CVE-2026-18664: nlnet labs nsd Improper Access Control

Vulnerability summary

CVE-2026-18664 is a critical (CVSS 9.1) Improper Access Control vulnerability in nlnet labs nsd. EPSS estimates a 0.26% chance of exploitation in the next 30 days, higher than 16.5% of scored vulnerabilities.

Timeline

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

Description

When ranges are used for access control (i.e. of the form 1.2.3.4-1.2.3.25), because NSD wrongly compares the IP address with the range on little endian systems, IPs that were meant to be allowed may be denied, and, IPs that were meant to be denied access could be allowed. An IPv4 address is compared with IPv4 ranges as unsigned 32 bit numbers directly with the endianness of the host, but the values to compare are in network byte order (big-endian). With IPv6 addresses the comparison is done in 4 times a unsigned 32 bit number comparison, again with the endianness of the host where all values are actually in network bye order.

• CWE-284Improper Access ControlThe product does not restrict or incorrectly restricts access to a resource from an unauthorized actor.
• CWE-697Incorrect ComparisonThe product compares two entities in a security-relevant context, but the comparison is incorrect.