Vulnerability-Lookup

CVE-2026-42189 (GCVE-0-2026-42189)

Vulnerability from cvelistv5 – Published: 2026-05-08 19:49 – Updated: 2026-05-11 14:23

Title

Russh: Pre-auth DoS via unbounded allocation in keyboard-interactive auth

Summary

Russh is a Rust SSH client & 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 ?

7.5 (High)


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

CWE

CWE-770 - Allocation of Resources Without Limits or Throttling
CWE-789 - Memory Allocation with Excessive Size Value

Assigner

GitHub_M

References

3 references

URL	Tags
https://github.com/Eugeny/russh/security/advisori…	x_refsource_CONFIRM
https://github.com/Eugeny/russh/commit/6c3c80a9b6…	x_refsource_MISC
https://github.com/Eugeny/russh/releases/tag/v0.60.1	x_refsource_MISC

Impacted products

1 product

Vendor	Product	Version
Eugeny	russh	Affected: < 0.60.1

Show details on NVD website

JSON

To clipboard

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FKIE_CVE-2026-42189

Vulnerability from fkie_nvd - Published: 2026-05-08 20:16 - Updated: 2026-05-14 18:07

Severity ?

7.5 (High) - CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

Summary

References

URL	Tags
security-advisories@github.com	https://github.com/Eugeny/russh/commit/6c3c80a9b6d60763d6227d60fa8310e57172a4d1	Patch
security-advisories@github.com	https://github.com/Eugeny/russh/releases/tag/v0.60.1	Product, Release Notes
security-advisories@github.com	https://github.com/Eugeny/russh/security/advisories/GHSA-f5v4-2wr6-hqmg	Exploit, Mitigation, Vendor Advisory
134c704f-9b21-4f2e-91b3-4a467353bcc0	https://github.com/Eugeny/russh/security/advisories/GHSA-f5v4-2wr6-hqmg	Exploit, Mitigation, Vendor Advisory

Impacted products

	Vendor	Product	Version
	russh_project	russh	*
	warpgate_project	warpgate	*

JSON

To clipboard

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GHSA-F5V4-2WR6-HQMG

Vulnerability from github – Published: 2026-04-24 15:39 – Updated: 2026-05-13 13:34

Summary

russh has pre-auth DoS via unbounded allocation in its keyboard-interactive auth handler

Details

Summary

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.

Vulnerability Details

In 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():

let n = map_err!(u32::decode(r))?;

// Bound both allocation and iteration by remaining packet data to
// prevent a malicious client from causing a multi-GB allocation or
// billions of loop iterations with a crafted count.
// Each response needs at least 4 bytes (length prefix).
let max_responses = r.remaining_len().saturating_add(3) / 4;
let n = (n as usize).min(max_responses);
let mut responses = Vec::with_capacity(n);
for _ in 0..n {
    responses.push(Bytes::decode(r).ok())
}

An 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<Bytes>) = ~6.4GB, causing an OOM crash.

Attack Flow

Attacker connects via TCP, completes key exchange (no credentials needed -- this is the anonymous DH handshake, not authentication)
Sends USERAUTH_REQUEST with method keyboard-interactive
Server handler returns Auth::Partial with prompts (standard for 2FA/TOTP)
Attacker sends USERAUTH_INFO_RESPONSE with n = 0x10000000 and no response data
Server calls Vec::with_capacity(268_435_456), OOM killed

No authentication is required. The allocation occurs before the handler validates any credentials. The attack is repeatable faster than the server can restart.

Affected Configurations

Any 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.

Source 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.

Confirmed End-to-End PoC

There is a complete Docker-contained PoC confirming the OOM kill: - Minimal russh server returning Auth::Partial for keyboard-interactive - Python client (paramiko for key exchange) sends malformed USERAUTH_INFO_RESPONSE - Container with 512MB memory limit; server is OOM-killed (exit code 137)

Available on request.

Proposed Fix

Cap the Vec::with_capacity allocation to what the remaining packet data can actually contain. Each response requires at least 4 bytes (length prefix), so:

let n = map_err!(u32::decode(r))?;

// Bound both allocation and iteration by remaining packet data to
// prevent a malicious client from causing a multi-GB allocation or
// billions of loop iterations with a crafted count.
// Each response needs at least 4 bytes (length prefix).
let max_responses = r.remaining_len().saturating_add(3) / 4;
let n = (n as usize).min(max_responses);
let mut responses = Vec::with_capacity(n);
for _ in 0..n {
    responses.push(Bytes::decode(r).ok())
}

This 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.

Severity

Pre-auth, remote, no credentials required, crashes the server process affecting all active sessions.

Severity ?

7.5 (High)


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

Show details on source website

JSON

To clipboard

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      "package": {
        "ecosystem": "crates.io",
        "name": "russh"
      },
      "ranges": [
        {
          "events": [
            {
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          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-42189"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-770",
      "CWE-789"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-04-24T15:39:37Z",
    "nvd_published_at": "2026-05-08T20:16:31Z",
    "severity": "HIGH"
  },
  "details": "## Summary\n\nA pre-authentication denial-of-service vulnerability exists in the server\u0027s 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\u0027s `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.",
  "id": "GHSA-f5v4-2wr6-hqmg",
  "modified": "2026-05-13T13:34:53Z",
  "published": "2026-04-24T15:39:37Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/Eugeny/russh/security/advisories/GHSA-f5v4-2wr6-hqmg"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-42189"
    },
    {
      "type": "WEB",
      "url": "https://github.com/Eugeny/russh/commit/6c3c80a9b6d60763d6227d60fa8310e57172a4d1"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/Eugeny/russh"
    },
    {
      "type": "WEB",
      "url": "https://github.com/Eugeny/russh/releases/tag/v0.60.1"
    }
  ],
  "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 has pre-auth DoS via unbounded allocation in its keyboard-interactive auth handler"
}

Sightings

Author	Source	Type	Date	Other

Nomenclature

Seen: The vulnerability was mentioned, discussed, or observed by the user.
Confirmed: The vulnerability has been validated from an analyst's perspective.
Published Proof of Concept: A public proof of concept is available for this vulnerability.
Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
Not confirmed: The user expressed doubt about the validity of the vulnerability.
Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.

Detection rules are retrieved from Rulezet.

Action not permitted

CVE-2026-42189 (GCVE-0-2026-42189)

FKIE_CVE-2026-42189

GHSA-F5V4-2WR6-HQMG

Summary

Vulnerability Details

Attack Flow

Affected Configurations

Confirmed End-to-End PoC

Proposed Fix

Severity

Tags

Sightings

Nomenclature