{"id":"GHSA-g2gp-3wwq-f4ph","aliases":[],"url":"https://o3.security/vulnerability/GHSA-g2gp-3wwq-f4ph","summary":"league/commonmark: Denial of service via adjacent inline attribute blocks","details":"### Impact\n\nWith 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)**.\n\nReaching 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.**\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** — 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.","published":"2026-08-06T20:39:52Z","modified":"2026-08-06T20:45:15.558862305Z","cvss":{"score":7.5,"severity":"HIGH","vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H"},"epss":null,"cisaKev":null,"exploitsKnown":0,"affectedPackages":[{"ecosystem":"Packagist","name":"league/commonmark","fixedVersion":"2.9.0"}],"fix":{"url":"https://github.com/thephpleague/commonmark/commit/2d4c0fafa62501be919262064cffa6d71687430b","label":"thephpleague/commonmark@2d4c0fa"},"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"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-08-06T20:45:15.558862305Z"}}