Source: cve@mitre.org
Sendmail allows local users to write to a file and gain group permissions via a .forward or :include: file.
Sendmail, a widely used email server, is vulnerable to a local privilege escalation. Attackers can exploit this flaw by manipulating .forward or :include: files to gain group-level permissions, potentially leading to complete system compromise and data exfiltration or remote code execution. This vulnerability is a significant threat due to the widespread use of Sendmail and the ease of exploitation.
Step 1: File Creation/Modification: The attacker creates or modifies a .forward or :include: file in their home directory. This file contains a malicious command or a reference to a malicious script.
Step 2: Payload Insertion: The attacker crafts a payload (e.g., a shell script) that will be executed with elevated privileges. This payload could be designed to add a new user to the system, modify system files, or install a backdoor.
Step 3: Email Delivery Trigger: The attacker sends an email to themselves or another user on the system. This triggers Sendmail to process the .forward or :include: file.
Step 4: Command Execution: Sendmail executes the commands specified in the .forward or :include: file. The malicious payload is executed with the permissions of the Sendmail user (e.g., mail or smmsp).
Step 5: Privilege Escalation: The attacker's payload, running with elevated privileges, performs the desired actions, such as adding a new user with root privileges or modifying system configurations.
The vulnerability stems from Sendmail's insecure handling of .forward and :include: files. These files allow users to specify where email should be delivered. Sendmail, when processing these files, doesn't adequately validate the permissions of the files or the commands executed within them. Specifically, it allows a local user to create or modify a .forward or :include: file in their home directory, and then specify a command (e.g., a shell script) that will be executed with the permissions of the Sendmail user (often mail or smmsp). Because the Sendmail user often belongs to a privileged group, this allows the attacker to gain elevated privileges. The root cause is a lack of proper input validation and permission checks when processing these configuration files, leading to an arbitrary command execution vulnerability.
While no specific APTs are directly linked to this CVE, the vulnerability's nature makes it attractive to any attacker seeking local privilege escalation. It is a common technique used in the initial stages of a compromise. This vulnerability is not listed in the CISA KEV database, likely due to its age and the availability of patches and mitigations.
Monitor file system activity for modifications to .forward and :include: files, especially in user home directories.
Analyze Sendmail logs for unusual activity, such as the execution of unexpected commands or scripts.
Look for the creation of new users or changes to system files that could indicate a compromise.
Network traffic analysis: While this is a local vulnerability, any outbound connections from the compromised system should be investigated, as this could indicate data exfiltration or command and control activity.
Check for the presence of suspicious files or scripts in user home directories or system directories that are executed by the Sendmail process.
Upgrade Sendmail to the latest version or a version that addresses this vulnerability. Apply all security patches.
Restrict the permissions of .forward and :include: files to prevent unauthorized modification. Ensure only the owner of the file can modify it.
Implement strong input validation and sanitization for any user-supplied data that is used by Sendmail.
Disable the use of shell commands within .forward and :include: files if not absolutely necessary. If shell commands are required, use safe alternatives.
Regularly audit Sendmail configuration files and user accounts.
Implement a host-based intrusion detection system (HIDS) to monitor for malicious activity.
Use a file integrity monitoring (FIM) tool to detect unauthorized changes to critical system files.