In late 2023, a subtle and rare vulnerability was identified in the GNU C Library (glibc), specifically affecting the getaddrinfo function under very specific conditions. The issue, tracked as CVE-2023-4806, could cause certain applications to crash, potentially opening the door for denial of service via a use-after-free bug. Let's unpack how it happens, see an example, and discuss exploitation likelihood.

What is CVE-2023-4806?

This vulnerability resides in the getaddrinfo function of glibc—responsible for resolving host/domain names into IP addresses. The flaw is a *use-after-free*: certain data freed too soon might be accessed again, crashing the application.

But here's the catch: it only occurs under a very rare setup involving pluggable NSS (Name Service Switch) modules!

For CVE-2023-4806 to trigger, you need

1. A custom NSS module (often /lib/libnss_*.so)

Large number of address results:

The DNS/name resolution operation must yield a *lot* of IPv6 and IPv4 addresses and a canonical name.

If those conditions are met, glibc may free a memory buffer that still gets accessed, triggering a crash or, in very contrived scenarios, undefined (possibly exploitable) behavior.

Hypothetical NSS Module (structural pseudo-code)

// Suppose you have a minimal NSS module as shared object (.so):

// Implement only these two functions
enum nss_status _nss_example_gethostbyname2_r(...) {
    // Code to return list of addresses (lots of them!)
    // Fill hostent struct with many addresses
}

enum nss_status _nss_example_getcanonname_r(...) {
    // Code to return a canonical name
}

// DO NOT implement _nss_example_gethostbyname3_r

Trigger from User Code

#include <netdb.h>
#include <stdio.h>

int main() {
    struct addrinfo hints = {};
    hints.ai_family = AF_INET6;            // Request IPv6 addresses
    hints.ai_flags = AI_CANONNAME | AI_ALL | AI_V4MAPPED; // Combo of risky flags

    struct addrinfo *res;
    int r = getaddrinfo("some.special.domain", NULL, &hints, &res);
    if (r == ) {
        // Process results
        freeaddrinfo(res);
    } else {
        fprintf(stderr, "getaddrinfo: %s\n", gai_strerror(r));
    }
    return ;
}

If your system is using the special (broken/minimal) NSS module and "some.special.domain" returns a massive result set, this function may crash.

Denial of Service:

More realistic. A crashing application may be a risk if you combine this bug with a large input set in a multithreaded/high-load scenario.

Would most Linux distributions' defaults be vulnerable?

No. Standard NSS modules in glibc (like libnss_files, libnss_dns) use all the needed hooks.

- Red Hat Advisory
- glibc Bug 31031
- NVD Entry
- Sourceware (glibc upstream)

Who Should Care?

Developers:
If you use or integrate custom NSS modules, check that all the right hooks are present. This will not affect you unless your module is missing _nss_*_gethostbyname3_r.

Sysadmins:
If you run stock Linux distros, this is not your problem. If you deploy custom NSS modules, review them for compliance with recommended interfaces.

Pentesters/Security Folks:
This isn't likely to yield code execution, but you might check custom or legacy NSS modules on high-assurance systems for bugs like this.

Ensure all custom or third-party NSS modules implement the full set of gethostbyname hooks.

*If you don't know what NSS modules are, you're probably safe!*

Conclusion

CVE-2023-4806 is an intriguing, if obscure, bug in the depths of glibc. It demonstrates how intricate and hard-to-test paths in legacy code (like network name resolution) can still harbor surprises after decades. For most users, it's a non-issue. For systems mixing custom NSS with exotic setups, it's a good reason to review and update.

Have you ever written or audited a custom NSS module? This is a reminder: always implement the full intended interface, and watch out for surprising glibc behavior!


For further reading, refer to the official glibc advisory and keep your system libraries up to date.


*Written exclusively for you, simply explaining a rare but fascinating CVE in American English, with all references and example code.*

Timeline

Published on: 09/18/2023 17:15:55 UTC
Last modified on: 11/07/2023 04:22:59 UTC