CVE-2026-44608 Use after free and crash under special conditions in RPZ code
Attackers can cause a crash in Unbound DNS resolver via use-after-free during RPZ zone transfers in multi-threaded environments.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-44608?
CVE-2026-44608 is a medium-severity use after free vulnerability in Nlnetlabs Unbound, published on 20 May 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official description
NLnet Labs Unbound 1.14.0 up to and including version 1.25.0 has a locking inconsistency vulnerability that when certain conditions are met (multi-threaded, RPZ XFR reload, RPZ zone with 'rpz-nsip'/'rpz-nsdname' triggers) it could result in heap use-after-free and eventual crash. An adversary can exploit the vulnerability if conditions are first met on a vulnerable Unbound, i.e., multi-threaded, an RPZ zone with 'rpz-nsip'/'rpz-nsdname' triggers and an ongoing XFR for that RPZ zone. Local RPZ files do not trigger the vulnerability. If the timing is right and an XFR happens at the same time another thread needs to read that RPZ zone, the reader may not hold the lock long enough and the thread applying the XFR may free objects that the reader is about to walk causing the use-after-free. Unbound 1.25.1 contains a patch with a fix to the locking code.
Weakness: CWE-413
Which versions are affected by CVE-2026-44608?
It affects 1 product.
Products include Nlnetlabs Unbound.
How do I fix CVE-2026-44608?
Apply the vendor's patch.
Upgrade to the fixed version. These bugs are found with fuzzing and sanitisers and avoided by memory-safe languages. More about Use after free.
Is CVE-2026-44608 being exploited?
There is no known exploitation of CVE-2026-44608 in the wild and no public exploit code.
Other vulnerabilities in Nlnetlabs Unbound
- CVE-2026-81642 Heap buffer overflow and possible Remote Code Execution when digesting DNSKEY
- CVE-2026-33278 Possible arbitrary code execution during DNSSEC validation
- CVE-2026-42944 Heap overflow with multiple NSID, COOKIE, PADDING EDNS options
- CVE-2026-42959 Crash during DNSSEC validation of malicious content
- CVE-2026-82717 CNAME synthesis could lead to heap corruption
Sources
Also known as ALPINE-CVE-2026-44608, AZL-87024, DEBIAN-CVE-2026-44608, ECHO-28d0-99d8-d11d, GHSA-fx8q-9cm5-75v9, UBUNTU-CVE-2026-44608.