Unbound 1.16.21.25.1 Ghost Domain TTL Extension Vulnerability
CVE-2026-42955 Published on July 22, 2026
Extra fix for CVE-2026-40622 to also clamp the TTL of A/AAAA records disallowing a one-time 'ghost domain' delegation renewal via glue records
In NLnet Labs Unbound 1.16.2 up to and including 1.25.1, a similar vulnerability as with CVE-2026-40622 in the 'ghost domain names' family of attacks was found in Unbound that could extend the ghost domain window by up to one cached TTL configured value for A/AAAA glue records. Similar to other 'ghost domain names' attacks, an adversary needs to control a (ghost) zone and be able to query a vulnerable Unbound. A single client A/AAAA query can cause Unbound to overwrite the cached expired parent-side glue rrset and essentially extend the ghost domain window by up to one cached TTL configured value ('cache-max-ttl'). In configurations where 'harden-referral-path: yes' is used (non-default configuration), no client query is required since Unbound implicitly performs that query. This is a variant of CVE-2026-40622 which only addressed the NS query.
Timeline
Issue reported by Qifan Zhang
NLnet Labs shares patch 19 days later.
Qifan Zhang verifies patch 3 days later.
Fixes released with version 1.25.2 39 days later.
Weakness Type
Operation on a Resource after Expiration or Release
The software uses, accesses, or otherwise operates on a resource after that resource has been expired, released, or revoked.
Products Associated with CVE-2026-42955
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Affected Versions
NLnet Labs Unbound:- Version 1.16.2 and below 1.25.2 is affected.
Exploit Probability
EPSS (Exploit Prediction Scoring System) scores estimate the probability that a vulnerability will be exploited in the wild within the next 30 days. The percentile shows you how this score compares to all other vulnerabilities.