In today's interconnected world, many devices and systems depend on effective communication to function efficiently. Many of these systems use the Avahi daemon, an open-source implementation of the Zeroconf protocol suite that is essential for facilitating network browsing and service discovery. Imagine the impact if a vulnerability were to be discovered in this critical software: countless devices and systems could be at risk.

In this article, we delve into a newly identified vulnerability in Avahi, dissecting the code and shedding light on the origin of the problem. The vulnerability, CVE-2023-38469, is a reachable assertion found in the avahi_dns_packet_append_record function. By understanding the underlying issue, security experts can better protect their systems and thwart potential attacks.

CVE-2023-38469 - Vulnerability Details

The CVE-2023-38469 vulnerability exists in Avahi's handling of the DNS packet record. More specifically, it occurs within the avahi_dns_packet_append_record function. This is a component of Avahi's core networking functionality, responsible for appending new records to DNS packets.

The code snippet below illustrates the problematic area

int avahi_dns_packet_append_record(AvahiDnsPacket *p, AvahiIfIndex idx, const AvahiRecord *r, int flush_cache) {
    unsigned char e[256];
    size_t size;

    size = avahi_encode_record(r, e, sizeof(e), idx, flush_cache);

    if (avahi_dns_packet_space(p) < size)
        return ;

    assert(size <= 256);
    memcpy(AVAHI_DNS_PACKET_FIELD(p, qdcount + 4 + 2 + p->index), e, size);

In the code above, a reachable assertion exists, which means that a logical expression is false when it should be true. This is problematic because it can lead to unexpected behavior and potential security issues, such as denial-of-service attacks.

The root of this vulnerability lies in the following line of code

assert(size <= 256);

The 'size' variable is assumed to be less than or equal to 256; however, no proper validation is performed before this assertion. Thus, if an attacker can exploit this assumption and cause the 'size' variable to exceed 256, the assertion would fail, leading to unexpected behavior and potential security issues.

Exploiting CVE-2023-38469

There are several potential ways to exploit this vulnerability, allowing an attacker to gain unauthorized access to a system or cause a denial-of-service attack. One possible method involves crafting a malicious DNS packet that causes the 'size' variable to exceed 256, triggering the assertion failure.

Additional References

For more information about Avahi, the Zeroconf protocol suite, and this particular vulnerability, visit the following resources:

1. The original Avahi project page:
2. Description of the Zeroconf protocol suite:
3. The official CVE entry for CVE-2023-38469:


As we have seen, CVE-2023-38469 is a serious vulnerability that could have far-reaching consequences for countless systems and devices relying on the Avahi daemon. By understanding the underlying issue and potential attack vectors, security professionals are better equipped to safeguard their networks and protect valuable assets.


Published on: 11/02/2023 15:15:08 UTC
Last modified on: 11/09/2023 19:58:53 UTC