CVE-2024-8932: Analysis of Integer Overflow Vulnerability in PHP's ldap_escape() Function on 32-bit Systems

Disclaimer: This post is meant for educational purposes only. Any exploitation attempts described in this post should be done within the scope of legal environments, with appropriate permissions.

Introduction

A recent addition to the Common Vulnerabilities and Exposures (CVE) list is the vulnerability CVE-2024-8932. This vulnerability affects PHP versions 8.1.* before 8.1.31, 8.2.* before 8.2.26, and 8.3.* before 8.3.14. Specifically, it occurs due to an integer overflow when passing uncontrolled long string inputs to the ldap_escape() function on 32-bit systems, resulting in an out-of-bounds write. In this post, we will deep dive into the vulnerability and discuss the exploit details.

Understanding the Vulnerability

At the core of the vulnerability is the ldap_escape() function in PHP. This function is used to escape strings for use in LDAP filters and DN strings, based on an RFC4515 and RFC4514 compliant manner. The function prototype is as follows:

string ldap_escape ( string $value [, string $ignore = "" [, int $flags =  ]] )

The vulnerability arises from an integer overflow on 32-bit systems when passing uncontrolled long string inputs to the function. This overflow can lead to an out-of-bounds write, allowing an attacker to potentially execute arbitrary code or cause a denial of service (DoS) on the affected system.

To understand the issue, let's examine a code snippet from the PHP source code:

/* Calculate escaped string length */
for (i = ; i < value_len; i++) {
    if (memchr(ignored_chars, value[i], sizeof(ignored_chars)) || !NEED_ESCAPE(value[i])) {
        out_len++;
    } else {
        out_len += 3;
    }
}

/* Check for integer overflow (issue CVE-2024-8932) */
if (out_len > UINT_MAX - 1) {
    RETURN_FALSE;
} else {
    out = safe_emalloc(1, out_len, 1);
}

In the above code snippet, an out_len integer is calculated to determine the required size of the escaped string. The issue occurs on 32-bit systems when the out_len significantly grows and causes an integer overflow. Since out_len is incremented without checking for potential overflows, this can lead to unexpected behavior.

Exploring the Exploit

Considering the nature of this vulnerability, let's explore an example exploit. Suppose we have the following PHP code:

<?php
// Uncontrolled long string input
$input = str_repeat("A", x7FFFFFFE);

// Passing the input to ldap_escape()
$output = ldap_escape($input);
?>

In this example, the $input string consists of x7FFFFFFE times the character "A". By passing this uncontrolled long string input to the ldap_escape() function, the out_len value will significantly grow. As a result, the out_len integer might wrap around to a smaller value than expected, causing an out-of-bounds write when the out buffer is allocated using safe_emalloc().

Mitigation and Conclusion

To mitigate this vulnerability, affected PHP versions should be updated to the patched versions: 8.1.31, 8.2.26, or 8.3.14. The official PHP release notes detail these updates and provide information on other fixes included in the patched versions.

The CVE-2024-8932 vulnerability emphasizes the importance of validating user inputs and handling integer overflows, particularly in languages like PHP that are widely used for web applications. By keeping systems and software up-to-date, it is possible to protect against known vulnerabilities and reduce the risk of potential exploitation.

Timeline

Published on: 11/22/2024 06:15:20 UTC