CVE-2026-56416
Possible heap buffer overflow when validator canonicalizes RDATA that contains domain name
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.
INFO
Published Date :
July 22, 2026, 2:17 p.m.
Last Modified :
Oct. 24, 2026, 1:10 p.m.
Remotely Exploit :
No
Source :
[email protected]
CVSS Scores
| Score | Version | Severity | Vector | Exploitability Score | Impact Score | Source |
|---|---|---|---|---|---|---|
| CVSS | 134c704f-9b21-4f2e-91b3-4a467353bcc0 | |||||
| CVSS 3.1 | MEDIUM | 206fc3a0-e175-490b-9eaa-a5738056c9f6 | ||||
| CVSS 3.1 | MEDIUM | [email protected] | ||||
| CVSS 3.1 | MEDIUM | MITRE-CVE |
Solution
- Update Unbound to a version later than 1.25.1.
- Ensure DNSSEC validation is enabled.
- Monitor DNS traffic for anomalies.
References to Advisories, Solutions, and Tools
Here, you will find a curated list of external links that provide in-depth
information, practical solutions, and valuable tools related to
CVE-2026-56416.
| URL | Resource |
|---|---|
| https://www.nlnetlabs.nl/downloads/unbound/CVE-2026-56416.txt | Vendor Advisory |
CWE - Common Weakness Enumeration
While CVE identifies
specific instances of vulnerabilities, CWE categorizes the common flaws or
weaknesses that can lead to vulnerabilities. CVE-2026-56416 is
associated with the following CWEs:
Common Attack Pattern Enumeration and Classification (CAPEC)
Common Attack Pattern Enumeration and Classification
(CAPEC)
stores attack patterns, which are descriptions of the common attributes and
approaches employed by adversaries to exploit the CVE-2026-56416
weaknesses.
We scan GitHub repositories to detect new proof-of-concept exploits. Following list is a collection of public exploits and proof-of-concepts, which have been published on GitHub (sorted by the most recently updated).
Results are limited to the first 15 repositories due to potential performance issues.
The following list is the news that have been mention
CVE-2026-56416 vulnerability anywhere in the article.
The following table lists the changes that have been made to the
CVE-2026-56416 vulnerability over time.
Vulnerability history details can be useful for understanding the evolution of a vulnerability, and for identifying the most recent changes that may impact the vulnerability's severity, exploitability, or other characteristics.
-
CVE Translated by [email protected]
Oct. 24, 2026
Action Type Old Value New Value Added Translation Title: Unbound de NLnet Labs, Description: En NLnet Labs Unbound hasta la versión 1.25.1 inclusive, cuando el validador construye la forma canónica de RDATA para un conjunto de RR PX/RP/MINFO/SOA cubierto por RRSIG, calcula la dirección del segundo nombre de dominio incrustado como ’datstart + dname_valid(datstart, ...)’ y lo pasa directamente a ’query_dname_tolower()’ sin verificar que un segundo nombre esté realmente presente en el RDATA. El analizador de formato de cable acepta RRs multi-dname cuyo RDATA termina después del primer nombre, por lo que un atacante que ejecuta un servidor autoritativo firmado con DNSSEC puede entregar un registro con un segundo nombre de dominio ausente (p. ej., registro SOA) y hacer que ’query_dname_tolower()’ recorra etiqueta por etiqueta bytes obsoletos en el ’env -> scratch_buffer’ por trabajador, más allá del final de esa asignación de pila si ’msg-buffer-size’ se ha reducido del valor predeterminado. Esto conduce a un desbordamiento de búfer de pila y en una compilación de lanzamiento el resultado depende en gran medida del contenido de la cola del búfer y del fragmento de pila adyacente. -
Initial Analysis by [email protected]
Jul. 24, 2026
Action Type Old Value New Value Added CPE Configuration OR *cpe:2.3:a:nlnetlabs:unbound:*:*:*:*:*:*:*:* versions up to (excluding) 1.25.2 Added Reference Type NLnet Labs: https://www.nlnetlabs.nl/downloads/unbound/CVE-2026-56416.txt Types: Vendor Advisory -
CVE Modified by 134c704f-9b21-4f2e-91b3-4a467353bcc0
Jul. 22, 2026
Action Type Old Value New Value Added SSVC {'id': 'CVE-2026-56416', 'role': 'CISA Coordinator', 'options': [{'exploitation': 'none'}, {'automatable': 'no'}, {'technicalImpact': 'partial'}], 'version': '2.0.3', 'timestamp': '2026-07-22T13:59:48.705600Z'} -
New CVE Received by [email protected]
Jul. 22, 2026
Action Type Old Value New Value Added Affected [{'vendor': 'NLnet Labs', 'product': 'Unbound', 'versions': [{'status': 'affected', 'version': '0', 'lessThan': '1.25.2', 'versionType': 'semver'}], 'defaultStatus': 'unaffected'}] Added 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. Added CVSS V3.1 AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:L Added CWE CWE-354 Added Reference https://www.nlnetlabs.nl/downloads/unbound/CVE-2026-56416.txt