russh is a Rust SSH client and server library that includes an SSH agent protocol implementation. Prior to 0.65.0, russh::keys::agent::server::serve in russh/src/keys/agent/server.rs accepts repeated SSH_AGENT_CONSTRAIN_CONFIRM records while adding a key and appends every Constraint::Confirm record to the key's retained constraint vector without a count or memory limit. A process that can connect to the agent socket, including the remote side of a forwarded agent session, can repeatedly add constrained keys and cause renewable memory growth that persists for the process lifetime because keys without a lifetime constraint do not expire. This issue is fixed in version 0.65.0.

Affected: crates.io: russh

### Summary

**All evidence, reproduction steps, and scripts in the report were verified against a real running instance. Nothing is fabricated or synthetic; every verdict is backed by captured output from an actual deployment.**

The SSH agent server that ships with russh (`russh::keys::agent::server::serve`) parses client key-addition requests and, for each `SSH_AGENT_CONSTRAIN_CONFIRM` (0x02) record it finds, pushes one `Constraint::Confirm` value into a per-key vector inside an unbounded loop, then stores the resulting pair permanently in its keystore (`russh/src/keys/agent/server.rs:296-303`). Neither the constraint vector nor the keystore has a size or count limit: the only boundary is the 256 KiB agent frame cap, and every 1-byte record a client sends becomes a 56-byte enum retained for the life of the process. Any process that can connect to the agent socket - the same trust model the project itself established when fixing a prior advisory about this same request handler - can therefore grow the agent's memory without bound and renew the growth at will: six 256 KiB requests retained 72.4 MiB (about 12.1 MiB per request, a roughly 48-fold amplification) while an equal number of unconstrained key additions retained nothing, measured against the unmodified server through its public API on v0.64.1. The earlier fix for that prior advisory deduplicates only the sibling `SSH_AGENT_CONSTRAIN_LIFETIME` (0x01) record type and leaves this parallel variant untouched, and since c…

russh: russh: Denial of Service via resource exhaustion in SSH agent russh: russh: Denial of Service via uncontrolled memory consumption in SSH agent russh: russh: Denial of Service via unbounded memory allocation in extension handling russh: russh: Denial of service via zero maximum packet size russh: russh: Unauthorized key signing via SSH agent confirmation bypass

Affected: Ubuntu:26.10: rust-russh

russh is a Rust SSH client and server library that includes an SSH agent protocol implementation. Prior to 0.65.0, russh::keys::agent::server::serve in russh/src/keys/agent/server.rs accepts repeated SSH_AGENT_CONSTRAIN_CONFIRM records while adding a key and appends every Constraint::Confirm record to the key's retained constraint vector without a count or memory limit. A process that can connect to the agent socket, including the remote side of a forwarded agent session, can repeatedly add constrained keys and cause renewable memory growth that persists for the process lifetime because keys without a lifetime constraint do not expire. This issue is fixed in version 0.65.0.