CVE-2026-59843 in Red Hat
Summary
by MITRE • 07/21/2026
A flaw was found in libssh. A remote authenticated peer can advertise a zero maximum packet size in SSH_MSG_CHANNEL_OPEN, causing later channel writes to loop indefinitely and consume CPU, leading to denial of service.
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Analysis
by VulDB Data Team • 07/21/2026
This vulnerability exists within the libssh library implementation of the Secure Shell protocol, representing a classic denial of service weakness that can be exploited by authenticated remote attackers. The flaw manifests when a malicious peer establishes an SSH connection and subsequently advertises a zero maximum packet size in the SSH_MSG_CHANNEL_OPEN message. This specific condition triggers a critical behavioral anomaly within the channel management subsystem where subsequent write operations become trapped in an infinite loop, consuming excessive cpu cycles and ultimately rendering the target system unavailable to legitimate users.
The technical root cause of this vulnerability stems from inadequate input validation within the ssh channel handling logic. When libssh receives the SSH_MSG_CHANNEL_OPEN message containing a zero maximum packet size value, the library fails to properly validate this parameter before proceeding with channel operations. This validation gap creates an execution path where the channel write function continuously attempts to process data with invalid packet size constraints, leading to indefinite looping behavior that exhausts system resources. The vulnerability specifically affects the channel write functionality and can be triggered by any authenticated peer who establishes a connection to the vulnerable ssh service.
From an operational impact perspective, this vulnerability presents a significant risk to ssh server availability and system stability. An authenticated attacker can leverage this flaw to consume cpu resources indefinitely without requiring additional privileges or complex exploitation techniques. The denial of service condition affects the entire ssh daemon rather than individual connections, potentially disrupting legitimate user access and system administration functions. Network administrators may observe sustained high cpu utilization on affected systems while legitimate users experience connection failures or severe performance degradation.
Security professionals should note this vulnerability aligns with CWE-129 Input Validation and CWE-400 Uncontrolled Resource Consumption patterns, demonstrating how improper parameter validation can lead to resource exhaustion attacks. The threat landscape for this vulnerability includes automated scanning tools that may attempt to identify vulnerable systems, as well as targeted attacks from authenticated adversaries seeking to disrupt ssh services. Organizations implementing ssh services should consider this weakness in their risk assessments and prioritize patching efforts accordingly.
The recommended mitigation strategy involves applying the latest security patches from libssh maintainers, which typically include proper validation of packet size parameters during channel establishment. System administrators should also implement monitoring solutions to detect unusual cpu usage patterns that may indicate exploitation attempts. Network segmentation and access controls can provide additional defense in depth by limiting exposure of ssh services to authenticated peers who might attempt such attacks. Additionally, implementing connection rate limiting and resource utilization monitoring can help detect and respond to exploitation attempts before they cause significant service disruption.