CVE-2026-33630
Description
A flaw was found in c-ares. A use-after-free / double-free vulnerability exists in the query-completion handling path, where a query callback is invoked while the query is still linked in internal lookup structures. A remote attacker can exploit this via ares_getaddrinfo() over TCP by sending crafted DNS responses that force an EDNS-downgrade retry followed by a connection reset, causing the internal completion handler to access freed memory. This leads to memory corruption and a crash (denial of service), with potential for further impact depending on the allocator and build configuration.
Statement
A use-after-free / double-free vulnerability was found in c-ares' query-completion handling. The flaw is remotely exploitable without application cooperation via ares_getaddrinfo() over TCP: a malicious or on-path DNS server can force a specific sequence of responses (FORMERR without OPT record, duplicate query ID response, TCP reset) that causes the internal completion handler to access freed memory. An attacker can force the client onto TCP by setting the truncation (TC) bit in a UDP response.
The consequence is memory corruption leading to a crash (denial of service).
This is a broader fix for the pattern previously addressed in CVE-2025-31498. All versions of c-ares prior to 1.34.7 are affected.
Mitigation
There is no complete mitigation for the remotely-triggered path. As a partial mitigation, use trusted DNS resolvers reached over a trusted transport (e.g., DNS-over-TLS). Avoid calling ares_cancel() from within a query callback to prevent the application-triggered path. Upgrade to c-ares 1.34.7 or later.
Common Vulnerability Scoring System (CVSS) Score Details
Info alert:Important note
CVSS scores for open source components depend on vendor-specific factors (e.g. version or build chain). Therefore, Red Hat's score and impact rating can be different from NVD and other vendors. Red Hat remains the authoritative CVE Naming Authority (CNA) source for its products and services (see Red Hat classifications).
CVSS v3 Score Breakdown
| Red Hat | NVD | cve.org | |
|---|---|---|---|
| Base Score | 7.5 | N/A | N/A |
| Attack Vector | Network | N/A | N/A |
| Attack Complexity | Low | N/A | N/A |
| Privileges Required | None | N/A | N/A |
| User Interaction | None | N/A | N/A |
| Scope | Unchanged | N/A | N/A |
| Confidentiality | None | N/A | N/A |
| Integrity Impact | None | N/A | N/A |
| Availability Impact | High | N/A | N/A |
Vector
Red Hat: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Understanding the Weakness (CWE)
Integrity
Technical Impact: Modify Memory
The use of previously freed memory may corrupt valid data, if the memory area in question has been allocated and used properly elsewhere.
Availability
Technical Impact: DoS: Crash, Exit, or Restart
If chunk consolidation occurs after the use of previously freed data, the process may crash when invalid data is used as chunk information.
Confidentiality
Technical Impact: Read Memory
Read operations on freed memory can sometimes leak sensitive information instead of causing a crash
Integrity,Confidentiality,Availability
Technical Impact: Execute Unauthorized Code or Commands
If malicious data is entered before chunk consolidation can take place, it may be possible to take advantage of a write-what-where primitive to execute arbitrary code. If the newly allocated data happens to hold a class, in C++ for example, various function pointers may be scattered within the heap data. If one of these function pointers is overwritten with an address to valid shellcode, execution of arbitrary code can be achieved.
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