
One 411 KB OSS-Fuzz input was reported as a new timeout — standalone 5.4 s on main vs 5.5 s on this branch, so that was load during the run; one
stack-overflow message changed wording. Cumulative parse time +0.1% (noise).

Check `pos - start > 999` instead of `maxLoop-- < 0` in `LinkParser.parseLabel` so backslash escapes (`\x`) count as 2 raw characters rather than 1 loop iteration. ---- CommonMark 0.31.2 section 4.7 ("Link reference definitions") specifies that a link label can have at most 999 raw characters inside the square brackets `[...]`. Previously, `LinkParser.parseLabel` attempted to enforce this limit by counting loop iterations: ```dart var maxLoop = 999; while (true) { if (maxLoop-- < 0) { return false; } ... if (char == $backslash) { advance(); ... } ... advance(); } ``` Whenever `char == $backslash`, a single iteration of the `while (true)` loop called `advance()` twice (once for `\` and once for the escaped character), consuming 2 raw characters while only decrementing `maxLoop` by 1. As a result, labels containing backslash escapes (such as `'[${'a' * 998}\[]: /url'`) had 1,000 raw characters inside `[...]` (up to 1,998 raw characters for 999 escapes) and were erroneously accepted as valid link reference definitions. In `LinkParser.parseLabel`, `start` records `pos` at the first character after the opening `[`. At the top of each iteration (before checking `char == $rbracket`), `pos - start` is the exact number of raw characters consumed inside `[...]` so far. Checking `if (pos - start > 999) return false;` accurately counts both characters of any `\` escape: - A 999-character label ending with an escape (`'${'a' * 997}\['`) reaches `]` at `pos - start == 999` and succeeds. - A 1,000-character label ending with an escape (`'${'a' * 998}\['`) reaches the next iteration at `pos - start == 1000` and immediately returns `false`.

gemini-code-assist

Code Review

This pull request updates the link label length validation logic in link_parser.dart to use a position-based check instead of a loop counter, ensuring that link reference definitions exceeding 999 characters are correctly ignored. Corresponding regression tests were added to regression_test.dart, and the CHANGELOG.md was updated to reflect this change. I have no feedback to provide.

