Skip to content

[markdown] Reject unescaped '(' in parenthesized link reference titles - #2771

Merged
jonasfj merged 2 commits into
mainfrom
markdown_link_reference_title_lparen
Nov 3, 2026
Merged

[markdown] Reject unescaped '(' in parenthesized link reference titles#2771
jonasfj merged 2 commits into
mainfrom
markdown_link_reference_title_lparen

Conversation

@jonasfj

@jonasfj jonasfj commented Oct 30, 2026 •
edited
Loading

Copy link
Copy Markdown
Member

In LinkParser._parseTitle, return false when delimiter == $lparen and an
unescaped ( (char == $lparen) is encountered.

Why this change is necessary

CommonMark 0.31.2 sections 4.7 and 6.3 specify that a link title enclosed in
parentheses (...) cannot contain unescaped ( characters.

Previously, while LinkSyntax._parseTitle was updated to reject unescaped (,
LinkParser._parseTitle (which parses link reference definitions
[label]: /url (title) across lines in LinkReferenceDefinitionSyntax) only
checked for the closing ) delimiter and allowed unescaped (.

Consequently:

  • On '[a]: /u\n(\n' * n, when LinkParser parsed [a]: /u on line 1, it saw
    ( on line 2 and scanned through all remaining [a]: /u and ( lines to the
    end of the block before failing to find ) and retreating BlockParser back
    to line 2.
  • On '[a]: /u (\n=\n' * n (or '[a]: /u\n(\n=\n' * n), SetextHeaderSyntax
    only matches when parser.currentSyntax is ParagraphSyntax, so
    BlockSyntax.isAtBlockEnd is false on = while LinkParser is running
    under LinkReferenceDefinitionSyntax. Because _parseTitle did not reject
    unescaped (, each [a]: /u scanned past = and all subsequent [a]: /u
    lines to the end of the document before failing and retreating, after which
    ParagraphSyntax and SetextHeaderSyntax consumed the = heading and
    repeated the full scan from the next [a]: /u line in O(n^2) time.

How this change works

Adding:

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

in LinkParser._parseTitle immediately aborts the title parse on an unescaped
( (while \( continues to be handled by the char == $backslash branch).
Unclosed parenthesized link reference titles now stop at the next unescaped (
in O(1) lines instead of scanning the remainder of the document.

Behavior change

A parenthesized title in a link reference definition may no longer contain an
unescaped ( (CommonMark §6.3, same rule as for inline link titles). A
definition such as [a]: /u (c(d) is no longer a definition and renders as a
paragraph, as in cmark and commonmark.js; [a]: /u (c\(d) still is one.

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 176, [link]: test ((), which the suite expects not to be a
    definition; now matching commonmark.js.
  • 1 real-world difference: a PyPI README using the [//]: # (comment) idiom
    with a comment that spans three lines and contains an unescaped ( on the
    last one. main swallowed it as a definition; it is now a paragraph in which
    [//] resolves against an earlier [//]: # definition — byte-for-byte what
    commonmark.js renders for this document.
  • No differences in pub.dev or the other real-world docs. One 777 KB OSS-Fuzz
    input was reported as a new timeout — standalone 25.5 s on main vs 25.0 s on this branch, so that was load during the run. Cumulative parse time −0.2%.

@jonasfj
[markdown] Reject unescaped '(' in parenthesized link reference titles
90c5803 
In `LinkParser._parseTitle`, return `false` when `delimiter == $lparen` and an
unescaped `(` (`char == $lparen`) is encountered.

----

### Why this change is necessary

CommonMark 0.31.2 sections 4.7 and 6.3 specify that a link title enclosed in
parentheses `(...)` cannot contain unescaped `(` characters.

Previously, while `LinkSyntax._parseTitle` was updated to reject unescaped `(`,
`LinkParser._parseTitle` (which parses link reference definitions
`[label]: /url (title)` across lines in `LinkReferenceDefinitionSyntax`) only
checked for the closing `)` delimiter and allowed unescaped `(`.

