Skip to content

[markdown] Reject '<' in AutolinkSyntax - #2755

Merged
jonasfj merged 2 commits into
mainfrom
markdown_autolink_reject_lt
Oct 30, 2026
Merged

[markdown] Reject '<' in AutolinkSyntax#2755
jonasfj merged 2 commits into
mainfrom
markdown_autolink_reject_lt

Conversation

@jonasfj

@jonasfj jonasfj commented Oct 29, 2026 •
edited
Loading

Copy link
Copy Markdown
Member

Add < to the negated character class in AutolinkSyntax's regex pattern
(r'<(([a-zA-Z][a-zA-Z\-\+\.]+):(?://)?[^\s<>]*)>').

Why this change is necessary

CommonMark 0.31.2 section 6.4 ("Autolinks") specifies that an absolute URI
autolink consists of <, a scheme, :, and zero or more characters other than
ASCII control characters, space, <, and >, followed by >.

Previously, AutolinkSyntax's regex used r'<(([a-zA-Z][a-zA-Z\-\+\.]+):(?://)?[^\s>]*)',
which excluded > and whitespace (\s) but allowed <. On an input containing
repeated unclosed autolinks without spaces—such as '<http://a' * n—whenever
InlineParser reached a <, AutolinkSyntax.tryMatch matched <http://a and
then kept consuming all subsequent <http://a substrings to the very end of the
input before failing to find a closing >. Across n occurrences of <, this
took O(n^2) time.

How this change works

Adding < to the negated character class ([^\s<>]*) both makes
AutolinkSyntax conform to CommonMark 0.31.2 section 6.4 and ensures that an
unclosed <http://a immediately stops at the next < in O(1) characters.

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.

Results:

  • Rendered HTML differs for 196 inputs: 114 OSS-Fuzz inputs and 82 real-world
    files, which are 4 distinct documents (79 are copies of one aud_midi*
    install guide vendored into several packages). All are autolinks containing
    an unescaped <, for example
    <https://dev.azure.com/<your-azure-devops-organization>/_packaging/feed>
    and the DOI <https://doi.org/10.1175/1520-0426(1992)009<0067:edafsi>2.0.co;2>.
    Before they were linked with %3C in the href; now they are literal text.
  • This is what the spec requires and what other renderers do. CommonMark §6.5
    defines the absolute URI of an autolink as a scheme, :, and "zero or more
    characters other than ASCII control characters, space, <, and >";
    commonmark.js 0.31, markdown-it 14, and GitHub render all four documents
    byte-identically to this branch. The md4c regression case <http://a<b>
    (example-129) now produces the expected &lt;http://a<b>.
  • No new exceptions; 3 OSS-Fuzz timeouts resolved; cumulative parse time −0.3%.

gemini-code-assist[bot]
gemini-code-assist Bot reviewed Oct 29, 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 AutolinkSyntax regular expression to prevent matching < inside <...> autolinks, conforming to the CommonMark definition of an absolute URI. It also adds a corresponding test case and updates the changelog. The reviewer suggested further refining the regular expression to allow digits in the URI scheme (e.g., http2) and to exclude ASCII control characters to fully comply with RFC 3986 and CommonMark specifications.

Comment thread pkgs/markdown/lib/src/inline_syntaxes/autolink_syntax.dart
AutolinkSyntax()
: super(
r'<(([a-zA-Z][a-zA-Z\-\+\.]+):(?://)?[^\s>]*)>',
r'<(([a-zA-Z][a-zA-Z\-\+\.]+):(?://)?[^\s<>]*)>',

@gemini-code-assist gemini-code-assist Bot Oct 29, 2026

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.

medium

While adding < to the negated character class correctly prevents matching nested < characters, there are two other areas where the autolink regular expression can be improved to fully conform to the CommonMark specification and URI standards:

  1. Missing Digits in Scheme: According to RFC 3986 and CommonMark, a URI scheme can contain digits (e.g., git+ssh, http2). The current character class [a-zA-Z\-\+\.] does not allow digits.
  2. ASCII Control Characters: CommonMark 0.31.2 section 6.4 specifies that absolute URI autolinks cannot contain ASCII control characters. While \s excludes whitespace control characters, other control characters (such as \x00-\x08, \x0E-\x1F, and \x7F) are currently allowed by [^\s<>].

We can address both issues by updating the regex pattern.

