When you’re working with regular expressions on Linux, chances are high that you’ve run into the PCRE2 library, a critical part of many software stacks. But did you know that a small bug in PCRE2’s testing tool, pcre2test, could let attackers crash your system using nothing but a tricky negative number?

Let’s dive into CVE-2022-41409: what it is, why it matters, and exactly how it works under the hood.

[References and Further Reading](#7)

1. What is PCRE2 and pcre2test?

PCRE2 stands for Perl Compatible Regular Expressions, Version 2. It’s a powerful and widely used regex engine, found in everything from web servers to security tools.

pcre2test is a command-line utility bundled with PCRE2. It lets developers try out regular expressions, see matches, and debug their regex patterns. Think of it as a playground for testing your regular expression code.


2. About CVE-2022-41409

CVE-2022-41409 is an integer overflow vulnerability affecting pcre2test in PCRE2 before version 10.41.

> “Integer overflow vulnerability in pcre2test before 10.41 allows attackers to cause a denial of service or other unspecified impacts via negative input.”
> — NVD - CVE-2022-41409

Let’s break it down

- Integer Overflow: A math error that happens when a calculation results in a number bigger than the allowable limit, causing it to “wrap around” to an unexpected value.
- Negative Input: The bug can be triggered by passing a negative number where a positive is expected.
- DoS: Denial of Service—usually, this means that the program crashes outright, and maybe even affects larger systems running it.


3. Where’s the Bug?

In computer programs, numbers are everywhere: indexes, sizes, counters, and more. Passing negative numbers where positive numbers are expected is a classic way to poke holes in a program.

For this bug, an integer overflow happens deep inside the code that handles command-line arguments in pcre2test.

The root of the problem is unsafe handling of negative input values, e.g., using atoi() without checking bounds.

Here’s roughly what happens—let’s see it in C-like pseudocode

char *input = argv[1];
int length = atoi(input);  // Converts string to int

char *buffer = malloc(length);
// If input is "-1", length becomes -1

if (buffer == NULL) exit(1);

memset(buffer, , length); // Yikes! Negative size!

If you pass "-1", length becomes -1. But when you ask malloc for a negative number, it gets converted to a huge unsigned value — like 4GB on 32-bit, or even more on 64-bit systems! The memset that follows can then overwrite memory or crash the app.


Step 2. Run this command

pcre2test -m -4096

Replace -4096 with just about any negative integer.

The program tries to allocate a huge amount of memory (due to the integer overflow).

- It might fail and crash, or worse — mishandle memory, potentially opening the door to more attacks (like memory corruption or code execution).


5. Real-World Impact

The main threat here is Denial of Service (DoS). An attacker—either accidentally or on purpose—can crash any automated system that calls pcre2test with user input, or just exploits it directly on a server.

If you’re offering regex testing as part of a larger service (maybe a private cloud or DevOps tool), this attack could break automation pipelines or server daemons.

Unspecified impacts: If combined with other vulnerabilities, such integer overflows sometimes allow memory corruption, leading to further exploits. As of now, though, no remote code execution is known for this bug.


6. How to Fix and Protect Yourself

- Upgrade PCRE2: This vulnerability is fixed in PCRE2 version 10.41 and later. Download the latest version from here.
- Input Validation: If you maintain a tool using PCRE2, always check that inputs are positive integers!
- Audit Other Tools: Even if you don’t use pcre2test, check other CLI tools using PCRE2 for similar bugs.


7. References and Further Reading

- NVD Entry for CVE-2022-41409
- Upstream Fix (GitHub)
- PCRE2 Official Homepage


TL;DR:  
*CVE-2022-41409 is a classic integer overflow in pcre2test, triggered by negative command-line input. This results in a denial of service as the tool tries to allocate or use a massive memory buffer. Update your PCRE2 tools to stay safe!*

Have more questions? Ask below or read the official docs for more details. Stay secure!

Timeline

Published on: 07/18/2023 14:15:00 UTC
Last modified on: 07/27/2023 03:46:00 UTC