CVE-2026-86433 commonmark 1.5.0 before 2.8.4 Denial of Service via Attributes
Attackers can cause denial of service in the commonmark PHP library by submitting crafted attribute blocks that trigger quadratic-time parsing.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-86433?
CVE-2026-86433 is a high-severity denial of service vulnerability in thephpleague commonmark, published on 7 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official description
commonmark versions from 1.5.0 before 2.8.4 contain a denial of service vulnerability in the Attributes extension where AttributesListener::findTargetAndDirection() performs quadratic-time sibling list scanning. Unauthenticated attackers can submit approximately 32 KB of repeated attribute blocks to cause parsing to take over 5 seconds, exhausting server resources.
Weakness: CWE-407
Which versions are affected by CVE-2026-86433?
How do I fix CVE-2026-86433?
Upgrade to a fixed version.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2026-86433 being exploited?
There is no known exploitation of CVE-2026-86433 in the wild and no public exploit code.
Sources
Also known as DEBIAN-CVE-2026-86433, GHSA-f9pm-gcpp-fwjp, UBUNTU-CVE-2026-86433.