Suggested change
r'<(([a-zA-Z][a-zA-Z\-\+\.]+):(?://)?[^\s<>]*)>',
r'<(([a-zA-Z][a-zA-Z0-9\-\+\.]+):(?://)?[^\s<>\x00-\x1F\x7F]*)>',

@github-actions

github-actions Bot commented Oct 29, 2026 •
edited
Loading

Copy link
Copy Markdown

PR Health

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.

Coverage ✔️
File Coverage
pkgs/markdown/lib/src/inline_syntaxes/autolink_syntax.dart 💚 100 %

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.

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).

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.

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.

@jonasfj
[markdown] Reject '<' in AutolinkSyntax
39d01c3 
Add `<` to the negated character class in `AutolinkSyntax`'s regex pattern
(`r'<(([a-zA-Z][a-zA-Z\-\+\.]+):(?://)?[^\s<>]*)>'`).

----

### Why this change is necessary

CommonMark 0.31.2 section 6.4 ("Autolinks") specifies that an absolute URI
autolink consists of `<`, a scheme, `:`, and zero or more characters other than
ASCII control characters, space, `<`, and `>`, followed by `>`.

Previously, `AutolinkSyntax`'s regex used `r'<(([a-zA-Z][a-zA-Z\-\+\.]+):(?://)?[^\s>]*)'`,
which excluded `>` and whitespace (`\s`) but allowed `<`. On an input containing
repeated unclosed autolinks without spaces—such as `'<http://a' * n`—whenever
`InlineParser` reached a `<`, `AutolinkSyntax.tryMatch` matched `<http://a` and
then kept consuming all subsequent `<http://a` substrings to the very end of the
input before failing to find a closing `>`. Across n occurrences of `<`, this
took O(n^2) time.

### How this change works

Adding `<` to the negated character class (`[^\s<>]*`) both makes
`AutolinkSyntax` conform to CommonMark 0.31.2 section 6.4 and ensures that an
unclosed `<http://a` immediately stops at the next `<` in O(1) characters.
@jonasfj
jonasfj force-pushed the markdown_autolink_reject_lt branch from 64fa648 to 39d01c3 Compare October 30, 2026 09:22
@jonasfj
jonasfj requested a review from szakarias October 30, 2026 09:43
@jonasfj
jonasfj marked this pull request as ready for review October 30, 2026 09:43
@jonasfj
jonasfj requested a review from srawlins as a code owner October 30, 2026 09:43
@jonasfj
jonasfj removed the request for review from srawlins October 30, 2026 09:44
@jonasfj jonasfj changed the title [WIP] [markdown] Reject '<' in AutolinkSyntax [markdown] Reject '<' in AutolinkSyntax Oct 30, 2026
@jonasfj
jonasfj enabled auto-merge (squash) October 30, 2026 10:08
@jonasfj
jonasfj merged commit c7eafbe into main Oct 30, 2026
21 of 22 checks passed
@jonasfj
jonasfj deleted the markdown_autolink_reject_lt branch October 30, 2026 10:09
copybara-service Bot pushed a commit to dart-lang/sdk that referenced this pull request Nov 3, 2026
@natebosch dart-scoped@luci-project-accounts.iam.gserviceaccount.com
[deps] rev core, ecosystem, http, i18n, shelf, test, tools
c48ebb8 
Revisions updated by `dart tools/rev_sdk_deps.dart`.

core (https://github.com/dart-lang/core/compare/d7bd45c..24e09c4):
  24e09c4c  Sun Nov 1 21:12:19 2026 +0000  dependabot[bot]  Bump the github-actions group with 4 updates (dart-lang/core#1037)
  87ac1b4c  Fri Oct 30 16:02:43 2026 +0100  Michael Thomsen  [os_detect] Deprecate package:os_detect in favor of package:platform (dart-lang/core#1003)
  8debf335  Fri Oct 30 01:57:36 2026 +0530  Sahaja Pallapothula  Clarify Characters equality and Unicode normalization (dart-lang/core#1031)

