CVE-2026-59850 in libssh
Summary
by MITRE • 07/21/2026
A flaw was found in libssh. If data packets are processed after a channel is closed, channel data callbacks can be invoked after the associated data has already been freed, leading to crashes or possible use-after-free conditions.
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Analysis
by VulDB Data Team • 07/21/2026
This vulnerability exists within the libssh library, a widely used open-source implementation of the ssh protocol that provides secure remote access capabilities across numerous applications and systems. The flaw represents a classic use-after-free condition that occurs when the library fails to properly manage memory references during channel lifecycle operations. When a channel is closed but pending data packets continue to be processed, the system attempts to invoke channel data callbacks on memory locations that have already been deallocated, creating a dangerous state where arbitrary code execution or system crashes may occur.
The technical root cause of this vulnerability stems from inadequate synchronization and memory management within the ssh channel handling mechanisms. According to CWE-416, this represents a use-after-free vulnerability where resources are accessed after they have been freed, specifically affecting the callback invocation system that manages channel data processing. The flaw manifests when the library does not properly track whether a channel remains valid during asynchronous data packet processing, allowing callbacks to execute against deallocated memory structures. This type of memory corruption vulnerability is particularly dangerous because it can be exploited to achieve remote code execution or denial of service conditions.
The operational impact of this vulnerability extends across numerous systems that depend on libssh for secure communications, including but not limited to network infrastructure devices, authentication servers, and remote access applications. Attackers could potentially exploit this condition by sending malicious data packets to a closed channel, triggering the use-after-free scenario that leads to system instability or unauthorized code execution. The vulnerability affects both client and server implementations of the ssh protocol within the libssh library, making it particularly concerning for organizations relying on ssh-based authentication and communication protocols.
Mitigation strategies should focus on immediate patching of affected libssh versions, implementing proper input validation and channel state checking before callback execution, and deploying monitoring systems to detect anomalous channel closure patterns. Organizations should also consider implementing network segmentation and access controls to limit exposure to potential exploitation vectors. The vulnerability aligns with ATT&CK technique T1078 which covers valid accounts and legitimate credentials for persistence and privilege escalation. Additionally, implementing proper memory safety mechanisms such as bounds checking and automatic resource management would help prevent similar issues in the future. Regular security audits of cryptographic libraries and maintaining updated versions are essential practices to address such vulnerabilities in production environments.