In recent times, a critical vulnerability has been found in the OpenSSH server (sshd) version 9.1. With CVE-2023-25136, a double-free vulnerability has been identified during the handling of options.kex_algorithms in the default configuration. Although the vulnerability is fixed in OpenSSH 9.2, attackers looking to exploit earlier versions could potentially execute remote code, posing a significant security risk.

The objective of this post is to delve deeper into the vulnerability, examine the code snippets, and discuss possible exploit details. Let's start by understanding the problem at hand.

Understanding the Double-Free Vulnerability

Commonly observed in software written in languages like C and C++, a double-free vulnerability can occur when the developer mistakenly frees an already freed memory location, causing memory corruption or unexpected behavior. This mistake can be leveraged by attackers to execute arbitrary code, leading to remote code execution in certain scenarios.

In OpenSSH server 9.1, the aforementioned vulnerability resides in the way options.kex_algorithms are managed. Due to this flaw, an unauthenticated remote attacker can use the vulnerability to jump to any location within the sshd address space. To further put things into perspective, a third-party report claims that, theoretically, remote code execution is possible.

Analyzing the Code Snippet

The issue stems from the options.kex_algorithms handling in the OpenSSH server 9.1. Here is a simplified code snippet that demonstrates the vulnerability:

// Simplified version of the actual sshd code in OpenSSH 9.1
void free_kex_algorithms(struct ssh *ssh) {
    free(ssh->kex->kex_algorithms);
    ssh->kex->kex_algorithms = NULL;
}

int main() {
    struct ssh *ssh;
    // Initialize ssh and kex structures
    // Set initial values for kex_algorithms

    // ... Some other code

    free_kex_algorithms(ssh);
    ssh->kex->kex_algorithms = NULL;

    // ... Some other code

    free_kex_algorithms(ssh);

    return ;
}

In this code snippet, free_kex_algorithms() frees the kex_algorithms pointer and sets it to NULL. The problem arises when free_kex_algorithms() is called twice, causing the double-free vulnerability.

For your reference, the official OpenSSH website provides a detailed description of the vulnerability and its fix in version 9.2. Here are the sources:

- OpenSSH 9.2 Release Notes
- OpenSSH Source Repository

Exploit Details

The exploit details for CVE-2023-25136 are not widely known, and remote code execution is considered only theoretically possible. Nonetheless, it is essential to prioritize upgrading to OpenSSH 9.2 or later to avert any potential threats that may arise due to this vulnerability.

Additionally, administrators should keep monitoring the latest security updates and threats continuously. By regularly updating your software, you are ensuring the protection of your systems against any new vulnerabilities and threat actors.

Conclusion

The double-free vulnerability in the OpenSSH server 9.1 is a pressing issue that requires immediate attention. By upgrading to OpenSSH 9.2 and implementing security best practices, administrators can significantly reduce the risk of exploitation. It is essential to maintain a proactive approach to cybersecurity and always be vigilant to potential threats to ensure the safety of our systems and data.

Timeline

Published on: 02/03/2023 06:15:00 UTC
Last modified on: 03/09/2023 19:15:00 UTC