In Exim before 4.99.2, on systems using musl libc (not glibc), an attacker can crash the connection instance when malformed DNS data is present in PTR records. This is caused by a dn_expand oddity in octal printing.
Weakness
The code does not function according to its published specifications, potentially leading to incorrect usage.
Affected Software
| Name | Vendor | Start Version | End Version |
|---|---|---|---|
| Exim | Exim | * | 4.99.2 (excluding) |
| Exim4 | Ubuntu | upstream | * |
Potential Mitigations
References
- https://code.exim.org/exim/exim/commit/628bbaca7672748d941a12e7cd5f0122a4e18c81
- https://exim.org/static/doc/security/CVE-2026-40684.txt
- https://exim.org/static/doc/security/cve-2026-04.1/CVE2026-40684.assessment
- https://www.openwall.com/lists/oss-security/2026/04/30/21
- http://www.openwall.com/lists/oss-security/2026/05/01/11