{"id":"CVE-2026-42189","aliases":["DEBIAN-CVE-2026-42189","EUVD-2026-28822","GHSA-f5v4-2wr6-hqmg","UBUNTU-CVE-2026-42189"],"status":"published","title":"Russh: Pre-auth DoS via unbounded allocation in keyboard-interactive auth","description":"Russh is a Rust SSH client \u0026 server library. Prior to version 0.60.1, a pre-authentication denial-of-service vulnerability exists in the server's keyboard-interactive authentication handler. A malicious client can crash any russh-based server that implements keyboard-interactive auth (e.g., for 2FA/TOTP) with a single malformed packet, requiring no credentials. This issue has been patched in version 0.60.1.","severity":{"version":"3.1","score":7.5,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H","level":"high","assessor":"cna","from":"cve"},"epss":{"day":"2026-11-11","score":0.00481,"percentile":0.41942,"model":"v2026.06.15"},"facts":{"in_kev":false,"known_exploited":false,"public_exploit":false,"fix_available":true},"cwes":["CWE-770","CWE-789"],"published":"2026-04-24T15:39:37Z","modified":"2026-10-28T02:30:42Z","fixes":[{"ecosystem":"crates.io","package":"russh","fixed_in":["0.60.1"],"sources":["ghsa","osv"]},{"ecosystem":"Debian:14","package":"rust-russh","fixed_in":["0.64.1-1"],"sources":["osv"]}],"affected":[{"source":"ghsa","ecosystem":"crates.io","package":"russh","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"0.60.1"}]}]},{"source":"osv","ecosystem":"Debian:14","package":"rust-russh","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"0.64.1-1"}]}],"versions":["0.57.1-1","0.57.1-2"]},{"source":"osv","ecosystem":"crates.io","package":"russh","ranges":[{"type":"SEMVER","events":[{"introduced":"0"},{"fixed":"0.60.1"}]}]},{"source":"osv","ecosystem":"Ubuntu:26.10","package":"rust-russh","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"}]}],"versions":["0.57.1-2"]}],"products":[{"vendor":"eugeny","product":"russh"},{"vendor":"russh_project","product":"russh"},{"vendor":"warpgate_project","product":"warpgate"}],"references":[{"url":"https://github.com/Eugeny/russh/security/advisories/GHSA-f5v4-2wr6-hqmg","tags":["x_refsource_CONFIRM","exploit","Exploit","Mitigation","Vendor Advisory","advisory","web","report"]},{"url":"https://github.com/Eugeny/russh/commit/6c3c80a9b6d60763d6227d60fa8310e57172a4d1","tags":["x_refsource_MISC","Patch","fix","web"]},{"url":"https://github.com/Eugeny/russh/releases/tag/v0.60.1","tags":["x_refsource_MISC","Product","Release Notes","web"]},{"url":"https://nvd.nist.gov/vuln/detail/CVE-2026-42189","tags":["advisory"]},{"url":"https://github.com/advisories/GHSA-f5v4-2wr6-hqmg"},{"url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/42xxx/CVE-2026-42189.json","tags":["advisory"]},{"url":"https://security-tracker.debian.org/tracker/CVE-2026-42189","tags":["advisory"]},{"url":"https://github.com/Eugeny/russh","tags":["package"]},{"url":"https://ubuntu.com/security/CVE-2026-42189","tags":["report"]},{"url":"https://www.cve.org/CVERecord?id=CVE-2026-42189","tags":["report"]}],"sources":{"cve":[{"id":"CVE-2026-42189","title":"Russh: Pre-auth DoS via unbounded allocation in keyboard-interactive auth","description":"Russh is a Rust SSH client \u0026 server library. Prior to version 0.60.1, a pre-authentication denial-of-service vulnerability exists in the server's keyboard-interactive authentication handler. A malicious client can crash any russh-based server that implements keyboard-interactive auth (e.g., for 2FA/TOTP) with a single malformed packet, requiring no credentials. This issue has been patched in version 0.60.1.","status":"published","published":"2026-05-08T19:49:51.179Z","modified":"2026-05-11T14:23:49.308Z","cvss":[{"version":"3.1","score":7.5,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H","level":"high","assessor":"cna"}],"cwes":["CWE-770","CWE-789"],"updated_at":"2026-11-02T13:40:23.731501Z"}],"euvd":[{"id":"EUVD-2026-28822","description":"Russh is a Rust SSH client \u0026 server library. Prior to version 0.60.1, a