NLnet Labs Unbound up to 1.25.1 DNS-over-QUIC resource consumption

Summaryinfo

A vulnerability has been found in NLnet Labs Unbound up to 1.25.1 and classified as critical. Affected is an unknown function of the component DNS-over-QUIC. This manipulation causes resource consumption. This vulnerability is registered as CVE-2026-41637. Remote exploitation of the attack is possible. No exploit is available.

Detailsinfo

A vulnerability has been found in NLnet Labs Unbound up to 1.25.1 and classified as critical. Affected by this vulnerability is an unknown part of the component DNS-over-QUIC. The manipulation with an unknown input leads to a resource consumption vulnerability. The CWE definition for the vulnerability is CWE-400. The product does not properly control the allocation and maintenance of a limited resource, thereby enabling an actor to influence the amount of resources consumed, eventually leading to the exhaustion of available resources. As an impact it is known to affect availability. The summary by CVE is:

In NLnet Labs Unbound 1.22.0 up to and including 1.25.1, client terminated DNS-over-QUIC (DoQ) queries are not accounted properly by Unbound resulting in low-cost inflation of the waiting number of replies for already in-flight resolution queries. This results in degradation of resolution service for new clients for already in-flight queries. A malicious actor can exploit the vulnerability by issuing DoQ queries for query names that need resolution and proceeding on immediately terminating the query by one of STOP_SENDING/RESET_STREAM/CONNECTION_CLOSE QUIC frames. Those terminated DoQ queries are not properly counted for and keep inflating the number of waiting replies for in-flight queries. When the maximum is reached, it results in silent query drops for new clients needing resolution for already in-flight queries. This vulnerability needs Unbound to be compiled with DoQ support ('--with-libngtcp2') and the 'quic-port' to be configured for the listening interfaces. Additionally, a malicious actor needs access to multiple source IPs to bypass the by-default configured 'wait-limit' option.

It is possible to read the advisory at nlnetlabs.nl. This vulnerability is known as CVE-2026-41637 since 06/22/2026. The exploitation appears to be easy. The attack can be launched remotely. The exploitation doesn't need any form of authentication. The technical details are unknown and an exploit is not publicly available. The attack technique deployed by this issue is T1499 according to MITRE ATT&CK.

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

• 🔒
• 🔒

CPE 2.2info

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• 🔒

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