CVE-2019-3856 in Libssh2
Summary
by MITRE
An integer overflow flaw, which could lead to an out of bounds write, was discovered in libssh2 before 1.8.1 in the way keyboard prompt requests are parsed. A remote attacker who compromises a SSH server may be able to execute code on the client system when a user connects to the server.
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Analysis
by VulDB Data Team • 08/07/2023
The vulnerability identified as CVE-2019-3856 represents a critical integer overflow flaw within the libssh2 library version 1.8.0 and earlier, specifically affecting the parsing of keyboard prompt requests during SSH authentication processes. This flaw resides in the library's handling of authentication prompts that are typically used in keyboard-interactive authentication methods, where the server sends a request to the client for additional authentication information such as passwords or one-time codes. The integer overflow occurs when the library processes the length field of these keyboard prompt requests without proper bounds checking, creating a scenario where an attacker can manipulate the data structure to cause memory corruption. This vulnerability falls under the CWE-190 category of Integer Overflow or Wraparound, which is a well-documented weakness in software security that occurs when an integer value exceeds its maximum representable value and wraps around to a smaller value, often causing unexpected behavior in memory management operations.
The operational impact of this vulnerability is severe and directly relates to the privilege escalation and code execution capabilities it provides to remote attackers. When a victim system connects to a compromised SSH server, the malicious server can craft specially crafted keyboard prompt responses that trigger the integer overflow condition. This overflow can result in an out-of-bounds write operation, where the library attempts to write data beyond the allocated memory boundaries of the keyboard prompt buffer. Such memory corruption can be exploited to overwrite adjacent memory locations, potentially including return addresses, function pointers, or other critical control data structures. The attack vector is particularly dangerous because it leverages the legitimate SSH client functionality, making it difficult to detect through standard network monitoring or intrusion detection systems. According to ATT&CK framework, this vulnerability maps to T1021.004 (SSH) and T1059.007 (Command and Scripting Interpreter: PowerShell) as it enables remote code execution through established SSH protocols and can be used to establish persistent access to compromised systems.
The exploitation of CVE-2019-3856 requires a remote attacker to have compromised access to an SSH server that is being used by victims to connect to. The attack chain begins with the attacker controlling the SSH server and sending malicious keyboard prompt requests to the client during authentication. The vulnerability is particularly concerning because it affects widely deployed SSH client libraries used across numerous operating systems and applications, including OpenSSH clients, various SSH GUI tools, and embedded systems that utilize libssh2 for SSH functionality. Organizations using affected versions of libssh2 are at risk regardless of their network security measures, as the vulnerability can be triggered without requiring additional network privileges or complex attack chains. The memory corruption resulting from the out-of-bounds write can potentially be leveraged to execute arbitrary code with the privileges of the user running the SSH client, which could range from simple information disclosure to full system compromise depending on the target environment. System administrators should prioritize updating their libssh2 installations to version 1.8.1 or later, as this release contains the necessary patches to prevent the integer overflow condition. Additionally, organizations should implement network segmentation, SSH server hardening measures, and monitor for unusual authentication patterns that might indicate exploitation attempts. The vulnerability highlights the importance of proper input validation and bounds checking in cryptographic libraries, as these components often handle sensitive data and are prime targets for sophisticated attacks.