A significant vulnerability (CVE-2023-39976) has been identified in the log_blackbox.c file of the libqb library, versions earlier than 2..8. This.Buffer Overflow vulnerability allows attackers the potential to crash the system or execute arbitrary code via long log messages. This is a result of the header size not being taken into consideration during log processing. In this post, we will be taking a more in-depth look at the vulnerability, including the actual code snippets and exploit details, alongside references to the original sources.

Code Snippet

The issue lies within log_blackbox.c, a part of the libqb library, specifically in the handling of log messages. Let's take a closer look at the vulnerable code:

static void
_log_blackbox_write_to_file(struct qb_log_target *t, struct timespec *ts, const char *buffer)
    int32_t rs;
    size_t total_size;
    total_size = sizeof(struct qb_log_entry) + strlen(buffer);
    if (blk_hdr->write_pt + total_size > blk_hdr->size) {

The total_size variable is calculated as the sum of the size of the qb_log_entry structure and the length of the log message buffer. However, the header size is not included in this calculation, which leads to a buffer overflow.

Exploit Details

The vulnerability is a classic buffer overflow attack that stems from improper handling of the log message length. To trigger the buffer overflow, an attacker can craft a long log message exceeding the available buffer size. This can either cause the system to crash or potentially enable the attacker to execute arbitrary code.

For instance, let's assume the attacker generates a log message of the following form

char evil_log[SIZE_OF_EVIL_LOG] = {'A', 'A', 'A', ..., 'A', '\'};
qb_log(LOG_DEBUG, "%s", evil_log);

Where SIZE_OF_EVIL_LOG is chosen in such a way that the resulting total_size (i.e., sizeof(struct qb_log_entry) + strlen(evil_log)) exceeds the available buffer.

By sending this oversized log message to the libqb's logging facility, the header size is not considered, and the buffer overflow is triggered, potentially leading to a crash or arbitrary code execution.


This vulnerability has been fixed in libqb version 2..8. The developers had addressed this issue by correctly incorporating the header size during the log message handling process. It is highly recommended to update your libqb version to 2..8 or later to protect your system from this vulnerability.

// Correctly calculate total_size by including sizeof(struct qb_log_hdr)
total_size = sizeof(struct qb_log_entry) + sizeof(struct qb_log_hdr) + strlen(buffer);

Original References

1. CVE-2023-39976
2. libqb Security Advisory


In this post, we discussed the CVE-2023-39976 vulnerability, a buffer overflow in the log_blackbox.c file of the libqb library. The vulnerability exists due to neglecting to account for the header size in log messages. By providing a long enough log message, attackers could exploit this vulnerability to crash the system or execute arbitrary code. To mitigate this issue, it is crucial to update to libqb version 2..8 or later where the vulnerability has been addressed. Remember to always keep your libraries updated and follow security best practices to protect your systems effectively.


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