CVE-2026-73489

Russh is a Rust SSH client & server library. Prior to 0.62.4, an authenticated SSH client can cause a denial of service by sending a pty-req channel request with more than 130 terminal-mode records. The parser in russh/src/server/encrypted.rs stores terminal modes in a fixed 130-entry [(Pty::TTY_OP_END, 0); 130] array but continues increasing the mode count, then constructs an out-of-bounds slice and panics before the application pty_request handler runs. The panic terminates the server session task without causing memory corruption. This issue is fixed in version 0.62.4.

Russh is a Rust SSH client & server library. Prior to 0.62.4, an authenticated SSH client can cause a denial of service by sending a pty-req channel request with more than 130 terminal-mode records. The parser in russh/src/server/encrypted.rs stores terminal modes in a fixed 130-entry [(Pty::TTY_OP_END, 0); 130] array but continues increasing the mode count, then constructs an out-of-bounds slice and panics before the application pty_request handler runs. The panic terminates the server session task without causing memory corruption. This issue is fixed in version 0.62.4.

Summary

A post-authentication denial-of-service panic in russh 0.62.2 (commit c4be19f1915c8682f4615c3fd50008512b474491, current default branch main as of 2026-07-22). An authenticated client sends a pty-req channel request carrying more than 130 terminal-mode records. The parser uses a fixed [(Pty::TTY_OP_END, 0); 130] array but increments its counter i for every valid record (logging "too many pty codes" without returning), then slices &modes[0..i] — an out-of-bounds slice that panics (range end index 131 out of range for slice of length 130) before the application pty_request handler runs.

This is reachable with the default server configuration and the default crypto config (curve25519-sha256 + chacha20-poly1305), requiring only an authenticated session channel — no caller-supplied parameter. It is reproduced end-to-end against the unmodified real russh 0.62.2 library (a real russh::client + russh::server over TCP, using the public Channel::request_pty(...) API); 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-4r3c-5hpg-58qr / CVE-2026-48110 is allocation-first string parsing, not the fixed-array slice overflow; it was fixed in 0.61.0 but this code path still overflows the fixed array).

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

Details

russh/src/server/encrypted.rs, pty-req handling (lines 1137–1201):

let mut modes = [(Pty::TTY_OP_END, 0); 130];     // fixed 130-entry array (line 1137)
let mut i = 0;
...
while !mode_bytes.is_empty() {
    let code = mode_bytes[0];
    if code == 0 { if mode_bytes.len() != 1 { return Err(...); } break; }
    if mode_bytes.len() < 5 { return Err(...); }
    let num = BigEndian::read_u32(&mode_bytes[1..5]);
    if let Some(code) = Pty::from_u8(code) {
        if i < 130 {
            modes[i] = (code, num);
        } else {
            error!("pty-req: too many pty codes");   // logs, does NOT return (line 1162)
        }
    }
    i += 1;                                          // keeps growing past 130
    mode_bytes = &mode_bytes[5..];
}
...
handler.pty_request(channel_num, &term, ..., &modes[0..i], self).await  // line 1201 — OOB when i > 130

Each terminal-mode record is exactly 5 bytes (1-byte opcode + 4-byte value), and i += 1 runs for every valid record (including repeats of the same opcode). The if i < 130 write-gate prevents an in-array overflow but the counter still grows unbounded, and the later &modes[0..i] slice has no corresponding bound. SSH packet-size limits do not bound the mode count to 130, so a single normal-sized pty-req can carry hundreds of mode records. The client's own request_pty serialization (russh/src/client/session.rs:

((1 + 5 * terminal_modes.len()) as u32).encode(&mut enc.write)?;   // line 129
for &(code, value) in terminal_modes {
    if code == Pty::TTY_OP_END { continue; }
    (code as u8).encode(&mut enc.write)?;
    value.encode(&mut enc.write)?;                                   // line 135
}

writes every record with no count cap, so 131 records reach the server as a legitimate authenticated channel request.

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 with the default crypto config. It runs an ATTACK case (131 mode records → panic) and a CONTROL case (130 mode records → parses fine), proving the panic is caused specifically by exceeding the 130-entry array.

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_c15_pty_modes_panic
RUST_BACKTRACE=1…

Russh is a Rust SSH client & server library. Prior to 0.62.4, an authenticated SSH client can cause a denial of service by sending a pty-req channel request with more than 130 terminal-mode records. The parser in russh/src/server/encrypted.rs stores terminal modes in a fixed 130-entry [(Pty::TTY_OP_END, 0); 130] array but continues increasing the mode count, then constructs an out-of-bounds slice and panics before the application pty_request handler runs. The panic terminates the server session task without causing memory corruption. This issue is fixed in version 0.62.4.

