CVE-2026-19538 Bypass of BLOCKED ACL items on proxy protocol port over TCP or TLS
Attackers can bypass BLOCKED ACL restrictions on the proxy protocol port by sending duplicate queries over persistent TCP or TLS connections.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-19538?
CVE-2026-19538 is a high-severity 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
The BLOCKED access control list items that are evaluated to deny access on the the proxy protocol port can be bypassed completely when connecting over TCP or TLS and sending the query twice on connection that is kept open.
Which versions are affected by CVE-2026-19538?
It affects 1 product.
Products include Nlnetlabs Nsd.
How do I fix CVE-2026-19538?
Apply the vendor's patch.
Is CVE-2026-19538 being exploited?
There is no known exploitation of CVE-2026-19538 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-2026-19401 Remote UDP DoS by sending multiple DNS Cookie options
- CVE-2026-12490 Bypass of client certificate verification with transfer over TLS
Sources
Also known as DEBIAN-CVE-2026-19538, GHSA-q27j-h4qm-x5f7, UBUNTU-CVE-2026-19538.