Details
## Summary
A russh **server** can be driven to unbounded heap growth (process OOM / kill) by
a peer that speaks only standard SSH messages, in the **default configuration**.
The peer starts a key re-exchange (sends `SSH_MSG_KEXINIT`) but never sends the
follow-up `SSH_MSG_KEX_ECDH_INIT`, leaving the server's kex state machine in
`SessionKexState::InProgress` **indefinitely**. While a rekey is in progress the
server's three message-drain paths are all gated off (`if !self.kex.active()`),
but the network-read path stays active, so every `SSH_MSG_CHANNEL_OPEN` the peer
sends is processed inline and appends one reply to an **unbounded** internal
queue (`priority_receiver`, an `UnboundedReceiver`) that is not dequeued until
the rekey completes. Because the peer decides whether the rekey ever completes,
the queue — and the server's memory — grows without bound.
This is reproducible end-to-end against a real russh server over a real
encrypted transport; see "Proof of concept". A one-line negative control (same
flood, no rekey) keeps memory flat, isolating the rekey window as the sole
trigger.
## Impact
- **Availability / DoS.** Server RSS climbs at roughly 3.3 KB per
`CHANNEL_OPEN`, driven entirely by the peer, until the process is OOM-killed.
In the reproduction one connection pushed the server from ~4 MB to **2.57 GB**
(and past **4.8 GB** against the stock `echoserver` example) and it was still
climbing when the flood was stopped.
- **Reached with standard messages, no special configuration.** Any handler is
affected, including one that rejects every channel (the reply enqueued on
rejection is exactly what accumulates). There is no per-connection cap on
in-flight channel opens or on the queue, and the queue's sender has no
backpressure.
- The peer keeps the connection alive simply by continuing to send, so the
inactivity timer never fires.
## Affected component
- `russh/src/server/session.rs` — `Session::run` `tokio::select!` loop. The
network-read arm is ungated; the three drain paths are gated on
`!self.kex.active()`.
- `russh/src/server/mod.rs` — `reply()` routes a non-kex message received during
a rekey straight to `server_read_encrypted` (inline processing).
- `russh/src/lib_inner.rs` — `ChannelOpenHandleInner`'s `accept`/`reject`/`Drop`
all `send` a reply on an `UnboundedSender`.
Verified on **v0.63.1** (commit `d3ae702`), which is the latest release. The
gating logic predates it.
## Details
`Session::run` (`russh/src/server/session.rs:631`) drives a `tokio::select!`
(`:713`). Three of its message-drain sites are gated on `!self.kex.active()`:
- the pre-`select!` batch drain of `priority_receiver`/`receiver`
(`session.rs:680`),
- the `priority_receiver.recv()` arm (`session.rs:762`),
- the `receiver.recv()` arm (`session.rs:770`, which also holds the only other
`priority_receiver` drain at `:777`).
The fourth arm, `r = &mut reading` (`session.rs:714`), is **ungated**: it reads
and processes one incoming packet every loop iteration regardless of rekey
state, calling `reply()` (`server/mod.rs:1128`).
During a **rekey**, `session.common.encrypted.is_some()`, so the strict-kex
message-ordering guard (`server/mod.rs:1143`, which is additionally gated on
`encrypted.is_none()`) does **not** apply. A non-kex message therefore falls
through `reply()` to `session.server_read_encrypted(handler, pkt)`
(`server/mod.rs:1232`) and is handled inline. For `SSH_MSG_CHANNEL_OPEN` this
reaches the channel-open handling, which hands the application a
`ChannelOpenHandle`.
Whether the handler accepts or (the trait default) rejects, the handle's
`accept` / `reject` / `Drop` all `send` a `Msg::ChannelOpenReply` on an
`UnboundedSender` (`russh/src/lib_inner.rs:560-603`; `Drop` sends
`AdministrativelyProhibited` at `:594-603`). That sender feeds
`priority_receiver`, declared `UnboundedReceiver<Msg>` (`session.rs:23`) and
created with `tokio::sync::mpsc::unbounded_channel()` (`session.rs:1522`). Its
only drain sites are the three arms gated off during the rekey. So each
`CHANNEL_OPEN` processed during the rekey window appends one reply (carrying a
`PendingChannelOpen` = channel params + mpsc `ChannelRef` + ids, a few KB
retained in practice) to a queue that is never dequeued.