Consequently:
- On `'[a]: /u\n(\n' * n`, when `LinkParser` parsed `[a]: /u` on line 1, it saw
  `(` on line 2 and scanned through all remaining `[a]: /u` and `(` lines to the
  end of the block before failing to find `)` and retreating `BlockParser` back
  to line 2.
- On `'[a]: /u (\n=\n' * n` (or `'[a]: /u\n(\n=\n' * n`), `SetextHeaderSyntax`
  only matches when `parser.currentSyntax is ParagraphSyntax`, so
  `BlockSyntax.isAtBlockEnd` is `false` on `=` while `LinkParser` is running
  under `LinkReferenceDefinitionSyntax`. Because `_parseTitle` did not reject
  unescaped `(`, each `[a]: /u` scanned past `=` and all subsequent `[a]: /u`
  lines to the end of the document before failing and retreating, after which
  `ParagraphSyntax` and `SetextHeaderSyntax` consumed the `=` heading and
  repeated the full scan from the next `[a]: /u` line in O(n^2) time.

### How this change works

Adding:
```dart
} else if (delimiter == $lparen && char == $lparen) {
  return false;
}
```
in `LinkParser._parseTitle` immediately aborts the title parse on an unescaped
`(` (while `\(` continues to be handled by the `char == $backslash` branch).
Unclosed parenthesized link reference titles now stop at the next unescaped `(`
in O(1) lines instead of scanning the remainder of the document.
gemini-code-assist[bot]
gemini-code-assist Bot reviewed Oct 30, 2026
View reviewed changes

@gemini-code-assist gemini-code-assist Bot left a comment

Copy link
Copy Markdown
Contributor

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

Code Review

This pull request updates the Markdown parser to prevent parenthesized link reference definition titles from containing an unescaped left parenthesis (, aligning with the CommonMark specification. The change is implemented in link_parser.dart and is accompanied by corresponding unit tests and a changelog update. I have no feedback to provide.

@github-actions

github-actions Bot commented Oct 30, 2026 •
edited
Loading

Copy link
Copy Markdown

PR Health

Publish ✔️
Package Version Status Publish tag (post-merge)
package:markdown ⭐ 7.4.1-wip WIP (no publish necessary)
  • 17 already published.
  • 25 WIP (no publish necessary).

Documentation at https://github.com/dart-lang/ecosystem/wiki/Publishing-automation.

This check can be disabled by tagging the PR with skip-publish-check.

Changelog Entry ✔️
Package Changed Files

Changes to files need to be accounted for in their respective changelogs.

This check can be disabled by tagging the PR with skip-changelog-check (for example, for non-user-visible changes when the package version is already at -wip).

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 ✔️

This check can be disabled by tagging the PR with skip-breaking-check.

API leaks ✔️

The following packages contain symbols visible in the public API, but not exported by the library. Export these symbols or remove them from your publicly visible API.

Package Leaked API symbol Leaking sources

This check can be disabled by tagging the PR with skip-leaking-check.

Unused Dependencies ✔️
Package Status
markdown ✔️ All dependencies utilized correctly.

For details on how to fix these, see dependency_validator.

This check can be disabled by tagging the PR with skip-unused-dependencies-check.

License Headers ✔️ 
// 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.
Files
no missing headers

All source files should start with a license header.

Unrelated files missing license headers
Files
pkgs/html/test/differential_test.dart
pkgs/html/test/dom_parsing_test.dart
pkgs/html/test/mxss_test.dart
pkgs/html/test/noah_ark_clause_limit_test.dart

This check can be disabled by tagging the PR with skip-license-check.

Coverage ✔️
File Coverage
pkgs/markdown/lib/src/link_parser.dart 💚 98 %

This check for test coverage is informational (issues shown here will not fail the PR).

This check can be disabled by tagging the PR with skip-coverage-check.

@jonasfj
jonasfj marked this pull request as ready for review November 2, 2026 13:59
@jonasfj
jonasfj requested a review from srawlins as a code owner November 2, 2026 13:59
@jonasfj
jonasfj requested review from szakarias and removed request for srawlins November 2, 2026 14:00
@jonasfj
jonasfj merged commit 820ce5f into main Nov 3, 2026
17 checks passed
@jonasfj
jonasfj deleted the markdown_link_reference_title_lparen branch November 3, 2026 12:04
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]>
Sign up for free to join this conversation on GitHub. Already have an account? Sign in to comment

Reviewers

@srawlins srawlins srawlins approved these changes
@sigurdm sigurdm sigurdm approved these changes
@szakarias szakarias Awaiting requested review from szakarias
+1 more reviewer
@gemini-code-assist gemini-code-assist[bot] gemini-code-assist[bot] left review comments
Reviewers whose approvals may not affect merge requirements

Assignees

No one assigned

Labels

package:markdown

Projects

None yet

Milestone

No milestone

Development

Successfully merging this pull request may close these issues.

3 participants

@jonasfj @srawlins @sigurdm