An SSH agent implementation in the russh library can spawn an unbounded number of long‑lived tasks when processing repeated SSH_AGENT_CONSTRAIN_LIFETIME records in a key‑addition request. Because the code clones the key blob into each task and never terminates tasks on later parse failures, an attacker who can speak the agent protocol locally or via forwarded connections can repeatedly send constrained add‑key requests to exhaust system memory and the task scheduler, eventually causing the agent process to be starved or terminated. The weakness is the uncontrolled resource consumption identified as CWE‑770.

The vulnerability exists in the russh library by Eugeny, affecting all versions prior to 0.64.1 that expose the SSH agent protocol. Systems that run russh with agent forwarding enabled or allow local access to the agent channel are susceptible. The fix is delivered in release 0.64.1.

The CVSS score of 4.3 reflects a moderate severity; the lack of an EPSS score makes the current exploitation probability unclear, but the attack vector is controllable locally or over agent forwarding, implying that compromise of a local user or of a host that forwards agent connections is required. The vulnerability is not listed in the CISA KEV catalog, and no official workaround is provided. An attacker who can speak the agent protocol locally or via forwarded connections can trigger the denial of service, but the exploitation requires repeated authenticated or unauthenticated interaction with the SSH agent. Due to the moderate CVSS score and the need for a local or privileged remote posture, the overall risk is moderate to high for environments that rely heavily on SSH agent forwarding.