Remediation/Mitigation Strategy for CVE-2025-26525: Arbitrary File Read via TeX Notation Filter

This document outlines the remediation and mitigation strategy for CVE-2025-26525, an arbitrary file read vulnerability affecting sites using a TeX notation filter where pdfTeX is available (e.g., those with TeX Live installed).

1. Vulnerability Description:

• CVE ID: CVE-2025-26525
• Description: Insufficient sanitizing in the TeX notation filter allows an attacker to potentially read arbitrary files on the server. This occurs because the TeX notation filter does not properly validate user-supplied input before passing it to pdfTeX. An attacker can craft malicious TeX code that, when processed by pdfTeX, will read the contents of a file on the system and include it in the generated output (e.g., PDF).
• Affected Systems: Any system using a TeX notation filter and has pdfTeX (part of TeX Live or a similar TeX distribution) installed. This typically affects websites, applications, or services that allow users to input TeX code and then render it into a PDF or other output format.
• Reported By: [email protected] (Fedora Project)

2. Severity:

• CVSS Score: 8.6 (High)
• CVSS Vector: Information provided is incomplete for a full calculation. Based on the available data:
  • Base Score: 8.6 (Assuming High Confidentiality Impact)
  • Attack Vector (AV): Network (Based on the information provided, assuming it can be exploited remotely)
  • Attack Complexity (AC): Low (Likely, as it’s a sanitization issue)
  • Privileges Required (PR): None (Assuming no authentication is needed)
  • User Interaction (UI): None (Assuming no user interaction is needed)
  • Scope (S): Changed (Likely, as it could allow unauthorized access to system files.)
  • Confidentiality Impact (C): High (Arbitrary file read)
  • Integrity Impact (I): None
  • Availability Impact (A): None
• Severity Level: High

3. Known Exploits:

• While specific exploit code is not explicitly detailed in the provided information, the nature of the vulnerability (arbitrary file read) implies that exploitation involves crafting malicious TeX code that leverages pdfTeX to read specified files. The complexity of the exploit depends on the specific implementation of the TeX notation filter and the available TeX packages/commands. A common attack vector would involve using \input, \usepackage, or similar TeX commands to include file contents.

4. Remediation Strategy:

The primary goal of the remediation strategy is to prevent attackers from reading arbitrary files through the TeX notation filter.

• Immediate Actions (Short-Term Mitigation):

  1. Disable the TeX Notation Filter (Recommended if Possible): The most effective immediate action is to disable the TeX notation filter if its functionality is not critical. This will eliminate the attack vector completely.
  2. Restrict pdfTeX Access: If disabling the filter isn’t feasible, restrict the permissions of the user account running pdfTeX. Prevent it from accessing sensitive files and directories. Use the principle of least privilege. Implement file system access controls.
  3. Input Sanitization (Partial Mitigation): Implement immediate input sanitization to block common file access commands and sequences in TeX, such as:
     • Deny use of commands such as \input, \usepackage, \include, \openin, \read, etc.
     • Filter out file paths (e.g., paths starting with /, C:\, .., etc.).
     • Check for and block malicious characters.
  4. Monitor System Logs: Enable detailed logging of all TeX filter operations and monitor these logs for suspicious activity, such as attempts to access unusual file paths.

• Long-Term Remediation:

  1. Comprehensive Input Sanitization & Validation: Implement a robust input sanitization and validation mechanism specifically designed for TeX code. This mechanism should:

     • Whitelist Approach: Define a very restrictive whitelist of allowed TeX commands and parameters. Reject anything not on the whitelist. This is the most secure approach.
     • Blacklist with Caution: If a whitelist is impractical, carefully maintain a blacklist of known dangerous commands and patterns. Be aware that blacklists are easily bypassed, so this approach is less secure. Regularly update the blacklist as new exploits are discovered.
     • Contextual Escaping: Properly escape all user-supplied input before passing it to pdfTeX. Use a library or function specifically designed for TeX escaping to prevent injection attacks.
     • Limit TeX Package Use: Restrict the available TeX packages to a minimal set required for the intended functionality. Remove any packages that are not necessary, as they can potentially introduce new vulnerabilities.
     • Sandboxing: Consider running pdfTeX in a sandboxed environment (e.g., using Docker, chroot, or a dedicated virtual machine). This will limit the damage that an attacker can cause, even if they manage to bypass the input sanitization. AppArmor or SELinux can further restrict file access.

  2. Regular Security Audits and Penetration Testing: Conduct regular security audits and penetration tests of the TeX notation filter to identify and address any remaining vulnerabilities.

  3. Update to Patched Version: If a patch is released by the maintainers of the TeX notation filter, apply it immediately.

5. Mitigation Strategy (If Remediation is Not Immediately Possible):

If a full remediation is not possible in the short term, implement the following mitigation measures to reduce the risk:

• Rate Limiting: Implement rate limiting on the TeX notation filter to prevent attackers from launching large-scale attacks.
• Web Application Firewall (WAF): Deploy a WAF with rules that can detect and block common TeX injection attacks. Configure the WAF to monitor and log suspicious requests. Regularly update the WAF rules.
• Intrusion Detection System (IDS): Configure an IDS to monitor network traffic for suspicious activity related to the TeX notation filter.
• Least Privilege: Ensure that the account under which the web application (or the process rendering the TeX code) runs has the minimum necessary privileges to perform its functions. Avoid running the process as root or other privileged accounts.
• User Education: Educate users about the risks of entering untrusted TeX code. If possible, implement a warning message to users before they submit TeX input.

6. Monitoring and Logging:

• Detailed Logging: Enable comprehensive logging of all TeX filter operations. Log all input, output, and any errors that occur during processing.
• Log Analysis: Regularly review logs for suspicious activity, such as attempts to access unusual file paths or the use of prohibited commands.
• Alerting: Configure alerts to notify administrators of any suspicious activity.

7. Communication Plan:

• Internal Communication: Inform the development, security, and operations teams about the vulnerability and the remediation/mitigation strategy.
• External Communication: If the vulnerability affects external users, develop a communication plan to inform them about the issue and any steps they need to take to protect themselves.

8. Verification:

• After implementing the remediation or mitigation strategy, thoroughly verify its effectiveness by conducting penetration testing and code reviews.
• Confirm that the input sanitization and validation mechanisms are working as expected.
• Verify that the restrictions on pdfTeX access are in place.
• Retest the exploit to confirm that it is no longer possible to read arbitrary files.

9. Continuous Improvement:

• Regularly review and update the remediation/mitigation strategy based on new threat intelligence and the results of security audits and penetration tests.
• Stay informed about new vulnerabilities and exploits related to TeX and pdfTeX.
• Continuously improve the security of the TeX notation filter.