CVE-2016-4009

What it actually does

An integer overflow vulnerability exists in the image resampling function. When processing a specially crafted image that specifies a negative new size, an integer overflow occurs during memory allocation calculation. This leads to a much smaller buffer being allocated on the heap than required. Subsequent data writing operations will then overflow this buffer, causing a heap-based buffer overflow. This condition can be leveraged by an attacker to achieve arbitrary code execution in the context of the application using the library.

Why it matters to the business

Exploitation of this vulnerability could allow a remote, unauthenticated attacker to take complete control of any server that uses this library to process user-supplied images. This can lead to a full system compromise, resulting in the theft of sensitive data, service disruption, and the use of the compromised system to launch further attacks against the internal network. Given the widespread use of this library in web applications and other services, the potential for impact is high.

How an attack unfolds

An attacker crafts a malicious image file designed to be processed by the vulnerable library. The image contains metadata specifying a negative value for a resize operation. When an application using the vulnerable library attempts to process this image, it calls the resampling function. The negative size value causes an integer overflow, leading to an undersized memory buffer being allocated on the heap. As the library writes the resampled image data, it writes beyond the bounds of this buffer, triggering a heap overflow. A skilled attacker can control the overflowed data to overwrite critical memory structures, such as function pointers, to divert the program's execution flow and run arbitrary code.

How to fix it

Upgrade the Pillow library to version 3.1.1 or later. For systems using specific packaged versions, such as Chainguard's mysql-8.0, upgrade to the patched version (e.g., 8.0.38-r0 or newer). If an immediate upgrade is not possible, implement strict input validation as a temporary mitigation to sanitize image resizing parameters before they are passed to the library, specifically disallowing negative values for dimensions.

How to confirm the fix worked

To confirm remediation, verify that the version of the Python Pillow library installed in all relevant environments is 3.1.1 or higher. Use package management tools like `pip list | grep Pillow` to check the installed version. To identify exposure, perform a dependency scan across all applications to find instances of vulnerable Pillow versions. A Software Bill of Materials (SBOM) can greatly aid in this discovery process.

Typical attack path

1. An attacker identifies a target application that processes user-uploaded images, such as a web application with a profile picture feature.
2. The attacker crafts a malicious image file containing specific parameters that trigger the integer overflow when the image is resampled.
3. The attacker uploads the malicious image to the target application.
4. The application's backend, using the vulnerable Pillow library, attempts to resize or process the uploaded image.
5. The library's resampling function is called, the integer overflow occurs, and a heap buffer overflow is triggered.
6. The overflow is exploited to execute a payload, giving the attacker remote control over the server with the permissions of the application process.

How to detect it

Detecting past exploitation can be challenging. Security teams should review application and system logs for unexpected crashes or segmentation faults related to the process that handles image manipulation. A stack trace involving the `ImagingResampleHorizontal` function would be a strong indicator. Monitor for anomalous behavior from application servers, such as spawning of shells (e.g., /bin/sh, cmd.exe) or unexpected outbound network connections, which could signify a successful compromise.

What your security team should do

• Prioritize upgrading the Pillow library to version 3.1.1 or newer on all systems, especially public-facing web servers that process user-uploaded images.
• Scan your software inventory for all instances of the Pillow library to identify all affected applications and ensure they are patched.
• Implement robust input validation for any file upload functionality as a defense-in-depth measure, ensuring that image metadata and processing parameters are within expected bounds before being handled by third-party libraries.