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[markdown] Reject unescaped '<' in bracketed inline link destinations - #2767

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[markdown] Reject unescaped '<' in bracketed inline link destinations#2767
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markdown_bracketed_destination_lt

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@jonasfj jonasfj commented Oct 30, 2026 •
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Reject unescaped < (char == $lt) alongside \n, \r, and \f inside
LinkSyntax._parseInlineBracketedLink.

Why this change is necessary

CommonMark 0.31.2 section 6.3 ("Links") defines a bracketed link destination as:

a sequence of zero or more characters between an opening < and a closing >
that contains no line endings or unescaped < or > characters

Previously, LinkSyntax._parseInlineBracketedLink broke out of the loop on >
and returned null on \n, \r, and \f, but did not reject unescaped <.
As a result:

  1. Invalid bracketed destinations containing unescaped < (such as
    [a](<foo<bar>)) were accepted in violation of CommonMark.
  2. On an input with repeated unclosed bracketed link destinations like
    '[a](<' * n, each [a](< scanned all the way to the end of the line across
    every subsequent [a](< before failing at EOF, taking O(n^2) time.

How this change works

Adding char == $lt to the rejection condition inside
_parseInlineBracketedLink:

if (char == $lf || char == $cr || char == $ff || char == $lt) {
  return null;
}

immediately aborts the bracketed destination scan when an unescaped < is
encountered (while \< escapes continue to be skipped by the preceding
if (char == $backslash) branch). On '[a](<' * n, each [a](< now stops at
the very next < in O(1) characters, reducing total scan time from O(n^2) to
O(n).

Behavior change

An inline link destination written in angle brackets may no longer contain an
unescaped <. CommonMark §6.3 defines it as "a sequence of zero or more
characters between an opening < and a closing > that contains no line
endings or unescaped < or > characters". Before, [a](<b<c>) produced a
link to b%3Cc; now it is literal text, as in cmark and commonmark.js.
[a](<b\<c>) still links to b%3Cc. This is also what stops '[a](<' * n
from rescanning to the end of the line at every ].

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:

  • 3 differences in the parser test suites, all cases the suites include
    precisely because they must not be links, and now matching
    commonmark.js: pulldown-cmark regression example 19 [](<<>) and the
    markdown-it / markdown-it-py commonmark_extras case [](<foo<bar>)
    (the escaped variant [](<foo\<bar>) in the same case still links).
  • 3 OSS-Fuzz inputs changed; in all 3 the changed region matches commonmark.js
    where it did not before.
  • No differences in pub.dev or the real-world docs.
  • One 130 KB OSS-Fuzz input was reported as a new timeout — standalone 5.5 s on main vs 5.4 s on this branch, so that was load during the run.
    Cumulative parse time −0.1%.

@jonasfj
[markdown] Reject unescaped '<' in bracketed inline link destinations
feb173f 
Reject unescaped `<` (`char == $lt`) alongside `\n`, `\r`, and `\f` inside
`LinkSyntax._parseInlineBracketedLink`.

----

### Why this change is necessary

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

Previously, `LinkSyntax._parseInlineBracketedLink` broke out of the loop on `>`
and returned `null` on `\n`, `\r`, and `\f`, but did not reject unescaped `<`.
As a result:
1. Invalid bracketed destinations containing unescaped `<` (such as
   `[a](<foo<bar>)`) were accepted in violation of CommonMark.
2. On an input with repeated unclosed bracketed link destinations like
   `'[a](<' * n`, each `[a](<` scanned all the way to the end of the line across
   every subsequent `[a](<` before failing at EOF, taking O(n^2) time.

### How this change works

Adding `char == $lt` to the rejection condition inside
`_parseInlineBracketedLink`:
```dart
if (char == $lf || char == $cr || char == $ff || char == $lt) {
  return null;
}
```
immediately aborts the bracketed destination scan when an unescaped `<` is
encountered (while `\<` escapes continue to be skipped by the preceding
`if (char == $backslash)` branch). On `'[a](<' * n`, each `[a](<` now stops at
the very next `<` in O(1) characters, reducing total scan time from O(n^2) to
O(n).
gemini-code-assist[bot]
gemini-code-assist Bot reviewed Oct 30, 2026
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Code Review

This pull request updates the inline link syntax parsing in link_syntax.dart to prevent inline link destinations enclosed in angle brackets (<...>) from containing an unescaped < character, aligning with the CommonMark specification. A corresponding entry has also been added to the changelog. There are no review comments, and the changes look correct, so I have no feedback to provide.

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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).

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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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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.
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Coverage ✔️
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pkgs/markdown/lib/src/inline_syntaxes/link_syntax.dart 💚 99 %

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@jonasfj
jonasfj marked this pull request as ready for review November 2, 2026 13:49
@jonasfj
jonasfj requested a review from srawlins as a code owner November 2, 2026 13:49
@jonasfj
jonasfj requested review from szakarias and removed request for srawlins November 2, 2026 13:49
@jonasfj
jonasfj merged commit 5ebd52b into main Nov 3, 2026
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@jonasfj
jonasfj deleted the markdown_bracketed_destination_lt branch November 3, 2026 12:03
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
@szakarias szakarias Awaiting requested review from szakarias
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