Quick answers
- Is CVE-2026-50251 exploited in the wild?
- No. Junglewise Threat Intelligence has no report of CVE-2026-50251 being exploited in the wild.
- How severe is CVE-2026-50251?
- Medium severity, with a CVSS score of 5.3, published 22 July 2026.
- Which products and versions are affected?
- NLnet Labs Unbound up to and including 1.25.1.
- Is there a fix for CVE-2026-50251?
- A patch was published on 2026-07-22: Version 1.25.2 released with fix Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.
- How is CVE-2026-50251 attacked?
- It is attacked over the network, in a software library.
- Am I affected by CVE-2026-50251?
- Check whether you run NLnet Labs Unbound at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.
Technical details
A vulnerability exists in NLnet Labs Unbound up to version 1.25.1 when 'unwanted-reply-threshold' is enabled. An attacker controlling a delegation can return in-bailiwick glue records of 0.0.0.0 or ::0. On systems that route these addresses (such as Linux routing them to loopback), Unbound's own listener may respond from a different IP (e.g., 127.0.0.1). This IP mismatch causes Unbound to flag the response as an 'unwanted reply,' incrementing a counter that, once reached, triggers a full cache flush of message and RRset caches. Because the counter resets after the flush, the attacker can sustain this loop indefinitely without spoofing packets. The fix in version 1.25.2 adds 0.0.0.0/::0 to the 'do-not-query-address' list by default.
Affected products
- NLnet Labs Unbound up to and including 1.25.1
Timeline
- 2026-07-22: advisory: Initial advisory published by NLnet Labs
- 2026-07-22: patched: Version 1.25.2 released with fix