PT-2026-45017 · Crates.Io · Russh

Published 2026-05-29 · Updated 2026-05-29 · CVE-2026-46702

OSV

CVSS v3.1

7.5

High

Base vector

AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

Summary

When SSH compression is enabled, russh accepted compressed packets whose on-wire size passed the normal transport packet-length checks but whose decompressed size was much larger. This allowed a remote peer to send oversized post-decompression packets that should have been rejected.

In current releases, this is a remote denial-of-service / resource-exhaustion issue in the post-decompression receive path.

In older releases before 0.58.0, the same remote decompression path used CryptoVec, which appears to make the historical impact worse.

Details

The normal SSH transport read path enforces a packet-length limit before the packet body is read:

• russh/src/cipher/mod.rs

However, RFC 4253 compression is applied to the SSH payload field only. The packet length field and MAC are computed over the compressed payload, so a packet that is reasonably sized on the wire can still expand to a much larger message body after decompression.

In russh, compressed packet bodies are later decompressed in:

• russh/src/compression.rs
• russh/src/client/mod.rs
• russh/src/server/session.rs

Before the fix, Decompress::decompress() grew its output buffer by repeated doubling and did not enforce a separate post-decompression ceiling. That meant a peer could send a small compressed packet that passed the normal on-wire transport length checks and then inflate it into a much larger packet after decompression.

It was verified that an attacker-crafted compressed payload can stay below the normal 256 KiB implementation transport packet cap while still inflating above the intended post-decompression bound. In other words, this is not only a "large on-wire packet" issue.

Version detail:

• The underlying post-decompression bounds bug appears to affect russh as far back as 0.34.0.
• In historical releases >= 0.34.0, < 0.58.0, the remote decompression path still used CryptoVec. Remote compressed SSH traffic could drive that path, and under constrained memory that historical code path could abort the process.
• In current-style releases >= 0.58.0, non-secret packet/decompression buffers were moved off CryptoVec and onto Vec<u8>, but the post-decompression size still remained unbounded. So the bug class remained reachable remotely, but the maintained-line impact is a current remote DoS / oversized-packet-acceptance issue rather than the older CryptoVec-based abort story.
• The maintained-line fix was verified against 0.60.2.

Compression is not selected in a default-vs-default russh session because the default preference order puts none first. However, the default server configuration still advertises zlib and [e-mail], and server-side negotiation follows the client's preference order for common algorithms. A client that prefers compression can therefore negotiate it with a default russh server.

OpenSSH portable was checked at /home/mjc/projects/openssh-portable commit 45b30e0a5. OpenSSH enforces a 256 KiB transport packet cap before decompression, but it does not reuse that cap after decompression. Instead, decompression writes to an sshbuf, which is indirectly bounded by OpenSSH's SSHBUF SIZE MAX hard maximum of 0x8000000 bytes (128 MiB).

The patch direction should follow that model: add an explicit post-decompression ceiling of 128 MiB, rather than assuming the compressed transport packet cap also bounds decompressed payload size.

Relevant OpenSSH reference points:

• /home/mjc/projects/openssh-portable/packet.c: PACKET MAX SIZE (256 * 1024)
• /home/mjc/projects/openssh-portable/packet.c: uncompress buffer() inflates into compression buffer
• /home/mjc/projects/openssh-portable/sshbuf.h: SSHBUF SIZE MAX 0x8000000

RFC / OpenSSH Comparison

RFC 4253 section 6 defines the binary packet format:

• packet length
• padding length
• payload
• random padding
• MAC

RFC 4253 section 6.2 says that, when compression is negotiated, the payload field is compressed, and that packet length and MAC are computed from the compressed payload. The RFC also says implementations should check that packet length is reasonable to avoid denial-of-service and buffer-overflow attacks.

That means the pre-decompression transport packet length check is necessary but not sufficient. A correct implementation still needs a reasonable bound on the decompressed payload that becomes parser input.

OpenSSH provides such a bound indirectly through sshbuf's hard maximum. The russh fix should make the corresponding post-decompression bound explicit.

PoC

There were two kinds of proof:

• a wire-cap sanity test showing an attacker-crafted best-compressed DEBUG payload can stay below the normal SSH transport packet cap while still inflating beyond the intended post-decompression bound
• direct client and server receive-path tests that exercise the oversized post-decompression behavior itself

The current in-tree regression tests are:

• tests::compress::oversized debug payload can stay below wire cap
• compression::tests::oversized decompressed packet is rejected
• client::tests::compressed debug is ignored after client parses it
• client::tests::oversized compressed debug is rejected before client ignores it
• server::session::tests::compressed debug is ignored after server parses it
• server::session::tests::oversized compressed debug is rejected before server ignores it

The important behavior is:

1. An attacker-crafted best-compressed DEBUG payload can stay below the normal 256 KiB transport packet cap while still inflating beyond 128 MiB.
2. In the direct client and server receive paths, small compressed DEBUG packets are still ignored normally after parsing.
3. In the direct client and server receive paths, oversized compressed DEBUG packets are rejected before the implementation reaches the normal "ignore DEBUG" behavior.

