{
  "cveTags": [],
  "descriptions": [
    {
      "lang": "en",
      "value": "Russh is a Rust SSH client \u0026 server library. From version 0.34.0 to before version 0.61.1, 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. This issue has been patched in version 0.61.1."
    }
  ],
  "id": "CVE-2026-46702",
  "lastModified": "2026-06-10T22:17:00.580",
  "metrics": {
    "cvssMetricV31": [
      {
        "cvssData": {
          "attackComplexity": "LOW",
          "attackVector": "NETWORK",
          "availabilityImpact": "HIGH",
          "baseScore": 7.5,
          "baseSeverity": "HIGH",
          "confidentialityImpact": "NONE",
          "integrityImpact": "NONE",
          "privilegesRequired": "NONE",
          "scope": "UNCHANGED",
          "userInteraction": "NONE",
          "vectorString": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H",
          "version": "3.1"
        },
        "exploitabilityScore": 3.9,
        "impactScore": 3.6,
        "source": "security-advisories@github.com",
        "type": "Secondary"
      }
    ]
  },
  "published": "2026-06-10T22:17:00.580",
  "references": [
    {
      "source": "security-advisories@github.com",
      "url": "https://github.com/Eugeny/russh/security/advisories/GHSA-wwx6-x28x-8259"
    }
  ],
  "sourceIdentifier": "security-advisories@github.com",
  "vulnStatus": "Received",
  "weaknesses": [
    {
      "description": [
        {
          "lang": "en",
          "value": "CWE-770"
        }
      ],
      "source": "security-advisories@github.com",
      "type": "Primary"
    }
  ]
}

{
  "affected": [
    {
      "package": {
        "ecosystem": "crates.io",
        "name": "russh"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0.34.0"
            },
            {
              "fixed": "0.61.1"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-46702"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-770"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-05-29T19:37:14Z",
    "nvd_published_at": null,
    "severity": "HIGH"
  },
  "details": "### Summary\n\nWhen 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.\n\nIn current releases, this is a remote denial-of-service / resource-exhaustion issue in the post-decompression receive path.\n\nIn older releases before `0.58.0`, the same remote decompression path used `CryptoVec`, which appears to make the historical impact worse.\n\n### Details\n\nThe normal SSH transport read path enforces a packet-length limit before the packet body is read:\n\n- `russh/src/cipher/mod.rs`\n\nHowever, 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.\n\nIn `russh`, compressed packet bodies are later decompressed in:\n\n- `russh/src/compression.rs`\n- `russh/src/client/mod.rs`\n- `russh/src/server/session.rs`\n\nBefore 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.\n\nIt 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.\n\nVersion detail:\n\n- The underlying post-decompression bounds bug appears to affect `russh` as far back as `0.34.0`.\n- In historical releases `\u003e= 0.34.0, \u003c 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.\n- In current-style releases `\u003e= 0.58.0`, non-secret packet/decompression buffers were moved off `CryptoVec` and onto `Vec\u003cu8\u003e`, 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.\n- The maintained-line fix was verified against `0.60.2`.\n\nCompression 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 `zlib@openssh.com`, and server-side negotiation follows the client\u0027s preference order for common algorithms. A client that prefers compression can therefore negotiate it with a default `russh` server.\n\nOpenSSH 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\u0027s `SSHBUF_SIZE_MAX` hard maximum of `0x8000000` bytes (`128 MiB`).\n\nThe 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.\n\nRelevant OpenSSH reference points:\n\n- `/home/mjc/projects/openssh-portable/packet.c`: `PACKET_MAX_SIZE (256 * 1024)`\n- `/home/mjc/projects/openssh-portable/packet.c`: `uncompress_buffer()` inflates into `compression_buffer`\n- `/home/mjc/projects/openssh-portable/sshbuf.h`: `SSHBUF_SIZE_MAX 0x8000000`\n\n### RFC / OpenSSH Comparison\n\nRFC 4253 section 6 defines the binary packet format:\n\n- `packet_length`\n- `padding_length`\n- `payload`\n- random padding\n- MAC\n\nRFC 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.\n\nThat 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.\n\nOpenSSH provides such a bound indirectly through `sshbuf`\u0027s hard maximum. The `russh` fix should make the corresponding post-decompression bound explicit.\n\n### PoC\n\nThere were two kinds of proof:\n\n- 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\n- direct client and server receive-path tests that exercise the oversized post-decompression behavior itself\n\nThe current in-tree regression tests are:\n\n- `tests::compress::oversized_debug_payload_can_stay_below_wire_cap`\n- `compression::tests::oversized_decompressed_packet_is_rejected`\n- `client::tests::compressed_debug_is_ignored_after_client_parses_it`\n- `client::tests::oversized_compressed_debug_is_rejected_before_client_ignores_it`\n- `server::session::tests::compressed_debug_is_ignored_after_server_parses_it`\n- `server::session::tests::oversized_compressed_debug_is_rejected_before_server_ignores_it`\n\nThe important behavior is:\n\n1. An attacker-crafted best-compressed `DEBUG` payload can stay below the normal `256 KiB` transport packet cap while still inflating beyond `128 MiB`.