Connection denial of service via blocking channel delivery
Published Sep 2, 2026 · Updated Sep 3, 2026
Denial of service in golang.org/x/crypto/ssh before 0.56.0 allows remote attackers to deadlock an established SSH connection. The default arm of channel.handlePacket sends unexpected, well-formed messages with a valid channel ID into ch.msg, but no consumer reads that channel after establishment, so the blocking send stalls the multiplexer's read loop. Exploitation requires a malicious peer and an established channel; the bounded consequence is a hang of that connection and its associated SSH activity.
Summary
What happened
Denial of service in golang.org/x/crypto/ssh before 0.56.0 allows remote attackers to deadlock an established SSH connection. The default arm of channel.handlePacket sends unexpected, well-formed messages with a valid channel ID into ch.msg, but no consumer reads that channel after establishment, so the blocking send stalls the multiplexer's read loop. Exploitation requires a malicious peer and an established channel; the bounded consequence is a hang of that connection and its associated SSH activity.
The record
- CVE
- CVE-2026-56855
- Published
- Sep 2, 2026
- Updated
- Sep 3, 2026
- Vendor
- golang.org/x/crypto
- Product
- golang.org/x/crypto/ssh
- Classifications
- CWE-770, T1499.004
- Attack vector
- network
- Privileges
- Unavailable
Timeline
How it unfolded
- Sep 2, 2026CVE publishedPublication date reported by the CVE source.
- Sep 3, 2026Record updatedLatest update available in the CVE record.
Exploitability
Present is not the same as exploitable
Compare your product and version with the public record. A matching version still requires validation against your environment.
Is a vulnerable build present?
Compare these published version ranges with your installed build and any vendor patches.
- Affected versionversion=0 <0.56.0
What conditions does exploitation require?
What is affected?
Published CVSS scores
No CVSS assessment is available in this record.
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
No public exploit references are available in this record.
Labels summarize the accepted research assessment. They do not indicate a test against your environment.
Technologies
Your stack
See the directory against your own environment.
Your stack
Check the software in your environment
Book a demo to see how Hinoki identifies affected software and validates exploitability in your environment.
Book a demo