GHSA-g2gp-3wwq-f4ph league/commonmark: Denial of service via adjacent inline attribute blocks

• Packagist league/commonmark: >= 1.5.0, < 2.9.0 → fixed in 2.9.0

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.