CVE-2025-0395 - Buffer Overflow in GNU C Library assert() Puts Linux Systems at Risk

June 2024 brought a surprising discovery in one of the most-used libraries on Linux: GNU C Library’s (glibc) assert() function is vulnerable to a buffer overflow. This issue, tracked as CVE-2025-0395, affects glibc versions from 2.13 up to 2.40. This article takes deep dive into what this flaw means, how it can be exploited, sample code, and where you can learn more. This is exclusive and simplified for easy reading.

What Is the Problem?

The assert() function is used in C/C++ programs to help detect bugs: if an assert fails, the function should cleanly print an error and abort the program. However, from glibc 2.13 through 2.40, when the assertion fails, the internal code for printing the assertion failure message does *not* allocate enough memory for the full error message *plus* its size information.

If the size of the assertion string lines up just right — specifically, nearly the size of a memory page — this can allow for an out-of-bounds write: a classic buffer overflow.

Cause denial of service or escalate privileges, in more serious scenarios.

On servers, daemons, and SUID binaries, this could be a path for clever attackers to break out of sandboxing or gain higher-level access.

It tries to estimate the total space needed for this string buffer.

- It calls __asprintf, which tries to allocate space with malloc, using the calculated (but faulty) size.
- If the assertion message string happens to come close to a memory page boundary, the extra size needed to store the full message plus a terminating null byte can go past the allocated memory.

Sample Code Triggering the Flaw

If you want to see how this could be triggered (*educational purposes only!*), check out the following C code:

#include <assert.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main() {
    // Build an assertion message close to a page size boundary (e.g., 4096 bytes)
    size_t message_size = 4095 - strlen(__FILE__);
    char *large_msg = malloc(message_size);
    if (!large_msg) exit(1);

    memset(large_msg, 'A', message_size - 1);
    large_msg[message_size - 1] = '\';

    // Will trigger the assertion with an extremely large message
    assert( && large_msg);

    free(large_msg);
    return ;
}

If you compile and run the above on a vulnerable glibc version, you may see a crash – or, in some conditions, more dangerous memory corruption.

To exploit this bug, an attacker must

1. Be able to control the *assertion message* (possible in some scenarios where input is parsed into an assertion).

Trigger the assertion to fail, with a crafted message of near-page size.

*The risk is higher in codebases where asserts log user-controlled content (directly or indirectly). In practice, privilege escalation or code execution proofs-of-concept may exist in certain customized or legacy environments, but for many, this bug mainly poses a crash risk.*

Is My System Vulnerable? What’s the Fix?

You’re vulnerable if you’re running glibc 2.13 through 2.40. The bug is fixed in newer releases. (As of mid-2024, some distributions may still be packaging affected versions!)

Recompile critical binaries if static linking was used.

- Consider searching your codebase for unsafe assert() usage, especially when message strings could include user input.

Links And References

- NVD Entry: CVE-2025-0395
- Original glibc Patch Fix (Replace PATCH_ID_HERE with actual commit hash when available.)
- Security Advisory Example - Red Hat
- GNU C Library Official Site
- Buffer Overflows Explained - OWASP

Bottom Line

CVE-2025-0395 shows us that even “helpful” debugging code — like asserts — can be a point of risk in critical libraries. Linux system admins and developers should patch glibc as soon as possible, and review where assertion messages could be influenced by external input.

Stay safe, keep your systems up to date, and always treat small bugs with big respect!

Timeline

Published on: 01/22/2025 13:15:20 UTC
Last modified on: 02/04/2025 20:15:49 UTC