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[markdown] Reject unescaped '(' in parenthesized inline link titles - #2770

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[markdown] Reject unescaped '(' in parenthesized inline link titles#2770
jonasfj merged 1 commit into
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markdown_inline_link_title_lparen

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@jonasfj jonasfj commented Oct 30, 2026 •
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In LinkSyntax._parseTitle, return null when delimiter == $lparen and an
unescaped ( (char == $lparen) is encountered.

Why this change is necessary

CommonMark 0.31.2 section 6.3 ("Links") defines a parenthesized link title as:

a sequence of zero or more characters between an opening ( and a closing )
that contains no unescaped ( or ) characters

Previously, LinkSyntax._parseTitle checked for \ escapes and for
closeDelimiter ()), but did not reject unescaped ( when delimiter == $lparen. Consequently, on an input with repeated unclosed parenthesized link
titles such as '[a](b (' * n, each [a](b ( entered LinkSyntax._parseTitle
at (, scanned past all subsequent [a](b ( tokens to the end of the input
looking for ), and failed at EOF, taking O(n^2) time.

How this change works

Adding:

} else if (delimiter == $lparen && char == $lparen) {
  return null;
}

inside LinkSyntax._parseTitle (after the char == $backslash branch, so \(
remains valid) immediately aborts the title scan when an unescaped ( is
encountered. Each unclosed [a](b ( in '[a](b (' * n now stops at the ( of
the very next token in O(1) time, making the total scan O(n).

Behavior change

A parenthesized inline link title may no longer contain an unescaped (.
CommonMark §6.3 defines it as "a sequence of zero or more characters between
matching parentheses ((...)), including a ( or ) character only if it is
backslash-escaped". Before, [a](b (c(d)) produced a link with the title
c(d; now it is literal text, as in cmark and commonmark.js.
[a](b (c\(d)) still links with the title c(d.

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:

  • 6 differences in the parser test suites and benchmarks, all inputs that the
    suites use as not links and now matching commonmark.js: cmark and
    cmark-gfm regression examples [link](url ((title)), the markdown-it /
    markdown-it-py commonmark_extras case, and the goldmark / md_proc
    benchmark line [this is not a link]((((((((((()))))))))) (((((((((())))))))))).
  • 17 OSS-Fuzz inputs changed; for 10 the changed region now matches
    commonmark.js where it did not before, for 0 the reverse (the rest differ
    before and after for unrelated reasons).
  • No differences in pub.dev or the real-world docs. No new exceptions;
    2 OSS-Fuzz timeouts resolved. Cumulative parse time −0.8%.

gemini-code-assist[bot]
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Code Review

This pull request updates the Markdown package to adhere to the CommonMark specification regarding parenthesized inline link titles, specifically by disallowing unescaped parentheses within them. It includes a corresponding update to the changelog and adds new unit tests to verify the behavior. I have no feedback to provide.

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github-actions Bot commented Oct 30, 2026 •
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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 ✔️
Package Change Current Version New Version Needed Version Looking good?
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/html/test/differential_test.dart
pkgs/html/test/dom_parsing_test.dart
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Coverage ✔️
File Coverage
pkgs/markdown/lib/src/inline_syntaxes/link_syntax.dart 💚 99 % ⬆️ 0 %

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@jonasfj
jonasfj requested a review from szakarias November 2, 2026 13:58
@jonasfj
jonasfj marked this pull request as ready for review November 2, 2026 13:58
@jonasfj
jonasfj requested a review from srawlins as a code owner November 2, 2026 13:58
@jonasfj
jonasfj removed the request for review from srawlins November 2, 2026 13:58
@jonasfj
jonasfj force-pushed the markdown_inline_link_title_lparen branch from 98fcd17 to 9dd7212 Compare November 3, 2026 12:06
@jonasfj
jonasfj enabled auto-merge (squash) November 3, 2026 12:09
@jonasfj
[markdown] Reject unescaped '(' in parenthesized inline link titles
bb36e1a 
In `LinkSyntax._parseTitle`, return `null` when `delimiter == $lparen` and an
unescaped `(` (`char == $lparen`) is encountered.

----

CommonMark 0.31.2 section 6.3 ("Links") defines a parenthesized link title as:
> a sequence of zero or more characters between an opening `(` and a closing `)`
> that contains no unescaped `(` or `)` characters

Previously, `LinkSyntax._parseTitle` checked for `\` escapes and for
`closeDelimiter` (`)`), but did not reject unescaped `(` when `delimiter ==
$lparen`. Consequently, on an input with repeated unclosed parenthesized link
titles such as `'[a](b (' * n`, each `[a](b (` entered `LinkSyntax._parseTitle`
at `(`, scanned past all subsequent `[a](b (` tokens to the end of the input
looking for `)`, and failed at EOF, taking O(n^2) time.

Adding:
```dart
} else if (delimiter == $lparen && char == $lparen) {
  return null;
}
```
inside `LinkSyntax._parseTitle` (after the `char == $backslash` branch, so `\(`
remains valid) immediately aborts the title scan when an unescaped `(` is
encountered. Each unclosed `[a](b (` in `'[a](b (' * n` now stops at the `(` of
the very next token in O(1) time, making the total scan O(n).
@jonasfj
jonasfj force-pushed the markdown_inline_link_title_lparen branch from 9dd7212 to bb36e1a Compare November 3, 2026 12:10
@jonasfj
jonasfj merged commit 5da06a5 into main Nov 3, 2026
23 checks passed
@jonasfj
jonasfj deleted the markdown_inline_link_title_lparen branch November 3, 2026 12:11
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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