Description
NLnet Labs Unbound 1.14.0 up to and including version 1.25.0 has a vulnerability that results in heap overflow when encoding multiple NSID and/or DNS Cookie EDNS and/or EDNS Padding options in the reply packet. The relevant options ('nsid', 'answer-cookie', 'pad-responses' (default)) need to be enabled for the vulnerability to be exploited. An adversary who can query Unbound can exploit the vulnerability by attaching multiple NSID and/or DNS Cookie EDNS and/or EDNS Padding options to the query. A flaw in the size calculation of the EDNS field truncates the correct value which allows the encoder to overflow the available space when writing. Those two combined lead to a heap overflow write of Unbound controlled data and eventually a crash. Unbound 1.25.1 contains a patch with a fix to de-duplicate the EDNS options and a fix to prevent truncation of the EDNS field size calculation.
Problem types
CWE-197: Numeric Truncation Error
Product status
1.14.0 (semver) before 1.25.1
Timeline
| 2026-04-26: | Issue reported by Qifan Zhang |
| 2026-04-28: | NLnet Labs shares patch |
| 2026-04-29: | Qifan Zhang verifies patch |
| 2026-05-20: | Fixes released with version 1.25.1 |
Credits
Qifan Zhang (Palo Alto Networks)
References
www.nlnetlabs.nl/downloads/unbound/CVE-2026-42944.txt