pre-authentication denial-of-service vulnerability exists in the server's keyboard-interactive authentication handler. A malicious client can crash any russh-based server that implements keyboard-interactive auth (e.g., for 2FA/TOTP) with a single malformed packet, requiring no credentials. This issue has been patched in version 0.60.1.","status":"published","published":"2026-05-08T19:49:51Z","modified":"2026-05-11T14:23:49Z","cvss":[{"version":"3.1","score":7.5,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H","level":"high"}],"updated_at":"2026-11-02T19:05:11.205633Z"}],"ghsa":[{"id":"GHSA-f5v4-2wr6-hqmg","title":"russh has pre-auth DoS via unbounded allocation in its keyboard-interactive auth handler","description":"## Summary\n\nA pre-authentication denial-of-service vulnerability exists in the server's keyboard-interactive authentication handler. A malicious client can crash any russh-based server that implements keyboard-interactive auth (e.g., for 2FA/TOTP) with a single malformed packet, requiring no credentials.\n\n## Vulnerability Details\n\nIn `russh/src/server/encrypted.rs`, the function `read_userauth_info_response` decodes a `u32` count from the client's `SSH_MSG_USERAUTH_INFO_RESPONSE` and passes it directly to `Vec::with_capacity()`:\n\n```rust\nlet n = map_err!(u32::decode(r))?;\n\n// Bound both allocation and iteration by remaining packet data to\n// prevent a malicious client from causing a multi-GB allocation or\n// billions of loop iterations with a crafted count.\n// Each response needs at least 4 bytes (length prefix).\nlet max_responses = r.remaining_len().saturating_add(3) / 4;\nlet n = (n as usize).min(max_responses);\nlet mut responses = Vec::with_capacity(n);\nfor _ in 0..n {\n    responses.push(Bytes::decode(r).ok())\n}\n```\n\nAn attacker can send `n = 0x10000000` (268M) or larger in a minimal packet (~50 bytes after encryption). The server attempts to allocate `n * ~24 bytes` (size of `Option\u003cBytes\u003e`) = ~6.4GB, causing an OOM crash.\n\n## Attack Flow\n\n1. Attacker connects via TCP, completes key exchange (no credentials needed -- this is the anonymous DH handshake, not authentication)\n2. Sends `USERAUTH_REQUEST` with method `keyboard-interactive`\n3. Server handler returns `Auth::Partial` with prompts (standard for 2FA/TOTP)\n4. Attacker sends `USERAUTH_INFO_RESPONSE` with `n = 0x10000000` and no response data\n5. Server calls `Vec::with_capacity(268_435_456)`, OOM killed\n\nNo authentication is required. The allocation occurs before the handler validates any credentials. The attack is repeatable faster than the server can restart.\n\n## Affected Configurations\n\nAny russh-based server where the `Handler::auth_keyboard_interactive` implementation returns `Auth::Partial` (i.e., sends prompts to the client). The default handler returns `Auth::reject()` and is not affected.\n\nSource code review suggests that downstream projects using keyboard-interactive for multi-step auth (e.g., TOTP/2FA) follow the affected pattern, since returning `Auth::Partial` before credential verification is the intended API usage for prompting.\n\n## Confirmed End-to-End PoC\n\nThere is a complete Docker-contained PoC confirming the OOM kill:\n- Minimal russh server returning `Auth::Partial` for keyboard-interactive\n- Python client (paramiko for key exchange) sends malformed `USERAUTH_INFO_RESPONSE`\n- Container with 512MB memory limit; server is OOM-killed (exit code 137)\n\nAvailable on request.\n\n## Proposed Fix\n\nCap the `Vec::with_capacity` allocation to what the remaining packet data can actually contain. Each response requires at least 4 bytes (length prefix), so:\n\n```rust\nlet n = map_err!(u32::decode(r))?;\n\n// Bound both allocation and iteration by remaining packet data to\n// prevent a malicious client from causing a multi-GB allocation or\n// billions of loop iterations with a crafted count.\n// Each response needs at least 4 bytes (length prefix).