Recently, a vulnerability known as CVE-2020-22218 has been discovered in the popular open-source software library libssh2 (version 1.10.). This security risk allows attackers to access out-of-bounds memory, potentially leading to information disclosure or application crashes. In this post, we'll dive into what this vulnerability means, the code snippet that leads to the issue, some original references, and exploit details.


libssh2 is a widely used library that provides an implementation of the SSHv2 protocol. Its primary purpose is to enable developers to add SSH support to their applications quickly and securely. The library supports a wide range of encryption and authentication methods and can be used with both client and server applications. The vulnerability CVE-2020-22218 specifically affects the "_libssh2_packet_add" function, which is commonly used for adding data packets to an SSH session.

Code Snippet

The issue is caused by a lack of proper boundary checks in the "_libssh2_packet_add" function, found in the "packet.c" source file. The relevant code snippet is:

/* packet.c */


static int
_libssh2_packet_add(LIBSSH2_SESSION *session, unsigned char *data,
                    size_t datalen, int macstate)
    memcpy(session->packAdd+session->packAdd_length, data, datalen);
    session->packAdd_length += datalen;

This code essentially copies the data received from a network packet into an internal buffer within the libssh2 session structure. The vulnerability stems from the fact that there are no checks in place to ensure that the amount of received data does not exceed the buffer size, which could lead to an out-of-bounds memory access.

Original References

1. The full vulnerability details can be found in the Common Vulnerabilities and Exposures (CVE) database entry for CVE-2020-22218:
2. The libssh2 GitHub repository contains the latest source code and information for the project:
3. The libssh2 mailing list is a valuable resource for understanding any discussions related to its security:

Exploit Details

An attacker can exploit this vulnerability by sending a specially crafted network packet that causes an out-of-bounds memory access. The malicious packet is designed to have a data payload larger than the expected buffer size, and it results in a potential information disclosure or application crashes. In some cases, the attacker might also be able to execute arbitrary commands on affected systems.

To protect your systems from this vulnerability, it is essential to apply security patches released by the libssh2 project. Developers should also follow best practices when using the libssh2 library to minimize the risk associated with this type of vulnerability. This includes implementing proper error handling, input validation, and buffer length checks, as well as ensuring the latest versions of the library are being used.


In summary, CVE-2020-22218 is a severe security vulnerability affecting the libssh2 library, which could potentially lead to out-of-bounds memory access and information disclosure. By understanding the issue's origins, analyzing the relevant code snippet, and being aware of the exploitation process, developers and security professionals can protect their systems and applications from potential harm. Regularly updating your software components and following best practices are crucial steps in ensuring a secure digital environment.


Published on: 08/22/2023 19:16:00 UTC
Last modified on: 10/06/2023 15:15:00 UTC