markdown2 CVE-2026-88753

Exploitation requires the target application to invoke markdown2.extract_metadata() (i.e., metadata/front-matter parsing enabled) on attacker-supplied Markdown content, using markdown2 at or below v2.5.5. It does NOT affect applications that only render Markdown without metadata extraction. The limiting factor is reachability: although no authentication or user interaction is needed once the parsing endpoint is exposed, the vulnerable function is an opt-in feature rather than the default rendering path, and impact is confined to availability (CPU exhaustion), not data compromise.

Signals are moderate and internally consistent rather than alarming. The provided CVSS 3.1 vector (AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H, base 7.5) reflects a pure availability impact (CWE-400 resource exhaustion / ReDoS) with no confidentiality or integrity loss - the 'HIGH' rating is driven entirely by the A:H component, which is typical for DoS and should not be read as a code-execution-class threat. EPSS is low at 0.15% (4th percentile), and the CVE is NOT on CISA KEV, so there is no evidence of active or widespread exploitation. A gist reference (NanaSec) is present and, for a ReDoS, almost certainly contains proof-of-concept trigger input, so publicly available exploit code likely exists - but POC availability must not be conflated with active exploitation. No SSVC decision data was supplied. The key caveat the raw CVSS hides: the AV:N/PR:N/UI:N metrics assume the vulnerable extract_metadata() path is reachable with attacker-controlled Markdown, which is an opt-in metadata-parsing feature, not markdown2's default rendering path - so real-world reachability is narrower than the network-unauthenticated vector implies. Net: a genuine but lower-tier priority - patch in normal cycles, prioritize only if your application parses untrusted Markdown metadata.

An application uses markdown2's extract_metadata() to parse front-matter/metadata from user-submitted Markdown (e.g., a CMS, wiki, or comment system). An attacker submits a small crafted Markdown document whose metadata section triggers catastrophic regular-expression backtracking, pinning a CPU core and hanging the worker. A public gist appears to provide the triggering input, lowering the attack-complexity (consistent with AC:L).