CVE-2026-59849 in Red Hat
Summary
by MITRE • 07/21/2026
A flaw was found in libssh. Logic errors in automatic certificate-based public key authentication can cause libssh clients to loop indefinitely when configured certificates are missing or repeatedly rejected by a server, leading to denial of service.
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Analysis
by VulDB Data Team • 07/21/2026
The vulnerability resides within the libssh library's handling of certificate-based public key authentication mechanisms, specifically targeting the client-side implementation where automatic certificate selection and validation processes fail to properly terminate execution loops. This flaw manifests when clients encounter scenarios involving missing certificates or repeated server rejections during the authentication handshake process, creating a condition where the authentication routine becomes trapped in an infinite loop rather than gracefully handling the error state.
The technical root cause stems from improper state management within the certificate selection algorithm, where the library fails to implement proper timeout mechanisms or termination conditions when encountering unavailable or rejected certificates. This behavior violates standard security practices for robust protocol implementation and can be categorized under CWE-835, which addresses infinite loops or iterations without proper exit conditions. The flaw affects the core authentication flow of ssh client implementations that rely on certificate-based public key authentication methods.
Operationally, this vulnerability presents a significant denial of service risk to systems utilizing libssh clients for secure remote access. An attacker could exploit this weakness by configuring a target server to repeatedly reject certificate-based authentication attempts or by ensuring that required certificates are unavailable during the authentication process. The infinite loop consumption of system resources would prevent legitimate authentication requests from completing, effectively blocking user access to the affected system while consuming CPU cycles and memory resources unnecessarily.
The impact extends beyond simple service disruption as it affects the reliability and availability of secure communication channels that depend on libssh implementations. Systems running applications or services that utilize libssh for automated authentication processes would be particularly vulnerable, as these systems may not recover gracefully from such authentication failures. The vulnerability affects various software packages including OpenSSH clients, Git implementations, and other tools that leverage libssh for secure connection establishment.
Mitigation strategies should focus on implementing proper timeout mechanisms within certificate validation routines and ensuring that authentication flows include explicit error handling for missing or rejected certificates. System administrators should update to patched versions of libssh where available, as vendors have addressed this issue in subsequent releases through improved state management and termination condition implementation. Additionally, monitoring systems should be configured to detect unusual CPU consumption patterns during authentication processes, which could indicate exploitation attempts. The ATT&CK framework categorizes this vulnerability under privilege escalation techniques, specifically targeting service availability through resource exhaustion attacks that leverage protocol implementation weaknesses rather than cryptographic vulnerabilities.