In mid-2022, a critical vulnerability, CVE-2022-30065, was discovered in BusyBox, a compact and widely used utility suite for embedded Linux systems. The issue lies specifically in the awk applet, commonly used for text processing and scripting on lightweight systems. This post will break down what went wrong, how attackers can exploit it, code samples to illustrate the bug, and how you can protect your systems.

What is BusyBox Awk?

BusyBox is called “the Swiss Army Knife of Embedded Linux” because it combines tiny versions of common UNIX utilities into a single binary.

awk is one of these utilities, used for pattern scanning and processing. It reads input line by line, examines each line to see if it matches a set of patterns, and performs actions on the lines that match.

Potential impact: Denial of Service (DoS), possible remote code execution (RCE)

- Original reference: SSD Advisory – BusyBox Awk Code Execution
- Official announcement: BusyBox Bug Tracker

The Root Cause

The vulnerability appears in the copyvar() function inside the awk applet. When handling a specially crafted awk pattern, copyvar() can free a variable and then use it again, leading to a use-after-free condition. A UAF can crash the program or, in some situations, allow attackers to execute arbitrary code.

Below is a key snippet from the BusyBox 1.35’s awk source (simplified for clarity)

void copyvar(var *dst, const var *src) {
    if (dst == src)
        return;
    varfree(dst);     // <-- dst is freed
    *dst = *src;      // <-- still using dst!
    dst->x = NULL;
}

The problem? dst is freed and then immediately accessed (dereferenced) after.

You can exploit this bug by running the following crafted awk script against BusyBox 1.35 awk

busybox awk 'BEGIN { a=; b=a; a=; }'

This snippet looks naive, but the sequence of assignments creates the bad condition in memory. The b=a assignment results in copyvar() operations between variables, triggering the use-after-free under the hood.

On a vulnerable version of BusyBox (1.35.x), this will typically result in a segmentation fault (crash):

Segmentation fault (core dumped)

In some cases, more complex crafted input may lead to controlled memory reuse, opening avenues for code execution.

Denial of Service

The simplest exploit is crashing awk, which can be problematic in automated scripts or for embedded devices relying on BusyBox awk for log processing, updates, or maintenance.

Code Execution (Theory)

If an attacker can run crafted awk patterns in your environment, and can control subsequent heap (malloc) allocations, they could potentially gain further control, possibly leading to arbitrary code execution.

No one has published a full remote code execution exploit, but UAF bugs in interpreters are notorious for this possibility, especially if combined with additional heap spraying tricks.

Mitigation & Fix

Upgrade to BusyBox 1.36. or later, where the bug has been fixed.

- Fix in upstream code
- Download latest BusyBox

References

- SSD-Disclosure: BusyBox Awk Use After Free
- CVE Details for CVE-2022-30065
- Patch Commit

Conclusion

CVE-2022-30065 is a great example of how a small code oversight, especially involving memory management, can cause serious software vulnerabilities, even in mature, “battle tested” open source projects. If you rely on BusyBox—particularly for IoT or embedded Linux—it’s vital to patch quickly. Regular updates, running security audits, and limiting execution of untrusted code are your best defenses!

Stay secure, and always keep an eye on your dependencies.


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Timeline

Published on: 05/18/2022 15:15:00 UTC
Last modified on: 06/01/2022 14:20:00 UTC