Researchers Expose Critical Remote Code Execution Flaw in Moodle

A critical security flaw was discovered in Moodle, a popular learning management platform, which resides in Moodle’s handling of calculated questions within quizzes, allowing instructors to define formulas that include variables to automatically generate variations of the same question. 

To evaluate these formulas and produce the answer, Moodle utilizes the eval() function, which isally should only process trusted code, but in Moodle’s case, the validation process designed to safeguard against malicious code injection is insufficient.   

The core issue lies in how Moodle validates the formulas before passing them to eval(), which aims to prevent the execution of arbitrary code by disallowing certain characters and functions within the formula. 

But this process falters because it prematurely replaces variables with the number 1.0 during the initial validation stage, which essentially blinds the validation to the potential dangers posed by the variables themselves. 

By cleverly crafting a formula that incorporates seemingly harmless variable references, attackers can bypass the validation altogether, although appearing safe due to the variable replacements, and can in fact encode and execute arbitrary code when eval() is called.

The effectiveness of this exploit hinges on the specific PHP version in use. Older PHP versions supported a curly brace array access notation that permitted a much simpler exploit. 

For instance, the expression (1){phpinfo()} would appear as the innocuous (1)1.0 to the validation function, consequently leading to the execution of the phpinfo() function (or any other function designated within the curly braces). 

Fortunately, this particular array access method has been deprecated and removed entirely since PHP version 8, the current standard on most operating systems, which necessitates a more elaborate approach for exploiting the vulnerability in current Moodle versions.

A technique bypasses validation logic in PHP code using mathematical functions and bitwise operations. By exploiting the behavior of string concatenation, the acos function, and the XOR operator, the authors generate arbitrary strings. 

These strings are then used to construct function names and call them using variable functions. The technique leverages a quirk in PHP’s variable substitution mechanism to circumvent the need for direct function calls. 

While this method allows for limited code execution, it is constrained by the requirement for functions that take a single numeric argument and the lack of ability to pass complex commands. 

Despite these limitations, the vulnerability can still be exploited to execute functions like DELETE_COURSE, potentially leading to unintended consequences.

Red Team Pentesting discovered a remote code execution vulnerability in Moodle by exploiting a syntax error in the PHP interpreter. They found that curly braces could be used to access object properties, which allowed them to execute arbitrary functions. 

By manipulating the answer formula and preventing variable substitution, they were able to execute commands on the Moodle server, while Moodle addressed the vulnerability by restricting the allowed characters in variable names and formulas, preventing the exploitation of this method.

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