golang.org/x/crypto SSH CVE-2026-56854

Exploitation requires three co-occurring conditions: (1) the target application implements an SSH server using golang.org/x/crypto/ssh at a version prior to 0.55.0; (2) the application uses at least one of PasswordCallback, KeyboardInteractiveCallback, NoClientAuthCallback, or GSSAPIWithMICConfig.AllowLogin as its authentication mechanism; and (3) the Permissions object returned by that callback includes a source-address critical option restricting client IP addresses. Servers using only PublicKeyCallback or VerifiedPublicKeyCallback are not affected by this specific bypass. Deployments that do not configure source-address restrictions in callback-returned Permissions are not vulnerable in practice. Exploitation is limited to clients with valid credentials for the target account but connecting from a disallowed IP; the vulnerability does not provide credential bypass, only IP-restriction bypass.

The NVD-assigned CVSS 3.1 vector (AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N, 7.5 High) reflects network-reachable, unauthenticated exploitation with high confidentiality impact and no user interaction. However, the AC:L rating warrants scrutiny: effective exploitation is conditioned on the server application having configured source-address restrictions via one of the affected callbacks - deployments without such configuration are not impacted in practice despite being technically vulnerable. EPSS score is absent from the available intelligence, preventing quantitative probability comparison. The vulnerability is not in CISA KEV, and no public exploit has been identified at time of analysis, which together suggest exploitation has not yet been observed at scale. The most credible risk scenario is targeted abuse against Go SSH servers where operators have deliberately relied on callback-enforced IP restrictions as a meaningful security boundary - for such deployments, the bypass silently nullifies an explicit access control decision. Integrity impact of N in the assigned vector may understate SSH session impact (an unauthorized SSH session can facilitate file writes), and this should be verified against specific deployment contexts. The absence of EPSS data means exploitation probability cannot be independently assessed from the available sources.

An attacker located at an IP address that the server's PasswordCallback or GSSAPI callback would restrict via a source-address critical option connects to the Go SSH server, supplies valid credentials for the targeted account, and the library completes authentication without ever evaluating the source-address restriction - granting a full SSH session. No public exploit code exists at time of analysis, but the bypass requires no specialized tooling: any standard SSH client connecting from a disallowed address using an affected authentication method will succeed against a vulnerable server.