CVE-2019-3858 in Libssh2
Summary
by MITRE
An out of bounds read flaw was discovered in libssh2 before 1.8.1 when a specially crafted SFTP packet is received from the server. 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 • 08/04/2023
The vulnerability identified as CVE-2019-3858 represents a critical out-of-bounds read flaw within the libssh2 library version 1.8.0 and earlier. This issue specifically manifests when the library processes specially crafted SFTP packets received from a compromised SSH server. The flaw stems from inadequate input validation and boundary checking within the SFTP protocol implementation, creating a scenario where maliciously constructed packets can trigger memory access violations. The vulnerability is particularly concerning as it exists within a widely deployed cryptographic library that facilitates secure remote connections and file transfers across numerous applications and systems.
From a technical perspective, this vulnerability operates through a classic buffer over-read condition where the libssh2 library fails to properly validate the length and structure of incoming SFTP packets before attempting to process them. When a malicious server sends a crafted packet that exceeds expected boundaries, the library's parsing routines attempt to read memory locations beyond the allocated buffer space. This behavior can result in information disclosure, as the read operation may inadvertently expose sensitive data from adjacent memory regions, or it can cause the application to crash due to accessing invalid memory addresses. The flaw is categorized under CWE-125 as an out-of-bounds read, which represents a fundamental memory safety issue that can lead to both denial of service and potential information leakage.
The operational impact of CVE-2019-3858 extends beyond simple service disruption, as it enables remote attackers to exploit compromised SSH servers to target client systems running vulnerable versions of libssh2. This creates a sophisticated attack vector where an adversary who has already gained access to a SSH server can leverage this vulnerability to either cause persistent denial of service against client applications or to extract confidential information from the client's memory space. The attack requires the victim to be using a vulnerable version of libssh2 and to establish an SFTP connection to the compromised server, making it a targeted rather than mass-exploitable vulnerability. However, given the widespread use of libssh2 in various SSH client implementations, the potential attack surface remains substantial. The vulnerability aligns with ATT&CK technique T1021.004 for remote services and T1046 for network service scanning, as it enables attackers to leverage compromised SSH infrastructure for further exploitation.
Mitigation strategies for CVE-2019-3858 primarily focus on upgrading to libssh2 version 1.8.1 or later, where the out-of-bounds read vulnerability has been addressed through proper input validation and boundary checking. Organizations should prioritize patching all systems that utilize vulnerable versions of the library, particularly those handling SFTP connections to external servers. Additional defensive measures include implementing network segmentation to limit exposure, monitoring for unusual SFTP traffic patterns, and employing network-based intrusion detection systems to identify potential exploitation attempts. Security teams should also consider implementing strict certificate validation and using alternative secure communication protocols where possible to reduce reliance on potentially vulnerable SFTP implementations. The vulnerability demonstrates the critical importance of maintaining up-to-date cryptographic libraries and highlights the need for comprehensive vulnerability management programs that address both known and emerging threats in the SSH ecosystem.