Is CVE-2026-55973 exploited in the wild?

No. Junglewise Threat Intelligence has no report of CVE-2026-55973 being exploited in the wild.

How severe is CVE-2026-55973?

High severity, with a CVSS score of 7.5, published 22 July 2026.

Which products and versions are affected?

NLnet Labs Unbound 1.23.0 to 1.25.1.

Is there a fix for CVE-2026-55973?

A patch was published on 2026-07-22. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

How is CVE-2026-55973 attacked?

It is attacked over the network, in a software library.

Am I affected by CVE-2026-55973?

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.

CVE-2026-55973: NLnet Labs Unbound stack overflow in DNS error reporting

CVE-2026-55973 · Severity: high · CVSS 7.5 · Published 2026-07-22

NLnet Labs Unbound is a widely used tool for translating human-readable website names into IP addresses. A flaw in how it handles error reporting allows a remote attacker to crash the service by sending a specially crafted response from a domain they control. This results in a denial-of-service, preventing users from accessing websites or other internet services that rely on the affected server.

Technologies: NLnet Labs Unbound. Vendors: NLnet Labs.

Quick answers

Is CVE-2026-55973 exploited in the wild?: No. Junglewise Threat Intelligence has no report of CVE-2026-55973 being exploited in the wild.
How severe is CVE-2026-55973?: High severity, with a CVSS score of 7.5, published 22 July 2026.
Which products and versions are affected?: NLnet Labs Unbound 1.23.0 to 1.25.1.
Is there a fix for CVE-2026-55973?: A patch was published on 2026-07-22. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.
How is CVE-2026-55973 attacked?: It is attacked over the network, in a software library.
Am I affected by CVE-2026-55973?: 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 Unbound versions 1.23.0 through 1.25.1 when 'dns-error-reporting: yes' is configured. The issue stems from improper validation of the EDNS Report-Channel option (code 18) length in upstream responses. When processing a synthetic '_er.' report query name, the 'find_closest_of_type()' function fails to stop at the embedded root and instead uses the full option length to strip labels. This causes the iterator to walk past the intended memory boundary, feeding garbage bytes to 'dname_query_hash()' and overwriting the 'labuf' stack variable. An attacker controlling a delegated zone can trigger this crash with a single response, leading to a daemon termination (DoS). The issue is fixed in version 1.25.2.

Affected products

NLnet Labs Unbound 1.23.0 to 1.25.1

Timeline

2026-07-22: advisory
2026-07-22: disclosed
2026-07-22: patched

References

https://www.nlnetlabs.nl/downloads/unbound/CVE-2026-55973.txt

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