CVE-2023-28531 is a security vulnerability present in the 'ssh-add' feature of OpenSSH versions before 9.3. OpenSSH is an open-source suite of connectivity tools that provide encrypted communication sessions over a computer network using the Secure Shell (SSH) protocol. When using smartcard keys, ssh-add lacks the proper per-hop destination constraints enforcement intended by the user. As a result, an attacker can potentially exploit this vulnerability to bypass these constraints, leading to unauthorized access to data or networks.

In this article, we will discuss the details of this vulnerability, provide code snippets to illustrate the flaw, and outline the resolution steps. We will also include links to original references and documentation.

Description of Vulnerability

The vulnerability in ssh-add can occur when a user adds a smartcard key to the ssh-agent. Smartcard keys provide strong cryptographic authentication for SSH connections. The intent is for these keys to be added to the agent with per-hop destination constraints specified by the user, which limits the key usage to specific hosts or networks. However, due to a programming error in OpenSSH versions before 9.3, these constraints are not enforced as intended.

By exploiting this vulnerability, an attacker could impersonate the user to gain access to protected SSH resources without respecting the constraints originally set forth by the user.

Here's a code snippet illustrating the problem

# Adding the smartcard key to ssh-agent with intended constraints
ssh-add -c -s /path/to/smartcard_so -D "allowed-destination" /path/to/key

# ssh-add does not enforce constraints correctly
# Attacker can still use keys outside "allowed-destination"

References to Original Documentation and Exploit Details

Further technical details on this vulnerability can be found in the original documentation and references:

1. The official OpenSSH advisory for this vulnerability - OpenSSH Advisory
2. NIST's National Vulnerability Database (NVD) entry for CVE-2023-28531 - NVD Entry
3. A detailed write-up explaining the vulnerability and exploit details by a security researcher Security Researcher's Blog

Resolving the Vulnerability

The recommended resolution for this vulnerability is to upgrade OpenSSH to version 9.3 or later, where the issue has been fixed. Users can do this by following the steps below:

Check your current OpenSSH version

ssh -V

2. If your OpenSSH version is earlier than 9.3, update it to the latest version available for your operating system:

# Debian/Ubuntu-based systems:
sudo apt-get update
sudo apt-get upgrade openssh-client openssh-server

# RedHat/CentOS/Fedora-based systems:
sudo yum update openssh

# OpenSUSE-based systems:
sudo zypper update openssh

Restart the ssh-agent process and reload the system's SSH configuration

sudo systemctl restart ssh-agent
sudo systemctl reload ssh

Re-add your smartcard keys to the ssh-agent with the correct constraints

ssh-add -c -s /path/to/smartcard_so -D "allowed-destination" /path/to/key

Conclusion

CVE-2023-28531 is a serious vulnerability in the ssh-add feature of OpenSSH versions prior to 9.3. Users should update their OpenSSH installations to protect themselves from potential unauthorized access to their data or networks. The guidance in this article should help affected users understand the nature of the issue and resolve it effectively.

Timeline

Published on: 03/17/2023 04:15:00 UTC
Last modified on: 04/05/2023 17:44:00 UTC