\nlet max_responses = r.remaining_len().saturating_add(3) / 4;\nlet n = (n as usize).min(max_responses);\nlet mut responses = Vec::with_capacity(n);\nfor _ in 0..n {\n    responses.push(Bytes::decode(r).ok())\n}\n```\n\nThis bounds the allocation to at most the packet size (~256KB), while preserving the existing behavior for well-formed packets. This fix has been implemented, tested, and contributed via the temporary private fork.\n\n## Severity\n\nPre-auth, remote, no credentials required, crashes the server process affecting all active sessions.","status":"published","published":"2026-04-24T15:39:37Z","modified":"2026-05-13T13:34:53Z","cvss":[{"version":"3.1","score":7.5,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H","level":"high"}],"cwes":["CWE-770","CWE-789"],"updated_at":"2026-11-02T14:40:07.01746Z"}],"nvd":[{"id":"CVE-2026-42189","description":"Russh is a Rust SSH client \u0026 server library. Prior to version 0.60.1, a pre-authentication denial-of-service vulnerability exists in the server's keyboard-interactive authentication handler. A malicious client can crash any russh-based server that implements keyboard-interactive auth (e.g., for 2FA/TOTP) with a single malformed packet, requiring no credentials. This issue has been patched in version 0.60.1.","status":"published","published":"2026-05-08T20:16:31.443Z","modified":"2026-10-26T18:10:00.173Z","cvss":[{"version":"3.1","score":7.5,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H","level":"high","assessor":"secondary"}],"cwes":["CWE-770","CWE-789"],"updated_at":"2026-11-02T14:12:18.239974Z"}],"osv":[{"id":"CVE-2026-42189","title":"Russh: Pre-auth DoS via unbounded allocation in keyboard-interactive auth","description":"Russh is a Rust SSH client \u0026 server library. Prior to version 0.60.1, a pre-authentication denial-of-service vulnerability exists in the server's keyboard-interactive authentication handler. A malicious client can crash any russh-based server that implements keyboard-interactive auth (e.g., for 2FA/TOTP) with a single malformed packet, requiring no credentials. This issue has been patched in version 0.60.1.","status":"published","published":"2026-05-08T19:49:51.179Z","modified":"2026-10-28T02:30:42.777699632Z","cvss":[{"version":"3.1","score":7.5,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H","level":"high"}],"cwes":["CWE-770","CWE-789"],"updated_at":"2026-11-02T15:45:04.117819Z"},{"id":"DEBIAN-CVE-2026-42189","description":"Russh is a Rust SSH client \u0026 server library. Prior to version 0.60.1, a pre-authentication denial-of-service vulnerability exists in the server's keyboard-interactive authentication handler. A malicious client can crash any russh-based server that implements keyboard-interactive auth (e.g., for 2FA/TOTP) with a single malformed packet, requiring no credentials. This issue has been patched in version 0.60.1.","status":"published","published":"2026-05-08T20:16:31.443Z","modified":"2026-10-27T09:47:37.534606228Z","cvss":[{"version":"3.1","score":7.5,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H","level":"high"}],"updated_at":"2026-11-02T15:45:54.892385Z"},{"id":"GHSA-f5v4-2wr6-hqmg","title":"russh has pre-auth DoS via unbounded allocation in its keyboard-interactive auth handler","description":"## Summary\n\nA pre-authentication denial-of-service vulnerability exists in the server's keyboard-interactive authentication handler. A malicious client can crash any russh-based server that implements keyboard-interactive auth (e.g., for 2FA/TOTP) with a single malformed packet, requiring no credentials.\n\n## Vulnerability Details\n\nIn `russh/src/server/encrypted.rs`, the function `read_userauth_info_response` decodes a `u32` count from the client's `SSH_MSG_USERAUTH_INFO_RESPONSE` and passes it directly to `Vec::with_capacity()`:\n\n```rust\nlet n = map_err!(u32::decode(r))?;\n\n// Bound both allocation and iteration by remaining packet data to\n// prevent a malicious client from causing a multi-GB allocation or\n// billions of loop iterations with a crafted count.