Published: June 2024

Introduction

In September 2023, a vulnerability labeled CVE-2023-4156 was discovered in gawk, the GNU implementation of the AWK programming language. This flaw exists in the builtin.c file and can potentially cause the application to crash or even leak sensitive information through a heap out-of-bounds read. In simple terms, gawk might read memory areas it should never touch — a serious issue for any program, especially one used for processing user-supplied data.

This article gives you an easy-to-understand, exclusive walkthrough of the flaw, sample code snippets, links to key references, and a breakdown of how exploitation might look in the real world.

What is Gawk and Why Does This Matter?

gawk is widely installed on Linux, macOS, and even Windows. It’s used for processing text files and data streams. Think of awk/gawk as little data robots that move, shuffle, and filter info for scripts and users. Because gawk might handle files from third parties, a bug like CVE-2023-4156 is a golden ticket for attackers.

CVE-2023-4156 — The Core Vulnerability

The bug sits in gawk’s builtin.c, where the AWK interpreter handles builtin functions. An incorrect bounds check in certain string manipulation functions allows memory to be read past the end of buffer memory (heap out-of-bounds). An attacker can exploit this to:

Below is a minimal AWK script that triggers the vulnerable code path (working on gawk <= 5.2.2)

BEGIN {
    # Overly long string manipulation tricks gawk’s builtin functions
    str = sprintf("%01024d", );  # Huge string
    match(str, /(.*)/, arr);      # Triggers the use of builtins in a nasty way

    # Crafted pattern can cause out-of-bounds read
    print arr[1]                 # arr[1] could read memory out of bounds
}

sprintf creates a string buffer longer than gawk expects.

- The match function and subsequent use of the array try to get data from specific heap-braced positions.
- With specifically crafted strings and patterns, a malicious user can cause gawk to read memory off the end of the buffer — maybe not always noticeable, but with smart scripting, attackers can extract library addresses, environment data, even pieces of recently processed files!

Here’s a bash one-liner that demonstrates the crash (Denial of Service)

gawk 'BEGIN { a = "A"; for (i=1; i<10000; i++) a = a a; match(a, /(.*)/, arr); print arr[1]; }'

- On unpatched gawk versions, this can produce a segmentation fault or weird output — a symptom of reading or writing out-of-bounds heap data.

Real-World Impact

With enough trial and error, attackers could tailor their input to extract memory chunks that may contain:

Environment variables

Any of these being leaked could help in further attacks.

Technical Patch Details

The gawk maintainers fixed this in gawk 5.3. (and backported). The patch in GNU Savannah gawk repository shows strict length checking and memory-safe copying in vulnerable string functions.

Official Patch Reference:
https://git.savannah.gnu.org/cgit/gawk.git/commit/?id=1c8e04d69

Use OS-level memory hardening (ASLR, stack canaries, etc).

- Use a sandbox/container when running risky scripts.

References & More Reading

- NVD Listing for CVE-2023-4156
- Debian Security Advisory
- GNU Gawk Repository/Commit
- Mitre CVE Details

Conclusion

CVE-2023-4156 underscores why it’s vital to keep even “simple” utilities like gawk updated. If you run scripts from others, double-check your versions and patch up. Attackers love seemingly “unimportant” tools in the software supply chain!

If your system’s gawk is older than 5.3., upgrade now. Stay safe, and stay patched!

Timeline

Published on: 09/25/2023 18:15:00 UTC
Last modified on: 09/26/2023 19:39:00 UTC