Pre-authentication denial of service via queued PONG buffers
Published Feb 28, 2025 · Updated Jun 30, 2026
Denial of service in OpenBSD Project OpenSSH 9.5p1 through 9.9p1 allows remote attackers to interrupt sshd via pre-authentication pings. During key exchange, ssh_packet_read_poll_seqnr() queues a newly allocated PONG buffer for every SSH2_MSG_PING without bounding the outgoing list; the buffers remain until exchange completion. No authentication is required, and parallel connections can exhaust memory or consume CPU until sshd stops serving connections.
Summary
What happened
Denial of service in OpenBSD Project OpenSSH 9.5p1 through 9.9p1 allows remote attackers to interrupt sshd via pre-authentication pings. During key exchange, ssh_packet_read_poll_seqnr() queues a newly allocated PONG buffer for every SSH2_MSG_PING without bounding the outgoing list; the buffers remain until exchange completion. No authentication is required, and parallel connections can exhaust memory or consume CPU until sshd stops serving connections.
The record
- CVE
- CVE-2025-26466
- Published
- Feb 28, 2025
- Updated
- Jun 30, 2026
- Vendor
- OpenBSD Project
- Product
- OpenSSH
- Classifications
- CWE-770, T1499.004
- Attack vector
- network
- Privileges
- unauthenticated
Timeline
How it unfolded
- Feb 28, 2025CVE publishedPublication date reported by the CVE source.
- Jun 30, 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=9.5p1 <=9.9p1
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
Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.
Weakness, pattern, technique
Public exploit references
- Qualys OpenSSH memory and CPU denial-of-service proof of conceptproof of concept · demonstrated
Labels summarize the accepted research assessment. They do not indicate a test against your environment.
Technologies
Your stack
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Your stack
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