CVE-2019-3859 in Libssh2
Summary
by MITRE
An out of bounds read flaw was discovered in libssh2 before 1.8.1 in the _libssh2_packet_require and _libssh2_packet_requirev functions. A remote attacker who compromises a SSH server may be able to cause a Denial of Service or read data in the client memory.
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Analysis
by VulDB Data Team • 12/18/2025
The vulnerability identified as CVE-2019-3859 represents a critical out-of-bounds read flaw within the libssh2 library version 1.8.0 and earlier. This security weakness resides in the packet handling mechanisms of the SSH client implementation, specifically within the _libssh2_packet_require and _libssh2_packet_requirev functions. The flaw arises from insufficient input validation and boundary checking when processing SSH protocol packets, creating a scenario where maliciously crafted responses from a compromised SSH server can trigger memory access violations.
The technical implementation of this vulnerability stems from improper handling of packet length fields and sequence number validation within the SSH protocol stack. When the vulnerable functions process incoming SSH packets, they fail to adequately validate the packet boundaries before attempting to read memory regions. This allows an attacker who has gained control of an SSH server to craft specially malformed packets that cause the client application to read beyond allocated memory buffers. The flaw operates at the protocol parsing layer, making it particularly dangerous as it can be exploited during normal SSH client-server communication without requiring any special privileges or authentication.
From an operational impact perspective, this vulnerability creates significant risk for any system utilizing libssh2 versions prior to 1.8.1 in SSH client implementations. The primary attack vectors involve scenarios where an attacker has compromised an SSH server and can manipulate the communication channel to send malicious packets to SSH clients. This could result in denial of service conditions where client applications crash or become unresponsive, as well as potential information disclosure through memory read operations that might expose sensitive data from the client's memory space. The vulnerability affects a wide range of applications including automated deployment tools, configuration management systems, and any software that relies on libssh2 for SSH connectivity.
The security implications extend beyond simple service disruption to include potential data leakage and system compromise. This vulnerability aligns with CWE-129, which addresses improper validation of length fields, and relates to ATT&CK technique T1071.004 for application layer protocol manipulation. Organizations using vulnerable versions of libssh2 should prioritize immediate patching to version 1.8.1 or later, as this update includes proper bounds checking and input validation mechanisms. Additionally, network segmentation and monitoring of SSH traffic can help detect potential exploitation attempts, while implementing strict SSH server authentication controls can reduce the attack surface where such compromises might occur. The vulnerability demonstrates the critical importance of proper input validation in cryptographic libraries and highlights the need for robust memory safety practices in security-critical components.