CVE-2017-7692 in SquirrelMail
Summary
by MITRE
SquirrelMail 1.4.22 (and other versions before 20170427_0200-SVN) allows post-authentication remote code execution via a sendmail.cf file that is mishandled in a popen call. It's possible to exploit this vulnerability to execute arbitrary shell commands on the remote server. The problem is in the Deliver_SendMail.class.php with the initStream function that uses escapeshellcmd() to sanitize the sendmail command before executing it. The use of escapeshellcmd() is not correct in this case since it doesn't escape whitespaces, allowing the injection of arbitrary command parameters. The problem is in -f$envelopefrom within the sendmail command line. Hence, if the target server uses sendmail and SquirrelMail is configured to use it as a command-line program, it's possible to trick sendmail into using an attacker-provided configuration file that triggers the execution of an arbitrary command. For exploitation, the attacker must upload a sendmail.cf file as an email attachment, and inject the sendmail.cf filename with the -C option within the "Options > Personal Informations > Email Address" setting.
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Analysis
by VulDB Data Team • 08/23/2024
The vulnerability identified as CVE-2017-7692 represents a critical post-authentication remote code execution flaw in SquirrelMail versions prior to 20170427_0200-SVN. This vulnerability exists within the email client's delivery mechanism and specifically targets the Deliver_SendMail.class.php component. The flaw stems from improper handling of the sendmail configuration file within a popen call, creating a pathway for attackers to execute arbitrary shell commands on the affected server. The vulnerability is particularly concerning because it requires only authenticated access to the web application, making it exploitable by users with valid email accounts.
The technical root cause lies in the flawed implementation of command sanitization within the initStream function of the Deliver_SendMail.class.php file. The developers employed escapeshellcmd() to sanitize the sendmail command, which is fundamentally inadequate for this context. CWE-78, which addresses Improper Neutralization of Special Elements used in OS Commands, directly applies to this vulnerability since the sanitization method fails to properly escape all special shell characters including whitespaces. The specific injection point occurs within the -f$envelopefrom parameter of the sendmail command line, where the envelope from address is processed without proper input validation. This allows attackers to manipulate the command execution flow by injecting malicious parameters through the configuration file path.
The exploitation process requires a multi-step approach that begins with uploading a malicious sendmail.cf file as an email attachment. The attacker must then navigate to the "Options > Personal Informations > Email Address" setting and inject the malicious filename using the -C option to specify the configuration file. This configuration file can contain commands that will be executed by sendmail when it processes the email delivery. The vulnerability is particularly dangerous when the target server utilizes sendmail as its primary mail delivery agent, as the attack vector directly leverages the sendmail command execution mechanism. The attack is further facilitated by the fact that SquirrelMail's configuration allows for the specification of custom sendmail command parameters, creating a direct path for command injection.
The operational impact of this vulnerability extends beyond simple code execution, potentially allowing attackers to gain full control over the affected server. Successful exploitation can lead to complete system compromise, data exfiltration, and persistence mechanisms being established. The vulnerability affects organizations that rely on SquirrelMail for email services and have not updated to the patched versions. From an ATT&CK perspective, this vulnerability maps to T1059.001 (Command and Scripting Interpreter: PowerShell) and T1078 (Valid Accounts) as it requires legitimate user authentication to exploit. The attack chain involves initial access through valid credentials followed by privilege escalation and persistence within the email infrastructure. Organizations using older versions of SquirrelMail are particularly vulnerable as the patch was released in April 2017, and the vulnerability remains exploitable in unpatched environments.
Mitigation strategies should focus on immediate patching of all affected SquirrelMail installations to versions released after April 27, 2017, which contain the necessary fixes for the command injection vulnerability. Network segmentation and access controls should be implemented to limit the scope of potential exploitation, particularly restricting access to email administration interfaces. Input validation should be strengthened throughout the application, particularly in areas where external commands are invoked. The use of alternative mail delivery methods that do not rely on command-line sendmail implementations should be considered as a long-term solution. Security monitoring should be enhanced to detect suspicious email attachment uploads and configuration changes within email client settings. Additionally, organizations should implement regular vulnerability assessments and maintain up-to-date security patches for all web applications to prevent similar issues from occurring in the future.