The strongest PoC for severity is the unauthenticated server-side case. A malicious client can choose zlib in the initial key exchange, because the default server advertises it and server-side negotiation follows the client's preference order for common algorithms. After NEWKEYS, but before authentication, the client can send a transport-layer SSH MSG DEBUG packet whose compressed body is below the transport packet cap but whose decompressed body exceeds the post-decompression cap.

That demonstrates the AV:N/AC:L/PR:N/UI:N case directly: the attacker is a remote SSH client and does not need a successfully authenticated session.

fn compressed debug payload(payload len: usize) -> Vec<u8> {
  let mut payload = vec![b'A'; payload len];
  payload[0] = crate::msg::DEBUG;

  let mut encoder =
    flate2::write::ZlibEncoder::new(Vec::new(), flate2::Compression::best());
  encoder.write all(&payload).unwrap();
  let compressed = encoder.finish().unwrap();

  assert!(
    compressed.len() < 256 * 1024,
    "oversized post-decompression payload still fits under the wire cap"
  );
  compressed
}

fn incoming packet(compressed: Vec<u8>) -> SSHBuffer {
  let mut buffer = SSHBuffer::new();
  // maybe decompress() receives the clear SSHBuffer after packet framing,
  // and decompresses bytes after packet length + padding length.
  buffer.buffer.extend from slice(&[0; 5]);
  buffer.buffer.extend from slice(&compressed);
  buffer
}

#[test]
fn unauthenticated client zlib debug is rejected by server before auth() {
  let mut server = preauth server session after newkeys with zlib decompressor();
  let oversized = MAXIMUM DECOMPRESSED PACKET LEN + 1024;
  let buffer = incoming packet(compressed debug payload(oversized));

  let err = server.maybe decompress(&buffer).unwrap err();
  assert!(
    matches!(err, crate::Error::PacketSize(len) if len > MAXIMUM DECOMPRESSED PACKET LEN)
  );
}

The equivalent wire-level attack shape is:

1. Connect to a russh server using the default compression advertisement.
2. Send SSH MSG KEXINIT with compression client-to-server preference:
  zlib,[e-mail],none
3. Complete key exchange and send SSH MSG NEWKEYS.
4. Before any SSH MSG USERAUTH REQUEST, send a compressed SSH MSG DEBUG packet:
  - compressed packet body: < 256 KiB
  - decompressed packet body: > 128 MiB
5. Vulnerable behavior: russh accepts and inflates the packet, then reaches the
  normal DEBUG ignore path.
6. Fixed behavior: russh rejects during decompression with Error::PacketSize.

The direct receive-path client/server regression tests are still useful because they isolate the bug precisely. They construct the post-decryption compressed packet body passed to maybe decompress() and prove that the oversized packet is rejected before normal DEBUG ignore handling. The server-side pre-auth variant above is the one that justifies the highest CVSS framing for this bug.

The most important targeted checks are:

cargo test -p russh oversized debug payload can stay below wire cap -- --nocapture
cargo test -p russh oversized compressed debug is rejected before client ignores it -- --nocapture
cargo test -p russh oversized compressed debug is rejected before server ignores it -- --nocapture

Before the fix, both the direct client and direct server receive-path oversized checks went red because the compressed payload was accepted and decompressed instead of being rejected at the post-decompression boundary. After the fix, they pass.

Impact

Suggested CVSS v3.1 for current maintained releases:

• CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
• Score: 7.5

Reasoning:

• AV:N: reachable by a remote SSH peer
• AC:L: straightforward once compression is enabled
• PR:N, UI:N: no prior auth or user interaction required
• C:N, I:N: confidentiality or integrity impact was not demonstrated
• A:H: remote peer can cause oversized post-decompression packet processing and disconnect / denial of service

Affected versions:

• historical stronger case: russh >= 0.34.0, < 0.58.0
• current maintained remote DoS case: russh >= 0.58.0, including 0.60.3

Fix / Patch Direction

Add an explicit maximum decompressed SSH packet size and enforce it inside Decompress::decompress() before returning decompressed bytes to the client or server packet parser.

The intended ceiling is 128 MiB, matching OpenSSH portable's effective sshbuf hard maximum for post-decompression packet storage. The fix should reject decompression output larger than that bound with a packet-size error before normal message dispatch.

The fix should preserve normal compressed packet behavior below the cap, including DEBUG packets that are decompressed and then ignored through the existing normal path.

Patch branch:

fix/zlib-decompression-cap

Fix

Allocation of Resources Without Limits

Weakness Enumeration

CWE-770

Related Identifiers

CVE-2026-46702

GHSA-WWX6-X28X-8259

Affected Products

Russh

References · 3

• https://osv.dev/vulnerability/GHSA-wwx6-x28x-8259 · Vendor Advisory
• https://github.com/Eugeny/russh/security/advisories/GHSA-wwx6-x28x-8259 · Note
• https://github.com/Eugeny/russh · Note