league/commonmark: Denial of service via duplicate footnote definitions
Impact
The Footnote extension records one backref per footnote reference and then appends the entire backref list for every footnote definition block in the document, without ever de-duplicating or removing repeated definitions of the same label (GatherFootnotesListener, populated by NumberFootnotesListener). A document that references a single label N times and also supplies N duplicate [^a]: definitions of that label therefore produces N × N FootnoteBackref nodes, so output size, parse time, and peak memory are all O(N²).
Reaching the vulnerable path requires FootnoteExtension to be registered on the Environment. This is opt-in, but is a commonly enabled GFM-style feature; no other non-default configuration is required. An unauthenticated attacker can expand a ~10 KB request into a ~62 MB HTML response, ~3 s of CPU, and ~440 MB of peak memory — enough to OOM-kill a default 128 MB PHP worker and deny service. Availability impact only; no confidentiality or integrity effect. The Footnote extension was introduced in 1.5.0 (May 2020) with this backref logic present from the first commit, so all releases from 1.5.0 onward (including every 2.x through 2.8.x) are affected.
Workarounds
There is no library-level option to cap the number of footnotes, references, or definitions, so no configuration switch prevents the amplification. Integrators who cannot upgrade should:
- Disable the Footnote extension for untrusted input, or
- Enforce a strict input-size limit before conversion — but note this is a weak control here, since the ~10 KB payload that already triggers the 62 MB / ~440 MB blowup is well within typical request-body limits, so any cap must be aggressively small to help.
Upgrading to the patched release is the recommended remediation.
Software afectado
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| Plataforma | Paquete | Versiones vulnerables | Corregida en |
|---|---|---|---|
| Paquete de Composer | league/commonmark | >= 1.5.0, < 2.9.0 | 2.9.0 |
Referencias
- Vector CVSS
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H- Debilidad (CWE)
CWE-407- Plataforma
- Composer
- Paquete
league/commonmark- Publicada
- 06.08.2026
- Actualizado
- 11.10.2026 10:22
Fuentes y licencias
OSV / GitHub Advisory Database - GHSA-jfm3-95jq-q3rf - CC BY 4.0
Historial de vulnerabilidades: league/commonmark
Todas las vulnerabilidades publicadas de este paquete (Paquete de Composer), de la más reciente a la más antigua. Muestra si es un fallo aislado o un paquete que hay que vigilar.
- Entradas en total
- 18
- Primera → última
- 09.2019 09.2026
- Sin corrección
- 0
- Explotada en ataques
- 0
Distribución por gravedad
- Alta 10
- Media 8
Divulgaciones por año
- 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 duplicate footnote definitions esta entrada
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