\n// Each response needs at least 4 bytes (length prefix).\nlet max_responses = r.remaining_len().saturating_add(3) / 4;\nlet n = (n as usize).min(max_responses);\nlet mut responses = Vec::with_capacity(n);\nfor _ in 0..n {\n    responses.push(Bytes::decode(r).ok())\n}\n```\n\nAn attacker can send `n = 0x10000000` (268M) or larger in a minimal packet (~50 bytes after encryption). The server attempts to allocate `n * ~24 bytes` (size of `Option\u003cBytes\u003e`) = ~6.4GB, causing an OOM crash.\n\n## Attack Flow\n\n1. Attacker connects via TCP, completes key exchange (no credentials needed -- this is the anonymous DH handshake, not authentication)\n2. Sends `USERAUTH_REQUEST` with method `keyboard-interactive`\n3. Server handler returns `Auth::Partial` with prompts (standard for 2FA/TOTP)\n4. Attacker sends `USERAUTH_INFO_RESPONSE` with `n = 0x10000000` and no response data\n5. Server calls `Vec::with_capacity(268_435_456)`, OOM killed\n\nNo authentication is required. The allocation occurs before the handler validates any credentials. The attack is repeatable faster than the server can restart.\n\n## Affected Configurations\n\nAny russh-based server where the `Handler::auth_keyboard_interactive` implementation returns `Auth::Partial` (i.e., sends prompts to the client). The default handler returns `Auth::reject()` and is not affected.\n\nSource code review suggests that downstream projects using keyboard-interactive for multi-step auth (e.g., TOTP/2FA) follow the affected pattern, since returning `Auth::Partial` before credential verification is the intended API usage for prompting.\n\n## Confirmed End-to-End PoC\n\nThere is a complete Docker-contained PoC confirming the OOM kill:\n- Minimal russh server returning `Auth::Partial` for keyboard-interactive\n- Python client (paramiko for key exchange) sends malformed `USERAUTH_INFO_RESPONSE`\n- Container with 512MB memory limit; server is OOM-killed (exit code 137)\n\nAvailable on request.\n\n## Proposed Fix\n\nCap the `Vec::with_capacity` allocation to what the remaining packet data can actually contain. Each response requires at least 4 bytes (length prefix), so:\n\n```rust\nlet n = map_err!(u32::decode(r))?;\n\n// Bound both allocation and iteration by remaining packet data to\n// prevent a malicious client from causing a multi-GB allocation or\n// billions of loop iterations with a crafted count.\n// Each response needs at least 4 bytes (length prefix).\nlet max_responses = r.remaining_len().saturating_add(3) / 4;\nlet n = (n as usize).min(max_responses);\nlet mut responses = Vec::with_capacity(n);\nfor _ in 0..n {\n    responses.push(Bytes::decode(r).ok())\n}\n```\n\nThis bounds the allocation to at most the packet size (~256KB), while preserving the existing behavior for well-formed packets. This fix has been implemented, tested, and contributed via the temporary private fork.\n\n## Severity\n\nPre-auth, remote, no credentials required, crashes the server process affecting all active sessions.","status":"published","published":"2026-04-24T15:39:37Z","modified":"2026-09-10T03:51:01.59471546Z","cvss":[{"version":"3.1","score":7.5,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H","level":"high"}],"cwes":["CWE-770","CWE-789"],"updated_at":"2026-11-02T15:46:46.006631Z"},{"id":"UBUNTU-CVE-2026-42189","description":"Russh is a Rust SSH client \u0026 server library. Prior to version 0.60.1, a pre-authentication denial-of-service vulnerability exists in the server's keyboard-interactive authentication handler. A malicious client can crash any russh-based server that implements keyboard-interactive auth (e.g., for 2FA/TOTP) with a single malformed packet, requiring no credentials. This issue has been patched in version 0.60.1.","status":"published","published":"2026-05-08T20:16:00Z","modified":"2026-10-17T12:58:54.478942575Z","cvss":[{"version":"3.1","score":7.5,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H","level":"high"}],"updated_at":"2026-11-02T16:56:18.393417Z"}]}}