league/commonmark: Denial of service via adjacent inline attribute blocks
Impact
With the Attributes extension enabled, AttributesListener::findTargetAndDirection() resolves each attribute node's target by walking outward through its siblings. For a run of N adjacent inline attribute blocks placed at the start of a block (with nothing to their left), each node scans the entire sibling list to the far-right end before giving up and falling back to the parent. Each resolution is therefore Θ(N) and the whole run is Θ(N²).
Reaching the path requires AttributesExtension (opt-in, but first-party: League\CommonMark\Extension\Attributes\AttributesExtension). No other configuration matters — the quadratic walk runs unconditionally during parsing and is not gated by the attributes/allow allow-list, the on* hardening added in 2.7.0, or allow_unsafe_links. An unauthenticated attacker can submit a ~32 KB input ({#a} repeated 8,000 times) that takes over 5 seconds to convert, with time growing quadratically in input length — a cheap denial of service. Availability impact only. The Attributes extension was introduced in 1.5.0 (May 2020) with this outward-walk resolver present from the first commit, so all releases from 1.5.0 onward (including every 2.x) are affected.
Workarounds
There is no library-level configuration that gates the quadratic walk. Integrators who cannot upgrade can only reduce exposure indirectly:
- Disable the Attributes extension for untrusted input, or
- Impose a strict maximum input length before conversion — noting that because the cost is quadratic, even a modest cap must be small to meaningfully bound worst-case CPU.
Upgrading to a release containing the fix is recommended.
Affected software
Click a technology to see all vulnerabilities affecting it.
| Platform | Package | Vulnerable versions | Fixed in |
|---|---|---|---|
| Composer package | league/commonmark | >= 1.5.0, < 2.9.0 | 2.9.0 |
References
- CVSS vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H- Weakness (CWE)
CWE-407- Platform
- Composer
- Package
league/commonmark- Disclosed
- 06.08.2026
- Updated
- 11.10.2026 10:22
Sources and licenses
OSV / GitHub Advisory Database - GHSA-g2gp-3wwq-f4ph - CC BY 4.0
Vulnerability history: league/commonmark
All published vulnerabilities of this package (Composer package), newest first. Shows whether this is a one-off slip or a package that needs watching.
- Total entries
- 18
- First → latest
- 09.2019 09.2026
- Without a fix
- 0
- Exploited in attacks
- 0
Severity breakdown
- High 10
- Medium 8
Disclosures per year
- 2019: 1
- 2020: 0
- 2021: 0
- 2022: 1
- 2023: 0
- 2024: 1
- 2025: 1
- 2026: 14
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league/commonmark: Denial of service via adjacent inline attribute blocks this entry
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