A Vulnerability in EXIM Could Allow for Arbitrary Code Execution

A vulnerability has been discovered in EXIM which could allow for arbitrary code execution. Exim is a mail transfer agent (MTA) for hosts that are running Unix or Unix-like operating systems. Successful exploitation of this vulnerability could allow for arbitrary code execution in the context of the service account. An unauthenticated attacker could then install programs; view, change, or delete data; or create new accounts with full user rights.

While there have been no reported exploits, this is a zero-day vulnerability.

A vulnerability has been discovered in EXIM which could allow for arbitrary code execution. Details of this vulnerability are as follows:

Tactic: Initial Access (TA0001):
Technique: Drive-By Compromise (T1189):

• The specific flaw exists within the smtp service, which listens on TCP port 25 by default. The issue results from the lack of proper validation of user-supplied data, which can result in a write past the end of a buffer. An attacker can leverage this vulnerability to execute code in the context of the service account. (CVE-2023-42115)
• An integer underflow vulnerability exists in the libspf2 library that can allow remote code execution for an unauthenticated user (CVE-2023-42118).

Successful exploitation of this vulnerability could allow for arbitrary code execution in the context of the service account. An unauthenticated attacker could then install programs; view, change, or delete data; or create new accounts with full user rights.

We recommend the following actions be taken:

• Apply appropriate updates provided by EXIM to vulnerable systems immediately after appropriate testing. (M1051: Update Software)

  • Safeguard 7.1: Establish and Maintain a Vulnerability Management Process: Establish and maintain a documented vulnerability management process for enterprise assets. Review and update documentation annually, or when significant enterprise changes occur that could impact this Safeguard.
  • Safeguard 7.4: Perform Automated Application Patch Management: Perform application updates on enterprise assets through automated patch management on a monthly, or more frequent, basis.
  • Safeguard 7.7: Remediate Detected Vulnerabilities: Remediate detected vulnerabilities in software through processes and tooling on a monthly, or more frequent, basis, based on the remediation process.
  • Safeguard 9.1: Ensure Use of Only Fully Supported Browsers and Email Clients: Ensure only fully supported browsers and email clients are allowed to execute in the enterprise, only using the latest version of browsers and email clients provided through the vendor.

• Apply the Principle of Least Privilege to all systems and services. Run all software as a non-privileged user (one without administrative privileges) to diminish the effects of a successful attack. (M1026: Privileged Account Management)

  • Safeguard 4.7: Manage Default Accounts on Enterprise Assets and Software: Manage default accounts on enterprise assets and software, such as root, administrator, and other pre-configured vendor accounts. Example implementations can include: disabling default accounts or making them unusable.
  • Safeguard 5.4: Restrict Administrator Privileges to Dedicated Administrator Accounts: Restrict administrator privileges to dedicated administrator accounts on enterprise assets. Conduct general computing activities, such as internet browsing, email, and productivity suite use, from the user’s primary, non-privileged account.

• Block execution of code on a system through application control, and/or script blocking. (M1038: Execution Prevention)

  • Safeguard 2.5 : Allowlist Authorized Software: Use technical controls, such as application allowlisting, to ensure that only authorized software can execute or be accessed. Reassess bi-annually, or more frequently.
  • Safeguard 2.6 : Allowlist Authorized Libraries: Use technical controls to ensure that only authorized software libraries, such as specific .dll, .ocx, .so, etc., files, are allowed to load into a system process. Block unauthorized libraries from loading into a system process. Reassess bi-annually, or more frequently.
  • Safeguard 2.7 : Allowlist Authorized Scripts: Use technical controls, such as digital signatures and version control, to ensure that only authorized scripts, such as specific .ps1, .py, etc., files, are allowed to execute. Block unauthorized scripts from executing. Reassess bi-annually, or more frequently.

• Remove or deny access to unnecessary and potentially vulnerable software to prevent abuse by adversaries. (M1042: Disable or Remove Feature or Program)

  • Safeguard 2.3: Address Unauthorized Software: Ensure that unauthorized software is either removed from use on enterprise assets or receives a documented exception. Review monthly, or more frequently.
  • Safeguard 2.5: Allowlist Authorized Software: Use technical controls, such as application allowlisting, to ensure that only authorized software can execute or be accessed. Reassessbi-annually, or more frequently.
  • Safeguard 2.7: Allowlist Authorized Scripts: Use technical controls, such as digital signatures and version control, to ensure that only authorized scripts, such as specific .ps1, .py, etc., files, are allowed to execute. Block unauthorized scripts from executing. Reassess bi-annually, or more frequently.
  • Safeguard 4.1: Establish and Maintain a Secure Configuration Process: Establish and maintain a secure configuration process for enterprise assets (end-user devices, including portable and mobile, non-computing/IoT devices, and servers) and software (operating systems and applications). Review and update documentation annually, or when significant enterprise changes occur that could impact this Safeguard.
  • Safeguard 4.8: Uninstall or Disable Unnecessary Services on Enterprise Assets and Software: Uninstall or disable unnecessary services on enterprise assets and software, such as an unused file sharing service, web application module, or service function.
  • Safeguard 18.3: Remediate Penetration Test Findings: Remediate penetration test findings based on the enterprise’s policy for remediation scope and prioritization.
  • Safeguard 18.5: Perform Periodic Internal Penetration Tests: Perform periodic internal penetration tests based on program requirements, no less than annually. The testing may be clear box or opaque box.