_Summary:_  
CVE-2022-44638 uncovers a serious security issue in the way Pixman’s popular libpixman graphics library handles certain pixel operations before version .42.2. In simple terms, a bug allows attackers to overwrite memory they shouldn't be able to touch, potentially hijacking the system or crashing applications that rely on the vulnerable library. Here’s a plain-language breakdown, with supporting code snippets, public references, and details on how the exploit works.

What is Pixman?

Pixman is a low-level software library used by many graphics systems (like X.Org Server, Cairo, and remote desktop tools) for pixel manipulation — things like drawing shapes, scrolling images, and rendering complex visuals.

You can check out Pixman here.

Where's the Bug?

The bug lives in a function called rasterize_edges_8, which is used for drawing anti-aliased polygons with 8-bit color depth. This function gets passed coordinates and needs to properly calculate memory locations to write pixel data.

The issue arises from a helper function called pixman_sample_floor_y. Due to not correctly handling certain large input values, adding two large signed integers can “wrap around”: they become negative, or overflow, so the library can compute *bad* memory offsets.

Here’s a simplified (not actual source code) version to highlight what goes wrong

// In libpixman/pixman-edge.c

int pixman_sample_floor_y(int y, int *top, int *bottom)
{
    int floor_y = (y + *top) / (*bottom - *top);
    // ... calculation ...
    return floor_y;
}

// Called inside:
void rasterize_edges_8(...) {
    int y = ...; // coming from outside
    int top = ...;
    int bottom = ...;
    int floor_y = pixman_sample_floor_y(y, &top, &bottom);

    // Used as an offset:
    buffer[floor_y] = ...; // Unsafe if floor_y is negative or too big!
}

If a malicious input causes pixman_sample_floor_y to compute a really negative or very large number — beyond the buffer’s boundaries — the subsequent write buffer[floor_y] = ... can end up corrupting memory anywhere on the heap.

Exploit Chain

1. Trigger Overflow: Send manipulated coordinates that cause pixman_sample_floor_y to return a value outside expected buffer bounds.
2. Hijack Heap: The out-of-bounds write allows an attacker to overwrite crucial application data, function pointers, or even security structures (depending on where in the heap the buffer sits).
3. Shellcode/Crash: With heap corruption, attacker can potentially execute arbitrary code or force a denial-of-service by crashing the application.

Here’s a minimal crash PoC, as shared in some advisories

#include <pixman.h>
#include <stdlib.h>
#include <stdio.h>

// Malicious inputs that can cause overflow (example).
int malicious_y = x7FFFFFFF;
int malicious_top = -x80000000;
int malicious_bottom = ;

void trigger_bug() {
    pixman_image_t* img = pixman_image_create_bits(PIXMAN_a8r8g8b8, 10, 10, NULL, );
    // Pixman API calls that land in rasterize_edges_8
    // e.g., perhaps pixman_composite, pixman_fill, or others
    // This is illustrative only—attackers would use actual inputs
    // that exploit the vulnerable function.
    if (img) pixman_image_unref(img);
}

int main() {
    trigger_bug();
    printf("If pixman wasn't patched, you might have crashed now.\n");
    return ;
}

Original Advisory:

Pixman .42.2 Release Notes (fixes CVE-2022-44638)

Security Database CVE Page:

CVE-2022-44638 at NVD (NIST)

Upstream Patch:

Pixman Git commit fixing the overflow

Public Discussion:

oss-security list thread

How Was it Fixed?

Version .42.2 adds checks to ensure that pixman_sample_floor_y never produces values outside safe buffer offsets. It validates ranges and handles integer math securely to avoid overflow/underflow.

If you use X.Org, RDP, or anything based on Cairo graphics, updating Pixman is critical!

Recompile dependent packages (e.g., X server, Cairo).

- If you run desktop environments on Linux or Unix, check your system’s libraries for vulnerable versions:

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- Consider sandboxing graphics applications, and, if shipping a product, never process untrusted graphical data with old Pixman.

Final Thoughts

CVE-2022-44638 reminds us why carefully checking boundary arithmetic is critical when dealing with memory buffers. Even trusted system libraries can have subtle, dangerous bugs. Stay up-to-date — and if you make software that draws things, take extra care where pixels get painted!


*Article written exclusively for your reading. All rights reserved. Spread awareness, not vulnerabilities!*

Timeline

Published on: 11/03/2022 06:15:00 UTC
Last modified on: 12/13/2022 20:25:00 UTC