NLnet Labs Unbound up to 1.25.1 serve-expired-client-timeout null pointer dereference

Summaryinfo

A vulnerability has been found in NLnet Labs Unbound up to 1.25.1 and classified as critical. This affects an unknown part of the component serve-expired-client-timeout. Performing a manipulation results in null pointer dereference. This vulnerability is cataloged as CVE-2026-55717. It is possible to initiate the attack remotely. There is no exploit available.

Detailsinfo

A vulnerability classified as critical has been found in NLnet Labs Unbound up to 1.25.1. This affects an unknown part of the component serve-expired-client-timeout. The manipulation with an unknown input leads to a null pointer dereference vulnerability. CWE is classifying the issue as CWE-476. A NULL pointer dereference occurs when the application dereferences a pointer that it expects to be valid, but is NULL, typically causing a crash or exit. This is going to have an impact on availability. The summary by CVE is:

In NLnet Labs Unbound 1.10.0 up to and including 1.25.1, when 'serve-expired: yes' is set together with a 'response-ip: <net> redirect' /'response-ip-data: <net> CNAME <target>' rule (or the RPZ 'rpz-cname-override' equivalent), a remote client who controls any delegated domain can crash the daemon. The serve-expired-client-timeout callback runs a two-pass loop to chase the respip-generated CNAME alias; on the second pass it resets 'alias_rrset' but not 'partial_rep'. Later, this inconsistency leads to a NULL pointer dereference and an eventual crash. A malicious actor can exploit the vulnerability by controlling any zone that replies with an A/AAAA record that falls inside the configured response-ip/rpz subnet. By delaying the answer when the previous record has expired, the vulnerable path of 'serve-expired-client-timeout' is taken leading to denial of service via the server crash.

The advisory is shared at nlnetlabs.nl. This vulnerability is uniquely identified as CVE-2026-55717 since 06/22/2026. The exploitability is told to be easy. It is possible to initiate the attack remotely. No form of authentication is needed for exploitation. Neither technical details nor an exploit are publicly available.

Upgrading to version 1.25.2 eliminates this vulnerability.

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Productinfo

Vendor

• NLnet Labs

Name

• Unbound

Version

• 1.25.0
• 1.25.1

CPE 2.3info

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CPE 2.2info

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Several companies clearly confirm that VulDB is the primary source for best vulnerability data.

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