Published: 2024-06

Author: [Your Name]

The security of web servers depends greatly on the reliability of the PHP engine, which powers a majority of websites worldwide. Recently, a critical vulnerability — CVE-2023-0568 — was discovered in the PHP core path resolution logic. This subtle, one-byte mistake could let attackers access, or even change, data they shouldn’t be able to.

In this article, we’ll break down what CVE-2023-0568 is, look at how it works with easy-to-understand examples, and discuss what you can do to protect your systems.

PHP 8.2.x before 8.2.3

The bug centers on a core C function in PHP’s path resolution code. When attempting to resolve file paths that are almost as long as the system’s MAXPATHLEN, PHP allocates a buffer that is one byte too small. When a path fills this buffer, the next byte in memory (not technically part of the allocation) is overwritten with a NUL byte (\x00). This can lead to memory corruption, data leaks, or even file modification, depending on how and where PHP uses the memory.

How Does the Vulnerability Work?

At the heart of PHP’s file system access is path normalization: removing relative directory pointers, expanding symbolic links, and so on. This typically involves copying a file path string into a buffer for safe handling.

The bug: The code allocates memory based on the length of the path, *but forgets that strings in C need one more byte for the terminating NUL character*. If the path is the system’s max length, the buffer is one byte short!

C Code (Simplified Example)

char *realpath = emalloc(MAXPATHLEN);
strncpy(realpath, input_path, MAXPATHLEN);
// BAD: No space for final NUL if input_path is MAXPATHLEN long!

When PHP copies the path, it writes a NUL byte *beyond the allocated buffer* if input_path is exactly MAXPATHLEN bytes. The memory just after the buffer could belong to *anything* else: another variable, stack data, or even sensitive information.

Security Impact

- Information Disclosure: Sensitive variables (such as authentication tokens) stored after the buffer could get overwritten or exposed.
- Data Modification: If the overwritten byte changes data later used by PHP, it can cause logic errors or change behavior.
- Crash and Denial of Service: Clobbering adjacent memory might lead to random PHP process crashes, impacting website reliability.

This issue is especially dangerous if user input can directly or indirectly influence file path lengths.

Proof of Concept (PoC) Example

While exploiting this reliably is tricky and depends on system architecture and PHP’s memory layout, here’s a simple demonstration to show how the overflow can trash adjacent memory:

PHP PoC Script

<?php
// Fill a large filename close to system's MAXPATHLEN
$max_length = 4095; // Example MAXPATHLEN - check your system!
$fake_path = str_repeat("A", $max_length - strlen(getcwd()) - 1);

// This will cause PHP to allocate a buffer that's too small
try {
    realpath($fake_path);
} catch (Throwable $e) {
    // If there's a segmentation fault, or weird behavior, the system is likely vulnerable
    echo "Potential vulnerability triggered: ", $e->getMessage(), "\n";
}
?>

Note: You’ll need to adjust $max_length for your environment. On UNIX systems, MAXPATHLEN is often 4096.

Potential Exploit Scenario

Imagine an application that stores sensitive tokens right after the path buffer (in memory). When an attacker overwrites that one byte, they could make the token invalid, alter user permissions, or trigger unexpected behavior.

2. Limit Path Input Lengths

If you must accept user-supplied filenames, add checks to reject excessively long paths.

if (strlen($user_path) > 1024) {
    die("Path too long");
}

3. Harden Your Environment

Restrict file operations using open_basedir and run PHP processes with the lowest possible privileges.

PHP Security Advisory:

https://www.php.net/archive/2023.php#2023-02-16-3

CVE Description:

https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2023-0568

PHP Source Patch Example:

https://github.com/php/php-src/pull/10748/files

Conclusion

CVE-2023-0568 is a memorable reminder: even small, subtle bugs in low-level code can open big security holes! Always keep your PHP runtime up-to-date and pay special attention to how user input affects your filesystem code.

Do you have any questions about this bug or how to stay safe? [Leave a comment below or contact me!]

Timeline

Published on: 02/16/2023 07:15:00 UTC
Last modified on: 03/03/2023 18:02:00 UTC