CVE-2026-18664 Wrong interpretation of ACL ranges
Attackers can bypass IP-based access controls on NSD DNS servers due to endianness errors in range comparisons, allowing unauthorized access or blocking legitimate users.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-18664?
CVE-2026-18664 is a high-severity authentication bypass vulnerability in NLnet Labs NSD, published on 26 Aug 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official 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.
Which versions are affected by CVE-2026-18664?
It affects 1 product.
Products include Nlnetlabs Nsd.
How do I fix CVE-2026-18664?
Apply the vendor's patch.
Upgrade, and rotate credentials and sessions that may have been exposed. More about Authentication bypass.
Is CVE-2026-18664 being exploited?
There is no known exploitation of CVE-2026-18664 in the wild and no public exploit code.
Other vulnerabilities in NLnet Labs NSD
- CVE-2026-12244 Heap overflow and crash with crafted SVCB RR
- CVE-2026-12245 Denial of DNS over TLS service by any DoT client
- CVE-2026-19401 Remote UDP DoS by sending multiple DNS Cookie options
- CVE-2026-19538 Bypass of BLOCKED ACL items on proxy protocol port over TCP or TLS
- CVE-2026-12490 Bypass of client certificate verification with transfer over TLS
Sources
Also known as DEBIAN-CVE-2026-18664, GHSA-fw9f-54cp-j7fc, UBUNTU-CVE-2026-18664.