CVE-2012-4528

Step 1: Crafting the Malformed Request: The attacker constructs a multipart/form-data POST request. This request includes a deliberately invalid or malformed part, such as an incomplete header or an incorrect boundary string. Step 2: Prepending the Invalid Part: The invalid part is placed before the legitimate, crafted data containing the malicious payload. This is crucial for triggering the vulnerability. Step 3: Bypassing Rule Checks: The mod_security2 module processes the invalid part first. Due to the parsing error, the module's rule engine fails to correctly identify and block the subsequent malicious payload. Step 4: Delivering the Payload: The crafted, malicious data, now bypassing the security rules, is delivered to the PHP application. This data can contain anything from SQL injection payloads to code designed for remote code execution. Step 5: Exploitation: The PHP application processes the injected data, potentially leading to unauthorized access, data breaches, or complete server compromise.

The vulnerability stems from an improper handling of multipart requests within mod_security2. Specifically, the module fails to correctly parse and validate the structure of multipart form data. The flaw lies in how the module processes invalid or malformed parts of a multipart request before processing the valid, crafted data. This allows an attacker to prepend a deliberately invalid part to the request, which confuses the parsing logic. The module's rule engine then fails to correctly identify and block the subsequent malicious payload, allowing it to bypass security checks. The root cause is a logic error in the parsing and rule application process, where the order of operations and the handling of error conditions related to multipart parsing are flawed. This leads to a bypass of the security rules, enabling the injection of malicious data.