russh is a Rust SSH client and server library that includes an SSH agent protocol implementation. From 0.50.0-beta.12 until 0.65.0, the agent server's Connection::try_sign path in russh/src/keys/agent/server.rs recognizes SSH_AGENT_CONSTRAIN_CONFIRM but returns a hard-coded successful result instead of calling Agent::confirm. In applications that override Agent::confirm to require user approval or another policy check, a process that can reach the agent socket, including the remote side of a forwarded agent, can use a key added through ssh-add -c without that check and can authenticate or sign data as the user while the key remains loaded. The separate Agent::confirm_request hook still runs for every signing request, so checks implemented there remain effective, and implementations that retain the default always-successful Agent::confirm do not lose an approval check through this regression. This issue is fixed in version 0.65.0.

Affected: crates.io: russh

### Summary

The agent server in `russh::keys::agent::server` never calls `Agent::confirm` before signing with a key that was added with the confirm constraint. That constraint is `SSH_AGENT_CONSTRAIN_CONFIRM` (RFC 9987 §5.2.7.2), which is what `ssh-add -c` sends.

The call was replaced by a hard-coded `true` in 3e04597 ("Native async traits encore", #455), first released in 0.50.0-beta.12. Every release since, up to and including 0.64.1, signs requests for confirm-constrained keys without asking the `Agent` implementation. A user who added a key with `-c` is therefore never asked to confirm.

### Details

`Connection::try_sign` notes that the key needs confirmation, then awaits a ready future instead of the agent ([server.rs L348-L360 at v0.64.1](https://github.com/Eugeny/russh/blob/e5d80a07dc554480b6ea3b2e14adfc190407b627/russh/src/keys/agent/server.rs#L348-L360)):

```rust let agent = if needs_confirm { let (agent, ok) = { let _pk = key.clone(); Box::new(futures::future::ready((agent, true))) } .await; if !ok { return Ok((agent, false)); } agent } else { agent }; ```

Before [3e04597](https://github.com/Eugeny/russh/commit/3e04597dc54e36bfc85c21996f7d8d9d224e9826) the first statement was `let (agent, ok) = agent.confirm(key.clone()).await;`.

`try_sign` was checked in the published crates on crates.io: - Every russh release from 0.50.0-beta.12 to 0.64.1 (52 versions) has the hard-coded `true`. - russh 0.50.0-beta.8 to be…