ecosystem (https://github.com/dart-lang/ecosystem/compare/24b9ba8..f8faabf):
  f8faabf  Sun Nov 1 18:16:20 2026 +0000  dependabot[bot]  Bump the github-actions group with 2 updates (dart-lang/ecosystem#481)
  27c2fc8  Sat Oct 31 12:36:53 2026 +0100  Moritz  Fix publish workflow pins, add tag_prefix to health, and update docs (dart-lang/ecosystem#480)
  05641a9  Sat Oct 31 09:56:42 2026 +0100  Moritz  Restore deprecated publish.yaml workflow and firehose CLI entrypoint (dart-lang/ecosystem#479)
  78abf72  Fri Oct 30 08:24:07 2026 -0700  Kevin Moore  fix(repo_manage): use pub_semver for changelog -wip transitions (dart-lang/ecosystem#475)
  0bfa654  Fri Oct 30 09:33:57 2026 +0100  Moritz  Move publish check to health workflow and use individual package publishing workflows from `setup-dart` (dart-lang/ecosystem#454)
  adf7979  Thu Oct 29 23:46:49 2026 -0700  Kevin Moore  test(firehose): use hasLength, containsPair, and isFalse (dart-lang/ecosystem#477)

http (https://github.com/dart-lang/http/compare/c76dfb3..8c0d66a):
  8c0d66a  Fri Oct 30 09:54:41 2026 -0700  Brian Quinlan  chore(cupertino_http): check ffigen generation (dart-lang/http#2044)
  c8737d1  Fri Oct 30 09:11:46 2026 +0100  Moritz  chore(http): run Firefox tests with -headless instead of xvfb-run (dart-lang/http#2033)
  05d0b18  Fri Oct 30 07:08:19 2026 +0100  Moritz  fix(http2): make StreamException a StreamTransportException and forward ALPN errors to onError (dart-lang/http#2009)

i18n (https://github.com/dart-lang/i18n/compare/4c7b4ad..7475ce6):
  7475ce64  Sat Oct 31 02:49:28 2026 +0100  Moritz  [intl4x] Fix NumberFormat.percent ignoring digits (dart-lang/i18n#1115)

shelf (https://github.com/dart-lang/shelf/compare/62d3e4e..9946de5):
  9946de5  Sun Nov 1 12:27:59 2026 +0000  dependabot[bot]  build(deps): bump the github-actions group with 3 updates (dart-lang/shelf#554)

test (https://github.com/dart-lang/test/compare/6d74720..2a9e017):
  2a9e0175  Sun Nov 1 03:47:24 2026 +0000  dependabot[bot]  Bump the github-actions group with 6 updates (dart-lang/test#2813)
  55cd5cd5  Fri Oct 30 13:43:47 2026 -0700  Nate Bosch  Migrate publishing workflows from firehose to setup-dart (dart-lang/test#2812)
  10dc78c0  Fri Oct 30 11:21:53 2026 -0700  Nate Bosch  [checks] Describe what the stream did in failures (dart-lang/test#2811)
  c11ad06d  Fri Oct 30 08:59:57 2026 -0700  Nate Bosch  [checks] Add CheckFailure.describeNested (dart-lang/test#2793)
  0137b43e  Fri Oct 30 05:40:43 2026 +0530  Shriraj Jadhav  [checks] Use the Iterable.last member for the last expectation (dart-lang/test#2807)
  50c9f454  Thu Oct 29 15:11:39 2026 -0700  Nate Bosch  [checks] Collapse clauses for described conditions (dart-lang/test#2794)

