CVE-2026-19401 Remote UDP DoS by sending multiple DNS Cookie options
Remote attackers can crash NSD DNS server child processes by sending crafted UDP packets with specific DNS Cookie options, causing service disruption.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-19401?
CVE-2026-19401 is a high-severity denial of service 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
Any remote client can crash a (debugging/non-release build type) NSD serve child by sending it a special crafted message with a specially tuned number of DNS Cookie options (17 when UDP payload size is 512). By continuously crashing the serve childs, the remote client can severely hamper or, when positioned sufficiently close, deny all DNS service.
Which versions are affected by CVE-2026-19401?
It affects 1 product.
Products include Nlnetlabs Nsd.
How do I fix CVE-2026-19401?
Upgrade to a fixed version.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2026-19401 being exploited?
There is no known exploitation of CVE-2026-19401 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-2026-18664 Wrong interpretation of ACL ranges
- 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-2026-19401, GHSA-v29p-5fvf-m7p8, UBUNTU-CVE-2026-19401.