CVE-2026-33278: nlnet labs unbound Use After Free

Vulnerability summary

CVE-2026-33278 is a critical (CVSS 9.8) Use After Free vulnerability in nlnet labs unbound. EPSS estimates a 1.13% chance of exploitation in the next 30 days, higher than 65.9% of scored vulnerabilities.

Timeline

1. 20-05-2026
   PublishedVulnerability first disclosed.
2. 01-09-2026
   Last modifiedVulnerability information updated.

Description

NLnet Labs Unbound 1.19.1 up to and including version 1.25.0 has a vulnerability in the DNSSEC validator that enables denial of service and possible remote code execution as a result of deep copying a data structure and erroneously overwriting a destination pointer. An adversary can exploit the vulnerability by controlling a malicious signed zone and querying a vulnerable Unbound. When DS sub-queries need to suspend validation due to NSEC3 computational budget exhaustion (introduced in Unbound 1.19.1), Unbound deep-copies response messages to preserve them across memory region teardown. A struct-assignment bug overwrites the destination's pointer with the source's pointer. After the sub-query region is freed, the resumed validator dereferences this dangling pointer, triggering a crash or potentially enabling arbitrary code execution. Unbound 1.25.1 contains a patch with a fix to preserve the correct pointer when deep copying the data structure.

• CWE-416Use After FreeThe product reuses or references memory after it has been freed. At some point afterward, the memory may be allocated again and saved in another pointer, while the original pointer references a location somewhere within the new allocation. Any operations using the original pointer are no longer valid because the memory "belongs" to the code that operates on the new pointer.
• CWE-672Operation on a Resource after Expiration or ReleaseThe product uses, accesses, or otherwise operates on a resource after that resource has been expired, released, or revoked.