The Linux graphical landscape heavily depends on Xorg, a foundational open-source implementation of the X Window System. On April 2024, security researchers identified a critical vulnerability—CVE-2024-31083—that exposes Xorg servers to potential arbitrary code execution attacks by authenticated users. This write-up breaks down the vulnerability, explains how the exploit works with code snippets, and lists referenced sources.

What is CVE-2024-31083?

CVE-2024-31083 is a *use-after-free* flaw discovered in the ProcRenderAddGlyphs() function of the Xorg X11 server. This bug lurks in how newly-uploaded glyphs are managed and affects many Linux desktop environments.

Vulnerability in Simple Terms

When a client (for example, your desktop app) uploads new text glyphs (basically shapes of characters) to the X server using the RENDER extension, they go through a function called ProcRenderAddGlyphs(). Here’s where the trouble starts:

1. The code allocates new glyphs, but doesn’t manage reference-counting (this means it can’t track if more than one thing points to the same glyph).
2. If a client tricks the server into storing multiple references to the same glyph (using AllocateGlyph()), the server can later free the same glyph pointer twice or use a pointer after it’s already been freed.

This is a classic *Use-After-Free* bug, which can destabilize the server or, worse, allow a savvy attacker to run custom code on the system.

How Does the Exploit Work?

Since Xorg often runs with elevated privileges, successful exploitation can mean root access for an attacker.

Step-by-Step Attack

1. Connect as an authenticated client to the X server (should have permission to use the display, not hard for a local user).
2. Send a crafted RENDER AddGlyphs request. Structure it so two entries point to the same glyph pointer.

ProcRenderAddGlyphs() will mistakenly store duplicate pointers to the same glyph.

- When freeing glyphs, it frees the same memory twice or uses the memory after it’s freed (hence use-after-free).

Here’s a simplified version of the vulnerable logic found in ProcRenderAddGlyphs()

// Simplified excerpt, adapted for illustration
int ProcRenderAddGlyphs(ClientPtr client)
{
    // ...parsing input, preparing data...

    // Allocate space for each new glyph
    GlyphPtr *glyphs = malloc(num_glyphs * sizeof(GlyphPtr));
    for (int i = ; i < num_glyphs; i++) {
        // Problem: AllocateGlyph might return the same pointer for different glyph IDs
        glyphs[i] = AllocateGlyph(same_data);
    }

    // Later in the logic:
    for (int i = ; i < num_glyphs; i++) {
        FreeGlyph(glyphs[i]); // Here, the same pointer might be freed multiple times
    }

    // ...rest of function...
}

Exploit Example (Python Pseudocode)

An attacker might script this with python-xlib or raw sockets, but here’s a *pseudocode* of what happens:

# Connect to local X server
disp = open_display()

# Craft the AddGlyphs request where two glyph IDs share the same data/pointer
send_render_addglyphs(
    glyph_ids=[101, 102],
    glyph_datas=[same_bytes, same_bytes]
)
# The server will now store two references to the same pointer, leading to UAF

Is My System Vulnerable?

- Affected: Xorg X server up to 21.1.12 and likely some popular distros (check issue tracker).

How to Fix?

- Upgrade Xorg: Upstream patch released. Your distro should have packages.

Restrict access to X server (xhost -), use Wayland if possible.

- Isolate untrusted users/apps from Xorg.

References

- Xorg official advisory and patch
- Bug Track: freedesktop.org #147
- CVE Detail - CVE-2024-31083
- SecurityWeek coverage
- Original function source: ProcRenderAddGlyphs() in render.c

Closing Thoughts

CVE-2024-31083 is a powerful reminder that privileges and memory safety are still serious issues, even in old-school software like Xorg. If you run Linux with Xorg, patch as soon as possible. Keep graphical interfaces restricted to trusted users—use the principle of least privilege whenever you can.

Stay safe!
(If you want to test this vulnerability in-depth, do so responsibly, in isolation, and never on production systems.)

Timeline

Published on: 04/05/2024 12:15:37 UTC
Last modified on: 04/25/2024 18:15:08 UTC