CVE-2026-42959 Crash during DNSSEC validation of malicious content
An attacker can crash the Unbound DNS resolver by sending malicious DNSSEC replies, causing a denial of service.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-42959?
CVE-2026-42959 is a high-severity 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 up to and including version 1.25.0 has a denial of service vulnerability in the DNSSEC validator that can lead to a crash given malicious upstream replies. When Unbound constructs chase-reply messages for validation, the code uses the wrong counter to calculate write offsets for ADDITIONAL section rrsets. DNAME duplication could increase the ANSWER section count and authority filtering could decrease the AUTHORITY section count and create an uninitialized array slot. Combining these two, the validator later dereferences this uninitialized pointer, causing an immediate process crash. An adversary controlling a DNSSEC-signed domain can trigger this bug with a single query by configuring a DNAME chain with unsigned CNAMEs and a response containing unsigned AUTHORITY records alongside signed ADDITIONAL glue records. Unbound 1.25.1 contains a patch with a fix to use the proper counters to calculate the write offsets.
Weakness: CWE-824
Which versions are affected by CVE-2026-42959?
It affects 1 product.
Products include Nlnetlabs Unbound.
How do I fix CVE-2026-42959?
Upgrade to a fixed version.
Is CVE-2026-42959 being exploited?
There is no known exploitation of CVE-2026-42959 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-2026-82717 CNAME synthesis could lead to heap corruption
- CVE-2026-81634 Possible heap buffer overflow during DNSSEC canonicalization
Sources
Also known as ALPINE-CVE-2026-42959, AZL-87021, DEBIAN-CVE-2026-42959, ECHO-2fce-8b09-c05e, GHSA-5jp8-75xr-qrmf, ROOT-OS-DEBIAN-11-CVE-2026-42959 and 2 more.