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[markdown] Cap parenthesis nesting in bare inline link destinations - #2769

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Nov 3, 2026
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[markdown] Cap parenthesis nesting in bare inline link destinations#2769
jonasfj merged 1 commit into
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markdown_bare_destination_paren_cap

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@jonasfj jonasfj commented Oct 30, 2026 •
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Return null from LinkSyntax._parseInlineBareDestinationLink once more than
32 levels of parentheses are open, as cmark does.

Why this change is necessary

A bare inline link destination may contain balanced parentheses, so
_parseInlineBareDestinationLink scans forward counting them until it finds
the closing ) of the link. On input such as '[a]((' * n every ] starts a
scan that runs to the end of the line before giving up, and InlineParser
then rewinds to just after that ], so the line is parsed in O(n^2) time.

How this change works

CommonMark leaves the depth of nested parentheses unbounded, but the reference
implementation cmark (and therefore GitHub) rejects a destination once more
than 32 levels are open. Doing the same bounds every scan to the next 33
unmatched ( instead of the end of the line, and makes the output agree with
cmark on this input.

Behavior change

A bare inline link destination may contain at most 32 levels of nested
parentheses; [a]( + 33 × ( + u + 33 × ) + ) is literal text, 32
levels still link. CommonMark itself puts no bound on nesting; this is the
limit used by the reference C implementation cmark (and therefore
cmark-gfm / GitHub). commonmark.js does not have the limit, so the two
reference implementations disagree on this input and we follow cmark.

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:

  • 1 difference in the parser test suites: pulldown-cmark regression
    example 197, which nests 40 parentheses and is a link in commonmark.js
    but not in cmark; this branch now agrees with cmark.
  • 1 OSS-Fuzz input changed (nested []( runs deeper than 32).
  • No differences in pub.dev or the real-world docs.
  • One 522 KB OSS-Fuzz input was reported as a new timeout — standalone 5.0 s on main vs 6.2 s on this branch (the same 522 KB input re-times at 4.9 s / 3.1 s on a neighbouring branch), so that was load during the run.
    1 timeout resolved. Cumulative parse time −0.5%.

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Code Review

This pull request limits bare inline link destinations to a maximum of 32 levels of nested parentheses to match cmark behavior and prevent performance issues from rescanning unclosed destinations. This is implemented in LinkSyntax by returning null if the parenthesis count exceeds 33. Corresponding tests and a changelog entry have been added. I have no feedback to provide.

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Publish ✔️
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package:markdown ⭐ 7.4.1-wip WIP (no publish necessary)
  • 17 already published.
  • 25 WIP (no publish necessary).

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Breaking changes ✔️
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markdown Non-Breaking 7.3.1 7.4.1-wip 7.4.1-wip ✔️

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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
// BSD-style license that can be found in the LICENSE file.
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pkgs/markdown/lib/src/inline_syntaxes/link_syntax.dart 💚 99 % ⬆️ 0 %

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@jonasfj
jonasfj marked this pull request as ready for review November 2, 2026 13:56
@jonasfj
jonasfj requested a review from srawlins as a code owner November 2, 2026 13:56
@jonasfj
jonasfj requested review from szakarias and removed request for srawlins November 2, 2026 13:56
@jonasfj
[markdown] Cap parenthesis nesting in bare inline link destinations
0b17a55 
Return `null` from `LinkSyntax._parseInlineBareDestinationLink` once more than
32 levels of parentheses are open, as `cmark` does.

----

A bare inline link destination may contain balanced parentheses, so
`_parseInlineBareDestinationLink` scans forward counting them until it finds
the closing `)` of the link. On input such as `'[a]((' * n` every `]` starts a
scan that runs to the end of the line before giving up, and `InlineParser`
then rewinds to just after that `]`, so the line is parsed in O(n^2) time.

CommonMark leaves the depth of nested parentheses unbounded, but the reference
implementation `cmark` (and therefore GitHub) rejects a destination once more
than 32 levels are open. Doing the same bounds every scan to the next 33
unmatched `(` instead of the end of the line, and makes the output agree with
`cmark` on this input.
@jonasfj
jonasfj force-pushed the markdown_bare_destination_paren_cap branch from 57cb273 to 0b17a55 Compare November 3, 2026 12:06
@jonasfj
jonasfj merged commit f3a5c92 into main Nov 3, 2026
25 checks passed
@jonasfj
jonasfj deleted the markdown_bare_destination_paren_cap branch November 3, 2026 12:09
copybara-service Bot pushed a commit to dart-lang/sdk that referenced this pull request Nov 5, 2026
@natebosch dart-scoped@luci-project-accounts.iam.gserviceaccount.com
[deps] rev core, dartdoc, ecosystem, http, native, protobuf, tools, web
cc100c0 
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]>
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@sigurdm sigurdm sigurdm approved these changes
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