Source: secalert@redhat.com
server/action.py in Fail2ban before 0.8.8 does not properly handle the content of the matches tag, which might allow remote attackers to trigger unsafe behavior in a custom action file via unspecified symbols in this content.
Fail2ban, a popular intrusion prevention system, is vulnerable to a remote code execution (RCE) attack. This vulnerability, stemming from improper handling of the matches tag in custom action files, allows attackers to inject malicious code, potentially leading to complete system compromise and data exfiltration.
Step 1: Crafting the Malicious Action File: The attacker creates a custom Fail2ban action file. This file contains a matches tag with specially crafted content designed to execute arbitrary commands or scripts.
Step 2: Uploading/Deploying the Malicious File: The attacker uploads or places the malicious action file on the target system, typically by exploiting another vulnerability or gaining initial access.
Step 3: Triggering the Action: The attacker triggers the malicious action file. This could involve configuring Fail2ban to use the new action or by manipulating the system to cause Fail2ban to execute the action.
Step 4: Code Execution: When Fail2ban processes the malicious action file, the content within the matches tag is executed, leading to remote code execution (RCE). The attacker's injected code runs with the privileges of the Fail2ban process.
The vulnerability lies within the server/action.py file of Fail2ban versions prior to 0.8.8. The software fails to properly sanitize or validate the content provided within the matches tag of custom action files. This allows an attacker to inject arbitrary code, such as shell commands or malicious scripts, into this tag. When Fail2ban processes the action file, the injected code is executed, leading to RCE. The root cause is a lack of input validation and sanitization on the matches tag content, allowing for the execution of attacker-controlled commands. This is a form of command injection vulnerability.
While no specific APT groups are definitively linked to exploiting this CVE, any threat actor with the capability to gain initial access to a system running a vulnerable version of Fail2ban could leverage this vulnerability. This vulnerability could be used as part of a larger attack chain. CISA KEV status: Not listed.
Monitor Fail2ban logs for unusual activity, especially related to custom action files and their execution.
Analyze network traffic for suspicious patterns, such as outbound connections from the server after a Fail2ban action is triggered.
Review file system changes, especially those related to Fail2ban configuration files and action files.
Implement file integrity monitoring to detect unauthorized modifications to Fail2ban configuration files and action files.
Use a Security Information and Event Management (SIEM) system to correlate events and identify suspicious activity patterns.
Upgrade Fail2ban to version 0.8.8 or later.
Implement strict input validation and sanitization for all user-supplied data, especially within custom action files.
Review and audit all custom action files for potentially malicious content.
Restrict write access to Fail2ban configuration files and action files to authorized users only.
Implement a least-privilege model for the Fail2ban process.
Regularly update Fail2ban and all other software to patch known vulnerabilities.
Consider using a web application firewall (WAF) to filter malicious requests.