\n2. In the direct client and server receive paths, small compressed `DEBUG` packets are still ignored normally after parsing.\n3. In the direct client and server receive paths, oversized compressed `DEBUG` packets are rejected before the implementation reaches the normal \"ignore DEBUG\" behavior.\n\nThe 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\u0027s 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.\n\nThat 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.\n\n```rust\nfn compressed_debug_payload(payload_len: usize) -\u003e Vec\u003cu8\u003e {\n    let mut payload = vec![b\u0027A\u0027; payload_len];\n    payload[0] = crate::msg::DEBUG;\n\n    let mut encoder =\n        flate2::write::ZlibEncoder::new(Vec::new(), flate2::Compression::best());\n    encoder.write_all(\u0026payload).unwrap();\n    let compressed = encoder.finish().unwrap();\n\n    assert!(\n        compressed.len() \u003c 256 * 1024,\n        \"oversized post-decompression payload still fits under the wire cap\"\n    );\n    compressed\n}\n\nfn incoming_packet(compressed: Vec\u003cu8\u003e) -\u003e SSHBuffer {\n    let mut buffer = SSHBuffer::new();\n    // maybe_decompress() receives the clear SSHBuffer after packet framing,\n    // and decompresses bytes after packet_length + padding_length.\n    buffer.buffer.extend_from_slice(\u0026[0; 5]);\n    buffer.buffer.extend_from_slice(\u0026compressed);\n    buffer\n}\n\n#[test]\nfn unauthenticated_client_zlib_debug_is_rejected_by_server_before_auth() {\n    let mut server = preauth_server_session_after_newkeys_with_zlib_decompressor();\n    let oversized = MAXIMUM_DECOMPRESSED_PACKET_LEN + 1024;\n    let buffer = incoming_packet(compressed_debug_payload(oversized));\n\n    let err = server.maybe_decompress(\u0026buffer).unwrap_err();\n    assert!(\n        matches!(err, crate::Error::PacketSize(len) if len \u003e MAXIMUM_DECOMPRESSED_PACKET_LEN)\n    );\n}\n```\n\nThe equivalent wire-level attack shape is:\n\n```text\n1. Connect to a russh server using the default compression advertisement.\n2. Send SSH_MSG_KEXINIT with compression client-to-server preference:\n   zlib,zlib@openssh.com,none\n3. Complete key exchange and send SSH_MSG_NEWKEYS.\n4. Before any SSH_MSG_USERAUTH_REQUEST, send a compressed SSH_MSG_DEBUG packet:\n   - compressed packet body: \u003c 256 KiB\n   - decompressed packet body: \u003e 128 MiB\n5. Vulnerable behavior: russh accepts and inflates the packet, then reaches the\n   normal DEBUG ignore path.\n6. Fixed behavior: russh rejects during decompression with Error::PacketSize.\n```\n\nThe 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.\n\nThe most important targeted checks are:\n\n```bash\ncargo test -p russh oversized_debug_payload_can_stay_below_wire_cap -- --nocapture\ncargo test -p russh oversized_compressed_debug_is_rejected_before_client_ignores_it -- --nocapture\ncargo test -p russh oversized_compressed_debug_is_rejected_before_server_ignores_it -- --nocapture\n```\n\nBefore 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.\n\n### Impact\n\nSuggested CVSS v3.1 for current maintained releases:\n\n- `CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H`\n- Score: `7.5`\n\nReasoning:\n\n- `AV:N`: reachable by a remote SSH peer\n- `AC:L`: straightforward once compression is enabled\n- `PR:N`, `UI:N`: no prior auth or user interaction required\n- `C:N`, `I:N`: confidentiality or integrity impact was not demonstrated\n- `A:H`: remote peer can cause oversized post-decompression packet processing and disconnect / denial of service\n\nAffected versions:\n\n- historical stronger case: `russh \u003e= 0.34.0, \u003c 0.58.0`\n- current maintained remote DoS case: `russh \u003e= 0.58.0`, including `0.60.3`\n\n### Fix / Patch Direction\n\nAdd an explicit maximum decompressed SSH packet size and enforce it inside `Decompress::decompress()` before returning decompressed bytes to the client or server packet parser.\n\nThe intended ceiling is `128 MiB`, matching OpenSSH portable\u0027s 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.\n\nThe fix should preserve normal compressed packet behavior below the cap, including `DEBUG` packets that are decompressed and then ignored through the existing normal path.\n\nPatch branch:\n\n```text\nfix/zlib-decompression-cap\n```",
  "id": "GHSA-wwx6-x28x-8259",
  "modified": "2026-05-29T19:37:14Z",
  "published": "2026-05-29T19:37:14Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/Eugeny/russh/security/advisories/GHSA-wwx6-x28x-8259"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/Eugeny/russh"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H",
      "type": "CVSS_V3"
    }
  ],
  "summary": "russh: Post-decompression SSH packet size was not bounded, allowing remote oversized compressed packets"
}