CVE-2026-46702: Russh SSH Compression Denial-of-Service Vulnerability – Patch to 0.61.1

Technical summary

The vulnerability exists in russh's SSH transport layer compression handling. Specifically, the library validates compressed packet sizes against RFC standards before decompression but does not enforce size limits on the decompressed output. An attacker can construct a malicious compressed payload that compresses to a small on-wire size (bypassing packet-length checks) but decompresses to a packet far exceeding safe limits. This bypasses the normal resource-exhaustion protections. Releases before 0.58.0 face compounded risk because they used CryptoVec for decompression, amplifying potential impact. The issue is rooted in CWE-770 (Allocation of Resources Without Limits or Throttling).

Business impact

For organizations deploying russh-based SSH services, this vulnerability creates a straightforward denial-of-service risk. An unauthenticated remote attacker can trigger resource exhaustion, causing service unavailability without requiring valid credentials or user interaction. The impact scales with the number of concurrent SSH sessions and available server memory. This is particularly concerning for SSH infrastructure supporting high-availability requirements or cloud platforms where service disruption directly affects revenue or compliance obligations. Patching is essential for any public-facing SSH deployment relying on russh.

Affected systems

The vulnerability affects russh versions 0.34.0 through 0.61.0. Any application or service directly embedding the russh library as a dependency is potentially at risk if: (1) SSH compression is explicitly enabled in the deployment configuration, and (2) the application accepts remote SSH connections. This includes custom SSH servers built with russh, CI/CD systems using russh-based tooling, and embedded SSH functionality in larger applications. Applications using versions 0.58.0 and earlier face elevated historical risk due to CryptoVec usage. Verify your dependency tree and configuration to confirm exposure.

Exploitability

Exploitability is high. The attack requires no authentication, no user interaction, and no special privileges—an attacker needs only network access to an SSH port and the ability to send crafted compressed packets. The CVSS 3.1 score of 7.5 (High severity, availability impact only) reflects this low barrier. Attack execution is straightforward: connect to the SSH service, enable compression negotiation, and transmit specially crafted compressed payloads designed to decompress into oversized packets. The vulnerability is not currently listed in CISA's Known Exploited Vulnerabilities catalog, but the simplicity of the attack vector makes exploitation trivial once understood.

Remediation

Upgrade russh to version 0.61.1 or later. This release implements proper decompression size validation to ensure expanded packets do not exceed safe limits. For applications unable to upgrade immediately, consider: (1) disabling SSH compression at the service configuration level if it is not critical to operations, (2) restricting SSH access via network-level controls (firewall rules, VPN requirements) to reduce the attack surface, and (3) implementing resource limits and monitoring to detect unusual decompression patterns. However, patching is the definitive remediation.

Patch guidance

Upgrade russh to version 0.61.1 or later via your package manager. For Rust projects using Cargo, update your Cargo.toml dependency declaration and run 'cargo update'. Verify the upgrade completes successfully and test SSH connectivity in a non-production environment before deploying to production. Review your release notes to confirm no breaking changes affect your application. If you maintain a custom fork or vendored copy of russh, apply the upstream patch from the official repository. Test thoroughly, as SSH library changes carry operational risk.

Detection guidance

Monitor for signs of decompression-based denial-of-service attacks: (1) SSH server logs showing abrupt connection drops or resource exhaustion warnings, (2) sudden spikes in CPU or memory usage correlated with SSH connection attempts, (3) compression negotiation attempts followed by immediate disconnections or timeouts. Network-level detection is challenging because the malicious packets are valid SSH protocol syntax until decompressed. Implement SSH connection rate limiting and per-connection resource quotas to mitigate impact. If available, enable detailed SSH library logging to observe packet sizes before and after decompression, then baseline normal behavior.

Why prioritize this

This vulnerability should be prioritized for immediate patching in any organization operating SSH services with russh. The combination of unauthenticated remote exploitability, ease of attack execution, direct availability impact, and the non-KEV status (indicating it may not yet be widely exploited or monitored) makes it a high-priority fix. Organizations with SSH exposed to the public internet should patch urgently; those with SSH restricted to trusted networks should still upgrade but may have slightly more scheduling flexibility.

Risk score, explained

The CVSS 3.1 score of 7.5 reflects High severity driven by a fully open attack vector (network, no authentication, no user interaction) and direct availability impact (denial-of-service). The score is not Critical (8.0+) because the impact is limited to availability; confidentiality and integrity are not affected. This is an appropriate and conservative scoring for a resource-exhaustion vulnerability affecting a foundational protocol component.

Frequently asked questions