# CVE-2026-56416: NLnet Labs Unbound heap buffer overflow in DNSSEC validator

Source: https://junglewise.ai/threats/cve-2026-56416-nlnet-labs-unbound-heap-buffer-overflow-in-dnssec-validator  
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-2026-56416 · Severity: medium · CVSS 4.8 · Published 2026-07-22

NLnet Labs Unbound is a widely used DNS resolver that translates human-readable domain names into IP addresses. A vulnerability exists where a malicious DNS server can send specially crafted, signed records that cause the resolver to process memory incorrectly. This can lead to a service crash or unpredictable behavior, potentially impacting the reliability of internet connectivity for users relying on the affected resolver.

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

## Quick answers

**Is CVE-2026-56416 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-56416 being exploited in the wild.

**How severe is CVE-2026-56416?** Medium severity, with a CVSS score of 4.8, 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-2026-56416?** 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-2026-56416 attacked?** It is attacked over the network, in a hosted service.

**Am I affected by CVE-2026-56416?** 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 heap buffer overflow exists in Unbound's DNSSEC validator when processing RRSIG-covered RRsets for PX, RP, MINFO, or SOA records. The root cause is a failure to verify the presence of a second embedded domain name in the RDATA before passing a calculated pointer to 'query_dname_tolower()'. An attacker operating a DNSSEC-signed authoritative server can provide a truncated record that causes the parser to read past the intended buffer into stale bytes or beyond the heap allocation. This is particularly exploitable if 'msg-buffer-size' is configured below the default value. The issue is resolved in Unbound version 1.25.2.

## Affected products

- NLnet Labs Unbound up to and including 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-2026-56416.txt](https://www.nlnetlabs.nl/downloads/unbound/CVE-2026-56416.txt)

## Related threats

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- [NLnet Labs Unbound DNS decompression CPU exhaustion](https://junglewise.ai/threats/cve-2026-96405-nlnet-labs-unbound-up-to-and-including-version-1-26-1-limits)
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- [NLnet Labs Unbound cache side-channel in upstream source port probe](https://junglewise.ai/threats/cve-2026-109125-in-nlnet-labs-unbound-up-to-and-including-version-1-26-1-an)
- [NLnet Labs Unbound DNS complexity attacks via ReTrap](https://junglewise.ai/threats/cve-2026-85501-nlnet-labs-unbound-dns-complexity-attacks-via-retrap)