Russh is a Rust SSH client & server library. Prior to 0.62.4, an authenticated SSH client can cause a denial of service by sending a pty-req channel request with more than 130 terminal-mode records. The parser in russh/src/server/encrypted.rs stores terminal modes in a fixed 130-entry [(Pty::TTY_OP_END, 0); 130] array but continues increasing the mode count, then constructs an out-of-bounds slice and panics before the application pty_request handler runs. The panic terminates the server session task without causing memory corruption. This issue is fixed in version 0.62.4.

Russh is a Rust SSH client & server library. Prior to 0.62.4, an authenticated SSH client can cause a denial of service by sending a pty-req channel request with more than 130 terminal-mode records. The parser in russh/src/server/encrypted.rs stores terminal modes in a fixed 130-entry [(Pty::TTY_OP_END, 0); 130] array but continues increasing the mode count, then constructs an out-of-bounds slice and panics before the application pty_request handler runs. The panic terminates the server session task without causing memory corruption. This issue is fixed in version 0.62.4.

Summary

A post-authentication denial-of-service panic in russh 0.62.2 (commit c4be19f1915c8682f4615c3fd50008512b474491, current default branch main as of 2026-07-22). An authenticated client sends a pty-req channel request carrying more than 130 terminal-mode records. The parser uses a fixed [(Pty::TTY_OP_END, 0); 130] array but increments its counter i for every valid record (logging "too many pty codes" without returning), then slices &modes[0..i] — an out-of-bounds slice that panics (range end index 131 out of range for slice of length 130) before the application pty_request handler runs.

This is reachable with the default server configuration and the default crypto config (curve25519-sha256 + chacha20-poly1305), requiring only an authenticated session channel — no caller-supplied parameter. It is reproduced end-to-end against the unmodified real russh 0.62.2 library (a real russh::client + russh::server over TCP, using the public Channel::request_pty(...) API); 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-4r3c-5hpg-58qr / CVE-2026-48110 is allocation-first string parsing, not the fixed-array slice overflow; it was fixed in 0.61.0 but this code path still overflows the fixed array).

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

Details

russh/src/server/encrypted.rs, pty-req handling (lines 1137–1201):

let mut modes = [(Pty::TTY_OP_END, 0); 130];     // fixed 130-entry array (line 1137)
let mut i = 0;
...
while !mode_bytes.is_empty() {
    let code = mode_bytes[0];
    if code == 0 { if mode_bytes.len() != 1 { return Err(...); } break; }
    if mode_bytes.len() < 5 { return Err(...); }
    let num = BigEndian::read_u32(&mode_bytes[1..5]);
    if let Some(code) = Pty::from_u8(code) {
        if i < 130 {
            modes[i] = (code, num);
        } else {
            error!("pty-req: too many pty codes");   // logs, does NOT return (line 1162)
        }
    }
    i += 1;                                          // keeps growing past 130
    mode_bytes = &mode_bytes[5..];
}
...
handler.pty_request(channel_num, &term, ..., &modes[0..i], self).await  // line 1201 — OOB when i > 130

Each terminal-mode record is exactly 5 bytes (1-byte opcode + 4-byte value), and i += 1 runs for every valid record (including repeats of the same opcode). The if i < 130 write-gate prevents an in-array overflow but the counter still grows unbounded, and the later &modes[0..i] slice has no corresponding bound. SSH packet-size limits do not bound the mode count to 130, so a single normal-sized pty-req can carry hundreds of mode records. The client's own request_pty serialization (russh/src/client/session.rs:

((1 + 5 * terminal_modes.len()) as u32).encode(&mut enc.write)?;   // line 129
for &(code, value) in terminal_modes {
    if code == Pty::TTY_OP_END { continue; }
    (code as u8).encode(&mut enc.write)?;
    value.encode(&mut enc.write)?;                                   // line 135
}

writes every record with no count cap, so 131 records reach the server as a legitimate authenticated channel request.

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 with the default crypto config. It runs an ATTACK case (131 mode records → panic) and a CONTROL case (130 mode records → parses fine), proving the panic is caused specifically by exceeding the 130-entry array.

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_c15_pty_modes_panic
RUST_BACKTRACE=1…