# CVE-2026-55717: NLnet Labs Unbound DoS via NULL pointer dereference in serve-expired-client-timeout

Source: https://junglewise.ai/threats/cve-2026-55717-nlnet-labs-unbound-dos-via-null-pointer-dereference-in-serve  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-07-22. Updated: 2026-07-22  
License: CC BY 4.0 with a link back

CVE-2026-55717 · Severity: medium · CVSS 5.9 · Published 2026-07-22

Unbound is a widely used DNS resolver that translates human-readable domain names into IP addresses. A vulnerability exists where a remote attacker can crash the DNS service by manipulating responses from a domain they control. This results in a denial-of-service, preventing users and systems from resolving domain names and potentially disrupting internet connectivity for the affected network.

Technologies: [NLnet Labs Unbound](https://junglewise.ai/threats/technologies/unbound). Vendors: [NLnet Labs](https://junglewise.ai/threats/vendors/nlnet-labs).

## Quick answers

**Is CVE-2026-55717 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-55717 being exploited in the wild.

**How severe is CVE-2026-55717?** Medium severity, with a CVSS score of 5.9, published 22 July 2026.

**Which products and versions are affected?** NLnet Labs Unbound 1.10.0 to 1.25.1.

**Is there a fix for CVE-2026-55717?** A patch was published on 2026-07-22: Fixed in version 1.25.2 Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-55717 attacked?** It is attacked over the network, in a hosted service.

**Am I affected by CVE-2026-55717?** 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 NULL pointer dereference exists in the 'serve-expired-client-timeout' callback of NLnet Labs Unbound. When 'serve-expired' is enabled alongside 'response-ip' redirect or RPZ CNAME override rules, the callback's two-pass loop for chasing CNAME aliases fails to reset the 'partial_rep' pointer during the second pass, despite resetting 'alias_rrset'. An attacker controlling a delegated domain can trigger this inconsistency by returning an A/AAAA record within a configured response-ip subnet and delaying the response to force the 'serve-expired' code path. This leads to a daemon crash. The issue is fixed in Unbound version 1.25.2.

## Affected products

- NLnet Labs Unbound 1.10.0 to 1.25.1

## Timeline

- 2026-07-22: disclosed
- 2026-07-22: advisory
- 2026-07-22: patched: Fixed in version 1.25.2

## References

- [https://www.nlnetlabs.nl/downloads/unbound/CVE-2026-55717.txt](https://www.nlnetlabs.nl/downloads/unbound/CVE-2026-55717.txt)

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