CVE-2024-43767: Analyzing a Serious Heap Overflow in SkBlurMaskFilterImpl.cpp - Exploit and Mitigation Explained

Hello everyone! Today, we are going to take a closer look at CVE-2024-43767, a serious heap overflow vulnerability that was found in the prepare_to_draw_into_mask function of SkBlurMaskFilterImpl.cpp. This flaw has the potential to lead to remote code execution without requiring any additional execution privileges or user interaction. In this post, we will provide some code snippets, link the relevant references, and share details about the exploit and how it can be mitigated.

Vulnerability: CVE-2024-43767

In the file SkBlurMaskFilterImpl.cpp, the prepare_to_draw_into_mask function is implemented as part of the Skia Graphics Engine. Skia is a 2D graphics library that provides various graphics-related capabilities to applications and platforms. Within this particular function, improper input validation can trigger a heap overflow, leading to severe consequences such as remote code execution.

The relevant code snippet from SkBlurMaskFilterImpl.cpp is shown below

// SkBlurMaskFilterImpl.cpp
static sk_sp<SkSpecialImage> prepare_to_draw_into_mask(
                                     const SkSpecialImage* src,
                                     SkIPoint* srcPt,
                                     const SkIRect& bounds,
                                     uint32_t pad,
                                     SkColor color) {
    int w = bounds.width() + pad;
    int h = bounds.height() + pad;
    SkImageInfo info = SkImageInfo::MakeA8(w, h);
    // ...
}

In the above code snippet, the width (w) and height (h) of the image are calculated by adding a pad value to the bounds of the source image. This calculation could potentially cause issues if the addition results in an integer overflow. If such an overflow occurs, the heap buffer allocation (SkImageInfo::MakeA8(w, h)) that follows would be smaller than intended, allowing subsequent operations to write beyond the allocated memory, leading to a heap overflow.

Exploit Details

The exploit for this vulnerability relies on creating an image with dimensions such that adding the pad value will cause an integer overflow. By doing this, the attacker can trigger the improper input validation and ultimately cause a heap overflow. Once the heap buffer is overrun, the attacker could potentially gain the ability to execute arbitrary code on the target system.

To achieve this, the attacker would need to craft a specially-crafted image or document that embeds such an image and convinces the user to open it with an application that uses the Skia Graphics Engine. Since user interaction is not required for exploitation, this makes the vulnerability even more dangerous and increases the risk of exploitation.

Mitigation Steps

The recommended solution to prevent this vulnerability is to ensure input validation is properly implemented in the prepare_to_draw_into_mask function. One way to achieve this is to make sure the width and height calculations do not result in an integer overflow.

Below is an example of how this can be done

// SkBlurMaskFilterImpl.cpp
static sk_sp<SkSpecialImage> prepare_to_draw_into_mask(
                                     const SkSpecialImage* src,
                                     SkIPoint* srcPt,
                                     const SkIRect& bounds,
                                     uint32_t pad,
                                     SkColor color) {
    if (bounds.width() > INT_MAX - pad || bounds.height() > INT_MAX - pad) {
        // Handle the integer overflow case
        return nullptr;
    }

    int w = bounds.width() + pad;
    int h = bounds.height() + pad;
    SkImageInfo info = SkImageInfo::MakeA8(w, h);
    // ...
}

In the updated code snippet, we added an if-statement that checks if the addition of the pad to the image dimensions would cause an integer overflow. If an overflow is detected, the function returns a nullptr to gracefully handle the error scenario. This simple change in the code can help prevent the heap overflow and subsequent exploitation.

Conclusion

The vulnerability identified as CVE-2024-43767 in the prepare_to_draw_into_mask function of SkBlurMaskFilterImpl.cpp is a severe heap overflow that could lead to remote code execution without user interaction or additional execution privileges. To protect against this exploit, proper input validation must be implemented. It is crucial to apply the necessary patches or workarounds recommended by the developers.

For more information and guidance, you can refer to the original references here

- Skia Graphics Engine repository: GitHub
- CVE-2024-43767: Official CVE Details

Stay safe, and always keep your system up-to-date to protect against possible threats!

Timeline

Published on: 01/03/2025 01:15:07 UTC
Last modified on: 01/03/2025 23:15:06 UTC