CVE-2026-42944: nlnet labs unbound Numeric Truncation Error

Vulnerability summary

CVE-2026-42944 is a high (CVSS 8.7) Numeric Truncation Error vulnerability in nlnet labs unbound. EPSS estimates a 0.78% chance of exploitation in the next 30 days, higher than 54.9% of scored vulnerabilities.

Timeline

1. 20-05-2026
   PublishedVulnerability first disclosed.
2. 26-08-2026
   Last modifiedVulnerability information updated.

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.

• CWE-197Numeric Truncation ErrorTruncation errors occur when a primitive is cast to a primitive of a smaller size and data is lost in the conversion.
• CWE-787Out-of-bounds WriteThe product writes data past the end, or before the beginning, of the intended buffer.
• CWE-131Incorrect Calculation of Buffer SizeThe product does not correctly calculate the size to be used when allocating a buffer, which could lead to a buffer overflow.