NLnet Labs Unbound up to 1.25.1 Server Cookie siphash authentication replay

Summaryinfo

A vulnerability, which was classified as critical, was found in NLnet Labs Unbound up to 1.25.1. Affected by this issue is the function siphash of the component Server Cookie. Such manipulation leads to authentication replay. This vulnerability is listed as CVE-2026-54478. The attack may be performed from remote. There is no available exploit.

Detailsinfo

A vulnerability was found in NLnet Labs Unbound up to 1.25.1. It has been rated as critical. Affected by this issue is the function siphash of the component Server Cookie. The manipulation with an unknown input leads to a authentication replay vulnerability. Using CWE to declare the problem leads to CWE-294. A capture-replay flaw exists when the design of the product makes it possible for a malicious user to sniff network traffic and bypass authentication by replaying it to the server in question to the same effect as the original message (or with minor changes). Impacted is confidentiality, integrity, and availability. CVE summarizes:

In NLnet Labs Unbound 1.18.0 up to and including 1.25.1, when Unbound listens on a 'proxy-protocol-port' interface with 'answer-cookie: yes', the RFC 9018 server-cookie SipHash is computed over the proxy's wire address instead of the PROXYv2-declared client. One server cookie obtained through a given proxy node therefore validates for every PROXYv2-declared source behind that node. On a UDP+proxy-protocol front, an off-path attacker can harvest one cookie with a single legitimate query, then replay it under any spoofed source and pass DNS Cookie checks that were deployed to defeat this in the first place.

The advisory is shared for download at nlnetlabs.nl. This vulnerability is handled as CVE-2026-54478 since 06/22/2026. The exploitation is known to be easy. The attack may be launched remotely. No form of authentication is required for exploitation. There are known technical details, but no exploit is available. The MITRE ATT&CK project declares the attack technique as T1040.

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

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CPE 2.2info

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