CVE-2019-3855 in Xcode
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 packets are read from the server. 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 • 12/17/2025
The vulnerability identified as CVE-2019-3855 represents a critical integer overflow flaw within the libssh2 library version 1.8.0 and earlier, which operates at the core of secure shell communications. This issue specifically affects the packet reading mechanism when clients establish connections to ssh servers, creating a potential pathway for remote code execution attacks. The flaw manifests in how the library handles packet size calculations during the secure communication process, where an attacker can manipulate the data flow to trigger unintended memory operations.
The technical implementation of this vulnerability stems from improper handling of integer arithmetic within the packet processing routines of libssh2. When a malicious ssh server sends specially crafted packets with manipulated size fields, the library fails to properly validate these values before performing memory allocation or access operations. This integer overflow condition occurs during the calculation of packet buffer sizes, where the overflowed value can subsequently be used as an index or size parameter for memory operations. The vulnerability is classified under CWE-190 as an integer overflow in the calculation of buffer sizes, which directly leads to memory corruption conditions. The flaw enables attackers to write data beyond the bounds of allocated memory regions, creating opportunities for arbitrary code execution.
The operational impact of this vulnerability extends beyond simple data corruption, as it allows remote attackers to execute arbitrary code on vulnerable client systems when users establish connections to compromised ssh servers. This attack vector is particularly dangerous because it requires no local privileges or user interaction beyond the normal ssh connection process, making it an ideal candidate for automated exploitation. The vulnerability affects any system that utilizes libssh2 for ssh client functionality, including various operating systems, applications, and network devices that depend on this library for secure communications. The attack scenario involves a compromised ssh server sending maliciously crafted packets that trigger the integer overflow, leading to memory corruption that can be leveraged to gain control over the client system.
Mitigation strategies for CVE-2019-3855 primarily focus on updating the libssh2 library to version 1.8.1 or later, where the integer overflow conditions have been properly addressed through enhanced input validation and proper integer arithmetic handling. System administrators should prioritize patching all affected systems and applications that utilize libssh2, particularly those that establish ssh connections to untrusted servers. Additional protective measures include implementing network segmentation to limit exposure to potentially compromised ssh servers, configuring ssh clients to use strict host key checking, and monitoring network traffic for suspicious packet patterns. Organizations should also consider implementing intrusion detection systems that can identify potential exploitation attempts targeting this vulnerability. The ATT&CK framework categorizes this vulnerability under T1059 for execution through remote access tools and T1210 for exploitation of remote services, highlighting the need for comprehensive defensive strategies. Security teams should conduct thorough vulnerability assessments to identify all systems using vulnerable versions of libssh2 and ensure proper patch management processes are in place to prevent similar issues from arising in the future.