CVE-2026-66731 facil.io 0.7.5 - 0.7.6 HTTP/1.1 Chunked Transfer Encoding Parser Crash DoS
Remote attackers can crash the server by sending a malformed chunked request with a negative size, causing a memory fault in the parser.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- None listed
What is CVE-2026-66731?
CVE-2026-66731 is a high-severity denial of service vulnerability in boazsegev facil.io, published on 27 Jul 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official description
facil.io 0.7.5 through 0.7.6 contains a denial-of-service vulnerability in the HTTP/1.1 chunked transfer encoding parser that allows unauthenticated remote attackers to crash the server by sending a negative chunk size value. Attackers can send a single POST request with a Transfer-Encoding: chunked header containing a leading minus sign in the chunk size field, causing the parser in http1_parser.h to compute a large positive integer from the negated value, corrupting internal state and moving the read pointer into unmapped memory resulting in a fault.
Weakness: CWE-125
Which versions are affected by CVE-2026-66731?
How do I fix CVE-2026-66731?
No fix has been published for CVE-2026-66731 yet. Limit exposure of the affected software until one is available.
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-2026-66731 being exploited?
It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-66731 exists.
Public exploit code: 1 proofs of concept on GitHub.
Sources
Also known as GHSA-vg6c-748h-g68q.