DNS Cookie bypass via proxy-address hash binding
Published Jul 22, 2026 · Updated Jul 22, 2026
DNS Cookie bypass in NLnet Labs Unbound 1.18.0 through 1.25.1 allows remote attackers to bypass anti-spoofing checks via UDP PROXYv2. The server-cookie SipHash binds to the proxy's wire address rather than the PROXYv2-declared client, so one cookie validates for multiple declared sources behind that proxy. Exploitation requires proxy-protocol-port with answer-cookie enabled; an off-path attacker can obtain one cookie legitimately, replay it with a spoofed source, and enable reflected DNS traffic.
Summary
What happened
DNS Cookie bypass in NLnet Labs Unbound 1.18.0 through 1.25.1 allows remote attackers to bypass anti-spoofing checks via UDP PROXYv2. The server-cookie SipHash binds to the proxy's wire address rather than the PROXYv2-declared client, so one cookie validates for multiple declared sources behind that proxy. Exploitation requires proxy-protocol-port with answer-cookie enabled; an off-path attacker can obtain one cookie legitimately, replay it with a spoofed source, and enable reflected DNS traffic.
The record
- CVE
- CVE-2026-54478
- Published
- Jul 22, 2026
- Updated
- Jul 22, 2026
- Vendor
- NLnet Labs
- Product
- Unbound
- Classifications
- CWE-303, T1498.002
- Attack vector
- network
- Privileges
- unauthenticated
Timeline
How it unfolded
- Jul 22, 2026CVE publishedPublication date reported by the CVE source.
- Jul 22, 2026Record updatedLatest update available in the CVE record.
Exploitability
Present is not the same as exploitable
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Is a vulnerable build present?
Compare these published version ranges with your installed build and any vendor patches.
- Affected versionversion=1.18.0 <1.25.2
What conditions does exploitation require?
What is affected?
Published CVSS scores
CVSS describes severity. EPSS estimates exploitation probability.
Attacks
What attackers are doing with it
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Weakness, pattern, technique
Public exploit references
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Technologies
Your stack
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Your stack
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