github-actions · 2026-10-30T13:17:42Z

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// Copyright (c) 2026, the Dart project authors. Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
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pkgs/html/test/differential_test.dart
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Revisions updated by `dart tools/rev_sdk_deps.dart`. core (https://github.com/dart-lang/core/compare/24e09c4..36f57a9): 36f57a99 Tue Nov 3 14:26:17 2026 +0100 Sigurd Meldgaard [os_detect] Remove bin/os_detect.dart (dart-lang/core#1015) 86bacfca Tue Nov 3 05:15:19 2026 -0500 Justin White [characters] Fix RangeError on an unpaired tail surrogate at range start (dart-lang/core#1033) dartdoc (https://github.com/dart-lang/dartdoc/compare/db94437..2f6847b): 2f6847bd Thu Oct 29 11:47:04 2026 +0200 Sergey G. Grekhov Add tests for invalid doc comments placement (dart-lang/dartdoc#4301) 17f2f0a3 Thu Oct 29 10:16:36 2026 +0100 Sarah Zakarias Compare base elements and unify properties in doc-reference-target-differs (dart-lang/dartdoc#4307) dartdoc is held back at 2f6847b, before dart-lang/dartdoc#4312, because its mock SDK version changes conflict in the google3 3-way merge with the hotfix in cl/1005963642. ecosystem (https://github.com/dart-lang/ecosystem/compare/f8faabf..54e1673): 54e1673 Wed Nov 4 09:46:54 2026 +0100 Sigurd Meldgaard fix(firehose): resolve dartExecutable via package:cli_util (dart-lang/ecosystem#469) d754065 Tue Nov 3 10:09:45 2026 -0800 Kevin Moore refactor(corpus): use path.join and seal closed type hierarchies (dart-lang/ecosystem#476) http (https://github.com/dart-lang/http/compare/8c0d66a..1f869d0): 1f869d0 Tue Nov 3 15:51:52 2026 -0800 dependabot[bot] chore(deps): bump the github-actions group with 8 updates (dart-lang/http#2052) 6d7f402 Tue Nov 3 14:12:04 2026 -0800 Brian Quinlan feat(cupertino_http): support custom trust eval (dart-lang/http#2038) 73f8c36 Tue Nov 3 10:15:16 2026 -0800 Brian Quinlan chore(cupertino_http): ready 3.1.1 release (dart-lang/http#2056) native (https://github.com/dart-lang/native/compare/b442038..1838637): 183863758 Wed Nov 4 09:01:27 2026 -0300 Daco Harkes [hooks_runner] Add `PKG_CONFIG_PATH` to environment variables allowlist (dart-lang/native#3779) c9e8118d4 Wed Nov 4 08:16:20 2026 +1100 Liam Appelbe [jni] Fix release mode segfault (dart-lang/native#3777) 5999cf7e4 Wed Nov 4 08:16:16 2026 +1100 Liam Appelbe [objective_c] Prepare to publish (dart-lang/native#3778) d294d9942 Tue Nov 3 15:15:49 2026 +0100 Michael Goderbauer [hooks_runner] Write recorded_uses.json under directory lock in link() (dart-lang/native#3770) 33aa5ef04 Tue Nov 3 11:42:38 2026 +1100 Liam Appelbe [objective_c] Cheaper and safer object/block validity checks (dart-lang/native#3776) protobuf (https://github.com/dart-lang/protobuf/compare/1d52301..19191c8): 19191c8 Wed Nov 4 17:14:33 2026 +0100 Sigurd Meldgaard Use dart.tool.dart2wasm in json.dart so DDC uses json_web.dart (google/protobuf.dart#1108) tools (https://github.com/dart-lang/tools/compare/1579f8d..a342744): a3427446f Wed Nov 4 15:11:35 2026 +0100 Jonas Finnemann Jensen Update CODEOWNERS for pkgs/markdown (dart-lang/tools#2795) 08c9afdc0 Wed Nov 4 14:40:44 2026 +0100 Jonas Finnemann Jensen [markdown] Check URL boundary before matching in AutolinkExtensionSyntax (dart-lang/tools#2786) 0fc5edcae Wed Nov 4 14:35:48 2026 +0100 Jonas Finnemann Jensen [markdown] Skip unclosed inline HTML tags before matching RegExp (dart-lang/tools#2788) b1f6f9956 Wed Nov 4 14:31:55 2026 +0100 Jonas Finnemann Jensen [markdown] Defer shortcut reference link label extraction in LinkSyntax.close (dart-lang/tools#2785) ab135e072 Wed Nov 4 14:30:37 2026 +0100 Jonas Finnemann Jensen [markdown] Avoid quadratic trailing-paren scan in AutolinkExtensionSyntax (dart-lang/tools#2787) f329bf3ea Wed Nov 4 17:09:34 2026 +0530 Ananya Kaul [code_builder] Separate chained cascades after nullable casts (dart-lang/tools#2704) 0a57e3446 Wed Nov 4 05:28:40 2026 +0530 Aman Maurya fix(coverage): allow empty coverage files from skipped test suites (`#2667`) (dart-lang/tools#2668) e95958f6a Tue Nov 3 21:53:57 2026 +0000 Keerti Parthasarathy [unified_analytics] Add root_tool and leaf_tool to every event; key consent on root_tool (dart-lang/tools#2784) 5da06a508 Tue Nov 3 13:11:04 2026 +0100 Jonas Finnemann Jensen [markdown] Reject unescaped '(' in parenthesized inline link titles (dart-lang/tools#2770) f3a5c9277 Tue Nov 3 13:09:32 2026 +0100 Jonas Finnemann Jensen [markdown] Cap parenthesis nesting in bare inline link destinations (dart-lang/tools#2769) 820ce5fb7 Tue Nov 3 13:04:35 2026 +0100 Jonas Finnemann Jensen [markdown] Reject unescaped '(' in parenthesized link reference titles (dart-lang/tools#2771) 775c5d080 Tue Nov 3 13:03:43 2026 +0100 Jonas Finnemann Jensen [markdown] Require balanced parentheses before bare inline link titles (dart-lang/tools#2768) 5ebd52bc5 Tue Nov 3 13:03:08 2026 +0100 Jonas Finnemann Jensen [markdown] Reject unescaped '<' in bracketed inline link destinations (dart-lang/tools#2767) feb7ea928 Tue Nov 3 13:02:15 2026 +0100 Jonas Finnemann Jensen [markdown] Fix 999-character limit in LinkParser.parseLabel (dart-lang/tools#2766) 31888a5da Tue Nov 3 13:01:43 2026 +0100 Jonas Finnemann Jensen [markdown] Enforce 999-character limit in _parseReferenceLinkLabel (dart-lang/tools#2765) e016c5d70 Tue Nov 3 09:26:32 2026 +0100 dart-ecosystem-bot[bot] [boring] Pin the prebuilt binaries of boring-binaries-20261101-f869fdca423c (dart-lang/tools#2778) 8c7935573 Mon Nov 2 10:15:24 2026 -0800 Kevin Moore [html] Move WPT selector fixtures to third_party/wpt-selectors (dart-lang/tools#2772) web (https://github.com/dart-lang/web/compare/45166fd..f388bf5): f388bf5 Sun Nov 1 06:57:22 2026 +0000 dependabot[bot] Bump the github-actions group with 4 updates (dart-lang/web#611) 658ce2f Sat Oct 31 12:47:20 2026 +0100 Moritz Migrate to ecosystem health publish check and setup-dart publish workflow (dart-lang/web#610) Change-Id: Ifcae02c4712b4b27a5a8ab1c57d8dcf13a6700cf Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/574281 Reviewed-by: Brian Quinlan <[email protected]> Commit-Queue: Nate Bosch <[email protected]>

github-actions Bot added the package:markdown label Oct 30, 2026

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jonasfj marked this pull request as ready for review November 2, 2026 13:48

jonasfj requested a review from srawlins as a code owner November 2, 2026 13:48

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Merge branch 'main' into markdown_link_parser_label_limit

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jonasfj merged commit feb7ea9 into main Nov 3, 2026
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[markdown] Fix 999-character limit in LinkParser.parseLabel - #2766

[markdown] Fix 999-character limit in LinkParser.parseLabel#2766
jonasfj merged 2 commits into
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markdown_link_parser_label_limit

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