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

CISA KEV is observed exploitation evidence. FIRST EPSS is a dated prediction of exploitation in the next 30 days. Missing scores are unknown. This collector requests predictions only for explicit CVE aliases in the public NuGet archive; Microsoft release views retain their existing prediction scope.

No collected risk observation for these identifiers. This does not establish absence of exploitation or a zero probability.

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.

Source published

8 May 2026 19:49:51 UTC

Source modified

11 May 2026 14:23:49 UTC

Observed here

23 Oct 2026 15:48:27 UTC

Source terms

CVE Program Terms of Use

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

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.

Source published

May 8, 2026, 7:49:51 PM (timezone not stated)

Source modified

May 11, 2026, 2:23:49 PM (timezone not stated)

Observed here

7 Nov 2026 09:32:22 UTC

Source terms

ENISA EUVD · source terms and attribution apply

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

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.

Source published

8 May 2026 20:16:31 UTC

Source modified

26 Oct 2026 18:10:00 UTC

Observed here

27 Oct 2026 11:12:01 UTC

Source terms

NIST NVD usage terms; underlying source attribution retained

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This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: nvd · NIST NVD usage terms; underlying source attribution retained. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS 3.1 · [email protected] · Secondary · 7.5 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

## 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()`: ```rust 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 1. Attacker connects via TCP, completes key exchange (no credentials needed -- this is the anonymous DH handshake, not authentication) 2. Sends `USERAUTH_REQUEST` with method `keyboard-interactive` 3. Server handler returns `Auth::Partial` with prompts (standard for 2FA/TOTP) 4. Attacker sends `USERAUTH_INFO_RESPONSE` with `n = 0x10000000` and no response data 5. 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: ```rust 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.

Source published

24 Apr 2026 15:39:37 UTC

Source modified

10 Sep 2026 03:51:01 UTC

Observed here

24 Oct 2026 13:19:54 UTC

Source terms

GitHub Advisory Database · CC-BY-4.0

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This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: osv · GitHub Advisory Database · CC-BY-4.0. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS_V3 · CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

Publisher rating · HIGH