The vulnerability in the russh library allows an attacker who can communicate with the SSH agent socket to send repeated SSH_AGENT_CONSTRAIN_CONFIRM messages, each of which is appended unchecked to a key's constraint vector. Because there is no counter or memory limit, the agent will continue to allocate memory for every new constraint; keys without a lifetime constraint persist indefinitely, so the memory growth remains for the process lifetime, which can result in resource exhaustion and a denial of service. Based on the description, the likely attack vector is a program that can open the agent socket, which could be a local process or a remote host if agent forwarding is enabled.

The vendor Eugeny provides the russh SSH client and server library. Versions of russh prior to 0.65.0 are affected because they implement an unbounded constraint vector in the agent server component. The fix is released in version 0.65.0, so any deployment using earlier releases is at risk.

The CVSS score of 3.3 indicates low severity, and the vulnerability is not listed in the CISA KEV catalog, meaning widespread exploitation is not yet documented. However, because the attack requires only access to the SSH agent socket, which may be locally available or exposed through forwarded agent sessions, an attacker could potentially trigger memory exhaustion and a denial of service in the affected process. The exploit is straightforward for a local process and could be carried out remotely if SSH agent forwarding is enabled, but no additional network privileges or configuration are required beyond socket access. The EPSS score is not available, so the likelihood of exploitation in the wild is uncertain, though the low CVSS suggests priority for patching rather than immediate mitigation.