Go x/crypto/ssh EUVD-2026-70266

The attack is exploitable against any Go SSH server or client using golang.org/x/crypto/ssh prior to version 0.56.0 that accepts connections from untrusted peers. The specific condition is the window between a channel being registered in the mux's chanList (on receipt of an SSH_MSG_CHANNEL_OPEN) and the channel receiving its open confirmation - a normal, brief phase in every SSH channel negotiation. The CVSS PR:N suggests the vendor considers this exploitable before SSH authentication completes, though this is not independently confirmed by the description; if authentication is required before channel messages are processed, then only authenticated peers (PR:L) could trigger the bug. What limits exploitation: the deadlock affects only the targeted connection's goroutine, not other concurrent connections or the server process as a whole, so a single exploit attempt causes a single-connection DoS; mass DoS would require flooding many concurrent connections.

The CVSS 3.1 base score of 7.5 with vector AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N contains a significant internal inconsistency: the described impact is a connection deadlock - a pure availability failure - yet the vector assigns C:H and A:N. This appears to be a scoring error; the correct metrics should be C:N/I:N/A:H. Real-world severity under a corrected vector remains 7.5 (the score is numerically coincidental) but the risk character is entirely different: this is an unauthenticated denial-of-service, not an information disclosure. The attack is automatable (AV:N/AC:L/PR:N/UI:N), and repeated connection attempts could exhaust a server's connection-handling goroutines, meaning even a mid-sized municipal IT team in leszno running Go-based SSH infrastructure should treat this as a high-priority patch. No EPSS data was provided, and the vulnerability is not in CISA KEV. No public exploit code has been identified. SSVC factors: exploitation status is none identified, automatable is yes, technical impact is partial (per-connection DoS, not system-wide code execution), suggesting a patch-on-schedule posture for most deployments but urgency for exposed public SSH endpoints.

An attacker establishes a TCP connection to a Go SSH server built on golang.org/x/crypto/ssh and, during the channel negotiation phase before the channel open confirmation is received, sends a high volume of channel request packets targeting the newly registered but not yet established channel. With no throttle on the incomingRequests queue for unestablished channels, the connection-handling goroutine deadlocks, leaving the session permanently unresponsive. Repeating this against multiple connections can deny service to legitimate users without triggering any teardown or error recovery.