CVE-2022-4104 - Exploiting a JPEG Loop Bug in Dropbox’s Lepton for Denial-of-Service

Lepton, an open-source JPEG compression tool created by Dropbox, helps save storage space by using advanced, lossless compression. However, in 2022, researchers discovered a serious bug — CVE-2022-4104 — that allows attackers to freeze the tool using a specially crafted JPEG file. In this post, I’ll walk you through what went wrong, how it can be exploited, and offer practical code snippets and references for further study.

What is CVE-2022-4104?

CVE-2022-4104 refers to a vulnerability within Lepton’s JPEG decompression logic. Specifically, an attacker can submit a JPEG file designed to send Lepton into an endless loop, halting the system's progress forever. This is a classic *denial-of-service* (DoS) attack — no sensitive data is at risk, but affected systems could hang or crash, causing downtime or resource starvation.

Deep Dive: The Faulty Loop

Inside Lepton, there’s code that processes certain JPEG "marker" segments. When handling corrupted or rogue JPEG files, the exit condition for a main loop can become impossible to reach. Effectively, the code keeps waiting for something that never arrives, resulting in infinite looping.

Let’s look at a simplified version of the problematic logic

// WARNING: Vulnerable code snippet (simplified)

while (offset < jpegData.size()) {
    // Parse next JPEG segment inside jpegData at offset
    if (meetUnexpectedMarker()) {
        // Error handling sets a variable but doesn't break
        continue; // Skips exit logic!
    }

    // Main extraction logic
    offset = calculateNextOffset(offset);

    // (Exit condition is never met if marker is malicious)
}

A malicious JPEG can be crafted so that meetUnexpectedMarker() is always true, causing the offset variable to never advance. The loop runs forever, hanging the system or process.

Step-by-step Attack Overview

1. Create a Malformed JPEG: The attacker manipulates JPEG markers to cause the parser to encounter unexpected conditions repeatedly.
2. Feed JPEG to Lepton: By submitting this file, either as a user upload or over an API, the attacker triggers the loop on the server.
3. Resource Exhaustion: The Lepton process becomes unresponsive, potentially consuming CPU cycles permanently.
4. Impact: If automated pipelines are used (like in cloud storage environments), queued image conversions pile up, causing service interruption.

While we can’t show real attack code, here’s a description

- *Place a marker in the JPEG which is not handled, or whose length causes the parser logic to miscompute offset progression.*

Proof-of-Concept Script (Python)

Below is a Python snippet that opens Lepton with an intentionally malformed JPEG file, demonstrating that it hangs:

import subprocess
import time

malicious_jpeg = 'test_dos.jpg'  # Crafted JPEG that triggers the bug

# WARNING: Running this may freeze the lepton process!
proc = subprocess.Popen(['lepton', malicious_jpeg])
try:
    proc.wait(timeout=10)
    print("Lepton completed (unexpected).")
except subprocess.TimeoutExpired:
    print("Lepton process is hanging (vulnerable to CVE-2022-4104)!")
    proc.kill()

You can verify the vulnerability if Lepton hangs for over 10 seconds and needs to be killed.

Fix and Mitigation

Dropbox patched this in Lepton by adding strict marker validation and improved exit conditions. The proper way to avoid this:

Validate Inputs: Always check that offsets and markers are within expected ranges.

2. Timeouts: Cap processing time/loops on user-supplied data.
3. Upgrade: Use Lepton’s latest version with the patch.

References

- Official CVE Report on Mitre
- Dropbox Lepton GitHub Repository
- Lepton Release Notes and Changelog

In Summary

CVE-2022-4104 demonstrates how even “invisible” bugs like infinite loops can take potent form with user-supplied data. JPEGs seem harmless, but if you process a lot of them — especially in cloud pipelines — make sure you’re protected. Always validate external data, keep third-party tools up to date, and understand the code you trust to run on your servers.

Stay safe, and always keep an eye out for those infinite loops!

Timeline

Published on: 11/28/2022 19:15:00 UTC
Last modified on: 12/01/2022 23:00:00 UTC