A use-after-free / double-free vulnerability was found in c-ares' query-completion handling. The same flaw — a query's callback being invoked while the query is still linked in the channel's internal lookup structures — is present at multiple points in the resend/finish path (timeout handling, response handling, and query dispatch). If the query, or for ares_getaddrinfo() the owning host_query, is freed as a side effect of that callback, it is then accessed and/or freed a second time.

It is reachable in two ways:

1. Application-triggered: a callback that re-enters c-ares, for example by calling ares_cancel(). The callback frees the query and it is freed again after the callback returns.

2. Remotely, with no application cooperation: via ares_getaddrinfo() over TCP. A malicious or on-path DNS server that returns a FORMERR without an OPT record (forcing an EDNS-downgrade retry), then a second response with the same query id, then resets the connection, causes ares_getaddrinfo()'s internal completion handler to re-enter (issuing a follow-on lookup whose send fails on the reset connection and completes immediately) and access a host_query that has already been freed. An attacker can force a client onto TCP by setting the truncation (TC) bit in a UDP response.

This was reproduced deterministically under AddressSanitizer (heap-use-after-free in the ares_getaddrinfo() completion handler) against v1.34.6 and the development branch, using only the public API and the standard system allocator.

The same pattern was previously addressed at one call site under CVE-2025-31498; CVE-2026-33630 covers the remaining occurrences.

The consequence is memory corruption leading to a crash (denial of service); a use-after-free may have further impact depending on the allocator and build. Reliable code execution has not been demonstrated.

Fixed in c-ares 1.34.7.