GHSA-G2GP-3WWQ-F4PH
Vulnerability from github – Published: 2026-08-06 20:39 – Updated: 2026-08-06 20:39Impact
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": [
{
"package": {
"ecosystem": "Packagist",
"name": "league/commonmark"
},
"ranges": [
{
"events": [
{
"introduced": "1.5.0"
},
{
"fixed": "2.9.0"
}
],
"type": "ECOSYSTEM"
}
]
}
],
"aliases": [],
"database_specific": {
"cwe_ids": [
"CWE-407"
],
"github_reviewed": true,
"github_reviewed_at": "2026-08-06T20:39:52Z",
"nvd_published_at": null,
"severity": "HIGH"
},
"details": "### Impact\n\nWith the Attributes extension enabled, `AttributesListener::findTargetAndDirection()` resolves each attribute node\u0027s 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 \u0398(N) and the whole run is **\u0398(N\u00b2)**.\n\nReaching the path requires `AttributesExtension` (opt-in, but first-party: `League\\CommonMark\\Extension\\Attributes\\AttributesExtension`). No other configuration matters \u2014 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 \u2014 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.**\n\n### Workarounds\n\nThere is no library-level configuration that gates the quadratic walk. Integrators who cannot upgrade can only reduce exposure indirectly:\n\n- **Disable the Attributes extension** for untrusted input, or\n- **Impose a strict maximum input length before conversion** \u2014 noting that because the cost is quadratic, even a modest cap must be small to meaningfully bound worst-case CPU.\n\nUpgrading to a release containing the fix is recommended.",
"id": "GHSA-g2gp-3wwq-f4ph",
"modified": "2026-08-06T20:39:52Z",
"published": "2026-08-06T20:39:52Z",
"references": [
{
"type": "WEB",
"url": "https://github.com/thephpleague/commonmark/security/advisories/GHSA-g2gp-3wwq-f4ph"
},
{
"type": "WEB",
"url": "https://github.com/thephpleague/commonmark/commit/2d4c0fafa62501be919262064cffa6d71687430b"
},
{
"type": "PACKAGE",
"url": "https://github.com/thephpleague/commonmark"
},
{
"type": "WEB",
"url": "https://github.com/thephpleague/commonmark/releases/tag/2.9.0"
}
],
"schema_version": "1.4.0",
"severity": [
{
"score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H",
"type": "CVSS_V3"
}
],
"summary": "league/commonmark: Denial of service via adjacent inline attribute blocks"
}
Sightings
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