A critical flaw, CVE-2024-7006, was discovered in the way Libtiff—a popular library for processing TIFF image files—handles memory allocation errors in the tif_dirinfo.c source file. This vulnerability can be triggered by attackers who exploit memory allocation failures, leading to a *null pointer dereference*. The result is usually a segmentation fault, causing the application to crash and leading to a classical denial-of-service (DoS) condition.

In this detailed article, we'll break down what this bug means, how it happens, and show code snippets. We'll go step by step through a potential exploit scenario, all in simple language.

What is Libtiff?

Libtiff is a widely-used C library that deals with Tagged Image File Format (TIFF) image files. Many tools, image viewers, and even web services rely on Libtiff for handling TIFF files. That makes bugs in Libtiff a big deal across many platforms.

The Flaw

In certain versions of Libtiff (as of early 2024), the tif_dirinfo.c source file did not properly check the result of a memory allocation before using the returned pointer. If the allocation failed—perhaps due to intentionally constrained heap size or system error—Libtiff would proceed to use a NULL pointer, causing a crash.

Impact

- Denial of Service: An attacker can cause any application linked to Libtiff to crash by making it process a specially engineered TIFF file, or by running it on a system with limited memory.
- Simple Exploitation: The attacker does not need deep technical skills, just the ability to feed a TIFF file to an application.

Source: Where the Bug Lives

The problem sits in the function responsible for handling TIFF directory entries. Let’s look specifically at tif_dirinfo.c (simplified for clarity):

TIFFFieldArray *array = (TIFFFieldArray *)_TIFFmalloc(sizeof(...));
/* ... */
array->fields = (TIFFField **) _TIFFmalloc(nfields * sizeof(TIFFField*));
/* No check for NULL above */

If _TIFFmalloc fails (returns NULL), and array or array->fields is used, you get a null pointer dereference.

Example buggy code from Libtiff's tif_dirinfo.c

array->fields = (TIFFField **)_TIFFmalloc(nfields * sizeof(TIFFField*));
for (i = ; i < nfields; i++) {
    array->fields[i] = ... // This will crash if array->fields is NULL
}

If heap memory is restricted (intentionally or not), _TIFFmalloc can fail, and the application will crash as soon as it dereferences array->fields.

1. Restrict Heap Space

On Linux, use ulimit to limit the amount of memory the process can use.

ulimit -v 128000  # Limit to 128MB

2. Feed the Application a Crafted TIFF

Prepare a large or malformed TIFF file that forces Libtiff to attempt large (or repeated) allocations.

Any simple application using Libtiff could be crashed as follows

tiffinfo crashfile.tiff
# or
your-photo-app crashfile.tiff

You’ll see a Segmentation fault (core dumped) if you check logs or output.

Proof-of-Concept Code (in C)

You could simulate the bug by crafting a TIFF file that declares a huge number of directory entries, or simply by limiting memory as above and using any big TIFF file.

Why Does This Happen?

- Lack of NULL Checks: The main issue is that after memory allocation, the code does not check if _TIFFmalloc returned NULL before using the pointer.
- Assumed Sufficient Memory: The code assumes there will be enough memory for every allocation, which just isn’t always true, especially in resource-constrained or hostile environments.
- No Handling for Partial Allocation: If only part of an object is allocated, Libtiff doesn’t clean up properly, which can make things worse.

Example: Fixed Code

The way to fix this bug is *simple*: Always check if your memory allocation succeeded.

array->fields = (TIFFField **)_TIFFmalloc(nfields * sizeof(TIFFField*));
if (!array->fields) {
    _TIFFfree(array);
    return NULL;  // Or handle error
}

References & Disclosure

- Libtiff Official GitHub
- NVD entry for CVE-2024-7006
- oss-security mailing list (thread)

How to Stay Safe

1. Update Libtiff: Ensure you are using the latest patched version (see the official releases).

Conclusion

CVE-2024-7006 is a classic case where a simple oversight—forgetting to check for a NULL pointer—has big consequences. Attackers can trivially crash applications using Libtiff, causing a denial-of-service. Always check your allocations, and keep your dependencies up to date!

Timeline

Published on: 08/12/2024 13:38:40 UTC
Last modified on: 11/06/2024 10:15:04 UTC