CVE-2023-38470: A Discoverable Assertion Vulnerability Found in Avahi's Escape Label Function

A recent vulnerability, CVE-2023-38470, has been identified in Avahi, an open-source zero-configuration networking (Zeroconf) implementation commonly used for facilitating networked device discovery and communication. The vulnerability exists within the avahi_escape_label() function, where a reachable assertion can potentially allow malicious attackers to crash the service, causing denial of service (DoS) conditions.

This comprehensive long-read post will delve into the specifics of CVE-2023-38470, outlining the vulnerable code snippet, relevant links to original references, and details concerning the exploit.

Vulnerable Code Snippet

The issue resides within the avahi_escape_label() function in the files avahi-core/unescape.c and avahi-core/escape.c. The vulnerable code snippet appears as follows:

void avahi_escape_label(const char *l, char *r, size_t rl) {
    unsigned n;
    size_t i, k = ;

    assert(l);
    assert(r);

    for (i = ; i < rl && l[i]; i++) {
        n = (unsigned)(unsigned char) l[i];

        if (n == '\\') {
            r[k++] = '\\';
            r[k++] = '\\';
        } else if (n >  && n < 127 && !strchr(AVAHI_ESCAPED_CHARS, n)) {
            r[k++] = (char) n;
        } else {
            assert(k < rl-3);
            snprintf(r+k, 4, "\\%03o", n);
            k += strlen(r+k);
        }
    }

    r[k] = ;
}

In this code, the assumption made is that the input string "l" for the escape label function would require less than two times the remaining length of the output string "r". If a malicious user is able to manipulate the input such that it does not meet this assumption, an assertion failure occurs, leading to a crash.

Original References

1. The CVE announcement and detailed information can be found on the CVE website: CVE-2023-38470

2. The official Avahi repository on GitHub which contains the affected source code is available at: github.com/lathiat/avahi

Exploit Details

Although specific exploit code has not yet been disclosed, a knowledgeable attacker might create test cases that trigger the reachable assertion by crafting custom packets containing specifically designed hostnames. Upon being processed by the vulnerable Avahi daemon, these malicious hostnames could cause denial of service due to assertion failure and crashing of the service.

Mitigation

As CVE-2023-38470 has only recently been reported, it is critical for users of the Avahi package to keep an eye on updates and security patches. Furthermore, organizations and developers relying on Avahi might want to proactively scrutinize their usage of the avahi_escape_label() function, and perhaps implement additional checks and balances to safeguard against this vulnerability.

Conclusion

In light of CVE-2023-38470, it becomes all the more important for developers, security teams, and network administrators to prioritize regular scrutiny and updates of critical software components. In doing so, potential security threats, such as the vulnerability detailed in this post, may be more promptly identified and remediated. Until an official patch is released, it is essential to remain vigilant and exercise caution in deploying instances of the vulnerable Avahi service.

Timeline

Published on: 11/02/2023 15:15:08 UTC
Last modified on: 11/09/2023 19:58:39 UTC