# CVE-2026-56416: Possible heap buffer overflow when validator canonicalizes RDATA that contains domain name

CVE-2026-56416 is fixed in 12 products on 1 platform, first by DEBIAN-CVE-2026-56416, released July 28, 2026. It is not reported as exploited. Patch Radar priority: Routine.

- Web page: https://patchblox.com/security/cve/CVE-2026-56416
- Priority: Routine
- CVSS: 4.8 (Moderate)
- EPSS: 0.0012
- CISA KEV: not listed
- Exploited (vendor report): no
- Publicly disclosed: no
- Published: Jul 22, 2026
- Weakness: Improper Validation of Integrity Check Value
- Vendor advisory: https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-56416

## Description

In NLnet Labs Unbound up to and including version 1.25.1, when the validator builds the canonical RDATA form for an RRSIG-covered PX/RP/MINFO/SOA RRset, it computes the address of the second embedded domain name as 'datstart + dname_valid(datstart, ...)' and passes it straight to 'query_dname_tolower()' without checking that a second name is actually present in the RDATA. The wire-format parser accepts multi-dname RRs whose RDATA ends after the first name, so an attacker who runs a DNSSEC-signed authoritative server can deliver a record with an absent second domain name (e.g. SOA record) and cause 'query_dname_tolower()' to walk label-by-label through stale bytes in the per-worker 'env->scratch_buffer', past the end of that heap allocation if 'msg-buffer-size' has been lowered from the default. This leads to heap buffer overflow and on a release build the outcome relies heavily on the contents of the buffer tail and the adjacent heap chunk.

## Where it's fixed

| Platform | Product | Fixed in | Fix | Released |
|---|---|---|---|---|
| Linux | python3-unbound amzn 2023 | 1.17.1-1.amzn2023.0.13 | [ALAS2023-2026-2033](https://patchblox.com/security/data/fix/linux/alas2023-2026-2033.md) | Aug 3, 2026 |
| Linux | python3-unbound-debuginfo amzn 2023 | 1.17.1-1.amzn2023.0.13 | [ALAS2023-2026-2033](https://patchblox.com/security/data/fix/linux/alas2023-2026-2033.md) | Aug 3, 2026 |
| Linux | unbound Debian 13.5 (trixie) | 1.26.1-0+deb13u1 | [DEBIAN-CVE-2026-56416](https://patchblox.com/security/data/fix/linux/debian-cve-2026-56416.md) | Jul 28, 2026 |
| Linux | unbound Debian 14 | 1.25.2-1 | [DEBIAN-CVE-2026-56416](https://patchblox.com/security/data/fix/linux/debian-cve-2026-56416.md) | Jul 28, 2026 |
| Linux | unbound amzn 2023 | 1.17.1-1.amzn2023.0.13 | [ALAS2023-2026-2033](https://patchblox.com/security/data/fix/linux/alas2023-2026-2033.md) | Aug 3, 2026 |
| Linux | unbound opensuse-leap 16.0 | 1.25.2-160000.1.1 | [OPENSUSE-SU-2026:21550-1](https://patchblox.com/security/data/fix/linux/opensuse-su-2026-21550-1.md) | Aug 10, 2026 |
| Linux | unbound sles 15.6 | 1.25.2-150600.23.19.1 | [SUSE-SU-2026:3885-1](https://patchblox.com/security/data/fix/linux/suse-su-2026-3885-1.md) | Aug 31, 2026 |
| Linux | unbound sles 15.7 | 1.25.2-150600.23.19.1 | [SUSE-SU-2026:3885-1](https://patchblox.com/security/data/fix/linux/suse-su-2026-3885-1.md) | Aug 31, 2026 |
| Linux | unbound sles 16.0 | 1.25.2-160000.1.1 | [SUSE-SU-2026:23226-1](https://patchblox.com/security/data/fix/linux/suse-su-2026-23226-1.md) | Aug 10, 2026 |
| Linux | unbound-anchor amzn 2023 | 1.17.1-1.amzn2023.0.13 | [ALAS2023-2026-2033](https://patchblox.com/security/data/fix/linux/alas2023-2026-2033.md) | Aug 3, 2026 |
| Linux | unbound-anchor-debuginfo amzn 2023 | 1.17.1-1.amzn2023.0.13 | [ALAS2023-2026-2033](https://patchblox.com/security/data/fix/linux/alas2023-2026-2033.md) | Aug 3, 2026 |
| Linux | unbound-debuginfo amzn 2023 | 1.17.1-1.amzn2023.0.13 | [ALAS2023-2026-2033](https://patchblox.com/security/data/fix/linux/alas2023-2026-2033.md) | Aug 3, 2026 |
| Linux | unbound-debugsource amzn 2023 | 1.17.1-1.amzn2023.0.13 | [ALAS2023-2026-2033](https://patchblox.com/security/data/fix/linux/alas2023-2026-2033.md) | Aug 3, 2026 |
| Linux | unbound-devel amzn 2023 | 1.17.1-1.amzn2023.0.13 | [ALAS2023-2026-2033](https://patchblox.com/security/data/fix/linux/alas2023-2026-2033.md) | Aug 3, 2026 |
| Linux | unbound-libs amzn 2023 | 1.17.1-1.amzn2023.0.13 | [ALAS2023-2026-2033](https://patchblox.com/security/data/fix/linux/alas2023-2026-2033.md) | Aug 3, 2026 |
| Linux | unbound-libs-debuginfo amzn 2023 | 1.17.1-1.amzn2023.0.13 | [ALAS2023-2026-2033](https://patchblox.com/security/data/fix/linux/alas2023-2026-2033.md) | Aug 3, 2026 |
| Linux | unbound-utils amzn 2023 | 1.17.1-1.amzn2023.0.13 | [ALAS2023-2026-2033](https://patchblox.com/security/data/fix/linux/alas2023-2026-2033.md) | Aug 3, 2026 |
| Linux | unbound-utils-debuginfo amzn 2023 | 1.17.1-1.amzn2023.0.13 | [ALAS2023-2026-2033](https://patchblox.com/security/data/fix/linux/alas2023-2026-2033.md) | Aug 3, 2026 |

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Data as of 2026-11-09T13:31:49Z UTC. Source: Patchblox Patch Radar (https://patchblox.com/security/patch-radar). Patchblox finds out-of-date software on Windows, macOS and Linux and updates it through release rings: https://patchblox.com/demo

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