tools (https://github.com/dart-lang/tools/compare/3980dca..1579f8d):
  1579f8d69  Mon Nov 2 02:44:58 2026 +0530  Joshua  fix(coverage): reject package: and dart: URIs outside known roots in Resolver (dart-lang/tools#2764)
  f869fdca4  Sun Nov 1 10:26:34 2026 +0100  dart-ecosystem-bot[bot]  [boring] Update BoringSSL to ed9209cf (dart-lang/tools#2774)
  d10e2b87f  Sun Nov 1 10:26:11 2026 +0100  dart-ecosystem-bot[bot]  [boring] Pin the prebuilt binaries of boring-binaries-20261101-33e9e69722af (dart-lang/tools#2777)
  33e9e6972  Sun Nov 1 05:56:30 2026 +0000  dependabot[bot]  build(deps): bump the github-actions group with 4 updates (dart-lang/tools#2775)
  f84d0d0d2  Sun Nov 1 05:36:12 2026 +0000  dependabot[bot]  build(deps): bump ffigen from 22.0.0 to 23.0.0 in /pkgs/boring (dart-lang/tools#2776)
  14ad5e515  Fri Oct 30 10:48:58 2026 -0700  Kevin Moore  [mime] Harden BoundMultipartStream error routing, headers, and epilogue (dart-lang/tools#2629)
  7a780d48c  Fri Oct 30 12:57:03 2026 +0100  Jonas Finnemann Jensen  [markdown] Avoid rescanning failed plain-text runs in InlineParser (dart-lang/tools#2757)
  3671d33a9  Fri Oct 30 12:56:48 2026 +0100  Jonas Finnemann Jensen  [markdown] Index footnote definitions and labels on Document for O(1) lookup (dart-lang/tools#2760)
  6f38e9fda  Fri Oct 30 12:56:09 2026 +0100  Jonas Finnemann Jensen  [markdown] Bound the domain of extended URL autolinks to DNS limits (dart-lang/tools#2759)
  aa0c38544  Fri Oct 30 12:20:40 2026 +0100  Jonas Finnemann Jensen  [markdown] Avoid quadratic removeRange in _combineAdjacentText (dart-lang/tools#2758)
  f425baf46  Fri Oct 30 11:16:28 2026 +0100  Jonas Finnemann Jensen  [markdown] Avoid rescanning failed space runs in LineBreakSyntax (dart-lang/tools#2756)
  c7eafbec3  Fri Oct 30 11:09:45 2026 +0100  Jonas Finnemann Jensen  [markdown] Reject '<' in AutolinkSyntax (dart-lang/tools#2755)
  86ed0f902  Fri Oct 30 11:08:25 2026 +0100  Jonas Finnemann Jensen  [markdown] Avoid quadratic backtracking in headerPattern (dart-lang/tools#2754)
  8ea07517f  Fri Oct 30 07:45:42 2026 +0100  Moritz  [boring] Regenerate the x509-limbo expected failures in the refresh job (dart-lang/tools#2712)
  40235c365  Thu Oct 29 14:46:17 2026 +0100  Jonas Finnemann Jensen  [markdown] Make HtmlBlockSyntax.parseChildLines iterative (dart-lang/tools#2749)
  0ff03f643  Thu Oct 29 14:41:33 2026 +0100  Jonas Finnemann Jensen  [markdown] Avoid re-escaping nested image alt text in ImageSyntax.createNode (dart-lang/tools#2752)
  784654d01  Thu Oct 29 14:37:11 2026 +0100  Jonas Finnemann Jensen  [markdown] Add test cases covering parser and renderer edge cases (dart-lang/tools#2753)
  4f2718ad8  Thu Oct 29 14:36:52 2026 +0100  Jonas Finnemann Jensen  [markdown] Make Document._parseInlineContent iterative (dart-lang/tools#2750)
  cac1421d5  Thu Oct 29 14:17:06 2026 +0100  Jonas Finnemann Jensen  [markdown] Avoid quadratic duplicate heading ID probing in HtmlRenderer (dart-lang/tools#2748)
  66880bcd3  Thu Oct 29 14:11:27 2026 +0100  Jonas Finnemann Jensen  [markdown] Check footnote label and empty map before scanning footnote keys (dart-lang/tools#2751)
  7f8696523  Thu Oct 29 14:07:06 2026 +0100  Jonas Finnemann Jensen  [markdown] Avoid quadratic blank-line lookahead in CodeBlockSyntax (dart-lang/tools#2747)
  9da6bb291  Thu Oct 29 14:03:04 2026 +0100  Jonas Finnemann Jensen  [markdown] Avoid exponential and quadratic backtracking in tablePattern (dart-lang/tools#2746)

dartdoc is held back at db94437, before dart-lang/dartdoc#4312, because its
mock SDK version changes conflict in the google3 3-way merge with the hotfix
in cl/1005963642.

native is held back at e0ca7c6, before dart-lang/native#3742, because
deprecating `MacOSCodeConfig(targetVersion: ...)` causes a fatal
`deprecated_member_use` diagnostic in `pkg/dartdev/lib/src/native_assets.dart`.

web is held back at 4e60e61, before dart-lang/web#596, because it changes
`package:web` signatures (e.g. `innerHTML=`, `parseFromString`, `Worker()`)
from `JSAny` to `String` in ways that break callers in google3.

Change-Id: I53d2cbde741c45b8fd33bca42742549819dd332b
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/572780
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

@szakarias szakarias szakarias approved these changes
+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.

2 participants

@jonasfj @szakarias