Two facts make this unbounded and remote:
1. The server enters `InProgress` the moment it receives the peer's `KEXINIT`
(`server/mod.rs:1153-1158`, `begin_rekey`) and only leaves it upon receiving
the peer's `KEX_ECDH_INIT`. **The peer decides** whether to ever send that,
so the window is attacker-held.
2. There is no per-connection cap on channels or on the priority queue, and the
sender is unbounded (no backpressure).
### Root cause
The intended design was to *buffer* packets received during a rekey and replay
them afterwards: the fields `pending_reads: Vec<Vec<u8>>` and `pending_len: u32`
(`session.rs:26-27`) exist and are **drained** at kex completion
(`server/mod.rs:1194-1198`). But nothing ever **pushes** to `pending_reads` or
increments `pending_len` (they are dead — confirmed by grep across
`russh/src/`). Instead of being buffered, channel messages received during a
rekey are processed inline, and their replies pile up in the unbounded
`priority_receiver`. The missing piece is a bound on — or bounded deferral of —
channel processing while `kex.active()`.
## Proof of concept
Everything runs inside a container; nothing touches the host.
**Lab.** `poc/Dockerfile` builds `russh-lab:head` from Eugeny/russh @ `d3ae702`
(v0.63.1), default features (rust 1.91). A raw-SSH-client PoC
(`poc/poc_rekey_dos.rs`) implements curve25519-sha256 / ssh-ed25519 /
aes256-ctr / hmac-sha2-256 by hand, completes the handshake and a `publickey`
auth, then:
1. sends `SSH_MSG_KEXINIT` (server enters `kex.active()`),
2. **never** sends `KEX_ECDH_INIT` (rekey stalls, attacker-held),
3. floods `SSH_MSG_CHANNEL_OPEN`.
A minimal server (`poc/poc_server.rs`) uses the **trait-default**
`channel_open_session` (which rejects by dropping the handle), so the measured
growth is purely the undrained priority queue, not accepted-channel state.
`poc/run.sh` runs the attack leg and an identical **negative control** with no
rekey. Fresh run inside the lab, `N = 800,000` opens
(`results/rerun-2026-08-29.log`):
```
=== ATTACK (rekey stall) === (poc_server baseline_RSS=4208KB, N=800000)
t=2s server_RSS=667760KB
t=4s server_RSS=1599600KB
t=6s server_RSS=2556016KB
t=8s server_RSS=2566256KB (flood done; memory retained)
=== CONTROL (no rekey) === (poc_server baseline_RSS=4208KB, N=800000)
t=2s..t=12s server_RSS=4208KB (flat throughout)
```
- **ATTACK**: RSS `4,208 KB → 2,566,256 KB` (~2.57 GB) and retained after the
flood ends — ~3.3 KB per `CHANNEL_OPEN`, attacker-driven. (An earlier canonical
run reached 2.7 GB at 800k opens, and past 4.8 GB against the stock
`echoserver` example at 1.5 M opens — see `results/canonical-run.log`.)
- **CONTROL**: RSS flat at `4,208 KB`. Without the rekey window the replies are
drained normally; TCP backpressure (the client never reads the failure
replies) even throttles the flood.
The control isolates the rekey window as the sole trigger. Both legs exercise
the real server entry point (`Session::run` → `reply` → `server_read_encrypted`)
over a real encrypted transport.
Reproduce: `C=russh-lab N=800000 ./poc/run.sh` (see `poc/POC-README.md`).
## Remediation
The priority queue carries locally generated channel-open replies, which are
non-kex messages the server **must not send** during a rekey anyway (RFC 4253
§7.1). So the fix is to bound how much channel work is done during a rekey, not
to drain the queue mid-rekey. Any of:
- **(recommended, minimal)** cap the number of non-kex messages processed while a
rekey is in progress and disconnect a peer that exceeds it. A stalled/abusive
rekey is then torn down after a small constant instead of growing memory
without bound. See `patch/rekey-message-cap.patch` (a ~10-line change local to
`Session::run`, validated in the lab — the attack leg is disconnected after the
cap and RSS stays flat; a normal rekey, which completes in one round trip, is
unaffected).
- bound / deferred-buffer channel messages during a rekey using the existing
(currently dead) `pending_reads`/`pending_len` machinery, with a hard cap.
- bound the number of in-flight channel opens per connection and reject beyond it.
OpenSSH does not service new channels mid-kex; matching that intent closes the
whole family.
