e107 CMS Patch Fixes Critical Unauthenticated Remote Code Execution Flaw

The e107 content management system released version 2.3.7 to address a critical security vulnerability that allows attackers to execute arbitrary code on affected servers. The National Vulnerability Database records the issue as CVE-2026-55573, assigning it a maximum severity score of 9.8. This rating indicates that the flaw poses a severe risk to any system running the software prior to the patched release.

The vulnerability stems from how the application handles data during the installation process. Specifically, the install.php script decodes a POST parameter named previous_steps and passes it to the unserialize function without restricting allowed classes. This occurs both at the file scope and within the e_install constructor. Because the installer remains accessible after a standard installation is complete, the entry point for exploitation is always available to external actors.

Technical Details of the Exploit

According to the NVD record, the weakness is classified under CWE-502, which relates to deserialization of untrusted data. An attacker can construct a specific object graph using the bundled GuzzleHttp\Psr7\PumpStream class. This crafted data flows through paths.hash into site_path, where string coercion invokes an attacker-controlled callable. This chain of events allows the execution of arbitrary PHP functions with the privileges of the web server process.

The lack of class restriction during deserialization is the primary factor enabling this attack. By leveraging the PumpStream class, an unauthenticated attacker can bypass normal application logic. This leads to full server compromise, granting the attacker complete control over the underlying infrastructure. The attack does not require prior authentication or valid user credentials, making it accessible to any internet-connected threat actor.

Affected Systems and Mitigation

The vulnerability affects all versions of e107 prior to 2.3.7. The vendor, e107inc, has released version 2.3.7 to fix this issue. Administrators running older versions are immediately at risk. The fix specifically addresses the unsafe deserialization practices in the installer script. Users must upgrade to the latest version to eliminate the exposure.

There are no temporary workarounds mentioned in the advisory other than upgrading. Since the installer remains accessible post-installation, simply disabling the installer via file permissions is not a guaranteed mitigation if the application logic does not enforce it. The most effective defense is applying the vendor-provided update. Organizations should verify their current version against the vulnerability timeline to ensure compliance with security standards.

What should you do?

Administrators should immediately check their e107 installations to determine if they are running a version older than 2.3.7. If an older version is detected, they should apply the update to version 2.3.7 as soon as possible. Security teams should monitor web server logs for unusual activity targeting the install.php script. Organizations should also review their server configurations to ensure that unnecessary scripts are not publicly accessible.