CVE-2026-59843: libssh Denial of Service Vulnerability

CVE-2026-59843 Overview

CVE-2026-59843 is a denial of service vulnerability in libssh, a C library implementing the SSHv2 protocol. A remote authenticated peer can advertise a zero maximum packet size in an SSH_MSG_CHANNEL_OPEN message. This value causes later channel write operations to loop indefinitely and consume CPU resources. The flaw is classified under CWE-400 (Uncontrolled Resource Consumption).

The issue affects libssh deployments where authenticated peers can open channels. Red Hat has published errata and tracking bugs for this issue in enterprise Linux distributions shipping libssh.

Critical Impact
An authenticated remote peer can trigger an infinite loop in channel write code, exhausting CPU and causing service unavailability for SSH-dependent applications.

Affected Products

libssh library (versions prior to the fixed release)
Red Hat Enterprise Linux packages shipping libssh (see RHSA-2026:42922)
Applications and services embedding libssh for SSH client or server functionality

Discovery Timeline

2026-07-21 - CVE-2026-59843 published to NVD
2026-07-22 - Last updated in NVD database

Technical Details for CVE-2026-59843

Vulnerability Analysis

The vulnerability resides in libssh channel handling logic. When a peer opens a channel using SSH_MSG_CHANNEL_OPEN, the message includes a maximum_packet_size parameter declared by the initiator. libssh does not validate that this value is greater than zero before using it as a divisor or loop bound in subsequent channel write operations.

When a channel write proceeds with a zero maximum packet size, the write loop cannot make forward progress. Each iteration attempts to send zero bytes and repeats without terminating. This produces a tight CPU loop within the process handling the SSH session.

The attacker must be authenticated to the SSH endpoint, which limits exposure to environments where SSH access is granted to untrusted or semi-trusted users. Successful exploitation results in CPU exhaustion and denial of service for the affected process. Confidentiality and integrity are not impacted.

Root Cause

The root cause is missing input validation on the maximum_packet_size field of the SSH_MSG_CHANNEL_OPEN message. libssh accepts a peer-supplied value of zero and uses it in channel write logic without a lower bound check. This maps to [CWE-400] uncontrolled resource consumption.

Attack Vector

Exploitation requires network access to a libssh endpoint and valid credentials to authenticate. After authentication, the attacker sends a channel open request advertising a maximum packet size of zero. When the application writes data to that channel, libssh enters an unbounded loop. The vulnerability is exploitable across trust boundaries only where SSH authentication is expected to precede fully privileged access.

No verified proof-of-concept code is publicly available at the time of this writing. See the Red Hat CVE record and bug report #2498176 for upstream analysis.

Detection Methods for CVE-2026-59843

Indicators of Compromise

Sustained high CPU utilization by processes linked against libssh.so following an authenticated SSH session
SSH server or client processes that become unresponsive but do not crash
Session logs showing SSH_MSG_CHANNEL_OPEN from a peer immediately followed by process CPU spikes

Detection Strategies

Monitor per-process CPU usage of services embedding libssh and alert on sustained saturation of a single core
Inspect SSH protocol traces for SSH_MSG_CHANNEL_OPEN messages containing a maximum_packet_size value of zero
Correlate authenticated session events with CPU anomaly telemetry to identify malicious channel open patterns

Monitoring Recommendations

Deploy resource utilization monitoring on all hosts running SSH servers or clients that use libssh
Enable verbose libssh logging in exposed services to capture channel negotiation parameters
Track authentication events and follow-on session behavior for accounts with SSH access to critical infrastructure

How to Mitigate CVE-2026-59843

Immediate Actions Required

Apply vendor patches from Red Hat and other distributions shipping libssh (see RHSA-2026:42922)
Restrict SSH authentication to trusted users and enforce strong credential policies
Restart services linked against libssh after applying the updated library to ensure the patched code path is loaded

Patch Information

Red Hat has released fixed packages via RHSA-2026:42922. Reference the Red Hat CVE page for the authoritative list of updated package versions. Consult upstream libssh releases and downstream distribution advisories for non-Red Hat environments.

Workarounds

Limit SSH access to authenticated users from trusted network segments until the patch is applied
Impose per-process CPU limits using cgroups or systemd resource controls to contain the impact of an exploitation attempt
Configure connection and session rate limits on SSH endpoints to reduce the surface for authenticated abuse