Use-after-free in c-ares - CVE-2026-33630

Detailed vulnerability description

The vulnerability allows a remote attacker to cause a denial of service.

The vulnerability exists due to use-after-free in c-ares query-completion handling for ares_getaddrinfo() when processing crafted DNS responses over TCP. A remote attacker can send crafted DNS responses that trigger re-entrant completion handling to cause a denial of service.

An attacker can force the client to retry over TCP by setting the truncation bit in a UDP response, and the issue was observed when a malicious or on-path DNS server returned a FORMERR response without an OPT record, followed by a second response with the same query id and a connection reset.

How to mitigate CVE-2026-33630

Sources

• https://github.com/c-ares/c-ares/security/advisories/GHSA-6wfj-rwm7-3542
• https://github.com/c-ares/c-ares/commit/d823199b688052dcdc1646f2ab4cb8c16b1c644a