Go x/crypto EUVD-2026-31402

Exploitation requires that the target application imports and actively uses the golang.org/x/crypto/ssh/agent package and that it processes SSH agent protocol messages - specifically key addition operations such as AddIdentity - where the key type is ed25519 and the caller can supply arbitrary wire-format byte content. CVSS PR:N/UI:N confirms no authentication or user interaction is needed from the network layer, but practical exposure is gated on whether the SSH agent socket or listener accepts connections from untrusted sources. Exploitation is limited by the following: applications that do not expose ssh/agent functionality to external callers are not affected; agents restricted to localhost with tight UNIX socket permissions require local access or prior code execution to reach; and only the agent process crashes (SSVC Technical Impact: partial), not the underlying operating system. Running Go version and whether the module has been updated to 0.52.0 are the decisive limiting factors for patched environments.

The CVSS 5.3 Medium score reflects an entirely network-reachable (AV:N), low-complexity (AC:L), unauthenticated (PR:N/UI:N) attack path, but the impact is limited to low availability (A:L) with no confidentiality or integrity impact, consistent with a targeted process crash rather than remote code execution or data leakage. EPSS at 0.02% in the 5th percentile signals near-negligible exploitation probability by the broader threat community, corroborating the SSVC assessment of Exploitation: none. However, SSVC flags the attack as Automatable: yes, meaning an adversary could craft a reliable, scripted attack without manual interaction - this elevates operational urgency for internet-exposed SSH agent deployments. SSVC Technical Impact is Partial, meaning only the agent process is affected, not the underlying host. The 'Information Disclosure' tag in the source intelligence appears inconsistent with the CVSS impact vector (C:N/I:N/A:L), which shows no confidentiality impact; this discrepancy should be confirmed with the vendor. Overall, this is a moderate-priority issue: real-world risk is meaningful only for services that expose the ssh/agent protocol to untrusted callers, but the low EPSS and absence of KEV or POC suggest no urgency beyond standard patch cycle.

An attacker with network or local access to an SSH agent socket (for example, one exposed via agent forwarding or a misconfigured TCP listener) sends an AddIdentity or equivalent agent protocol message whose ed25519 private key field contains malformed wire bytes with an incorrect length or structure. The ssh/agent package casts these bytes directly into an ed25519.PrivateKey object, and when the application subsequently attempts to use that key (e.g., for a signing operation), the Go runtime panics and terminates the agent process. No public proof-of-concept code has been identified at time of analysis, but the automatable nature of the attack (SSVC: Automatable yes) means a reliable exploit is technically straightforward to construct.