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Introduction

A new vulnerability, CVE-2024-6197, has been discovered in one of the world’s most widely used data transfer libraries: libcurl. This bug lurks in the code responsible for parsing ASN.1 UTF-8 strings and, while the most common effect is a program crash, in special scenarios it could potentially lead to more serious consequences.

This post explains how the bug works, walks through example code, and discusses possible real-world impacts — using simple language for everyone to follow.

Breaking Down the Bug

The vulnerable function is called utf8asn1str(). Its job is to safely parse ASN.1 encoded UTF-8 strings. If it spots a problem, it returns an error — but here’s the kicker: when it bails out, it accidentally calls free() on a variable that shouldn’t be freed, specifically, a 4-byte buffer that’s stored on the stack (not on the heap).

Modern malloc implementations (like recent glibc) catch this and abort — but not all do.

- Some environments (legacy or embedded) might actually add that “fake” pointer to its free chunk list, meaning future allocations can overwrite stack memory.

Here’s a simplified version of the core logic

int utf8asn1str(const unsigned char *data, size_t len) {
    unsigned char buf[4]; // Local stack buffer
    if (len > 4) {
        // Error: input too large
        free(buf); // Oops! buf is on the stack
        return -1;
    }
    // proceed with parsing...
    return ;
}

What should happen:
free() should be called only on heap pointers that came from malloc() or similar.

What actually happens:
Here, it frees a pointer to a stack variable, which is a violation.

Exploit Details

If a malicious input triggers the error path, and the environment’s memory manager does not abort, here’s what can happen:

Potential code execution, if the attacker is lucky and the stars align

However, in most practical cases, the result is a crash.

If an attacker can reliably control subsequent allocations

…an attacker might be able to escalate to arbitrary code execution. This is unlikely but not impossible.

A simple way to trigger the bug (for research/educational purposes)

#include <stdio.h>
#include <stdlib.h>

// Simulating the vulnerable function
void vuln(const unsigned char* data, size_t len) {
    unsigned char buf[4];
    if (len > 4) {
        free(buf); // Crash or UB here!
        return;
    }
    // Normal processing
}

int main() {
    unsigned char payload[8] = {};
    vuln(payload, 8); // Triggers the bug
    return ;
}

Running this on glibc, you’ll get something like

free(): invalid pointer
Aborted (core dumped)

On non-hardened malloc implementations, the program might not crash immediately.

Who is affected?

Any software or device built with a vulnerable version of libcurl that processes ASN.1 data with utf8asn1str().

- Modern Linux/Unix:

Will typically abort immediately, reducing most risks to a crash (DoS).

- Legacy/Embedded:

Remote Attack Feasibility:

Depends on attacker’s ability to send crafted ASN.1 data to the target, and repeat the process with different payloads.

References

- libcurl Security Advisory: CVE-2024-6197
- ASN.1 and Curl Docs
- Common malloc/free Exploit Techniques

Mitigation

If you use libcurl with ASN.1 features (or you're not sure), update libcurl to the patched version immediately. The fix is simple: remove the invalid free() call.

Example of fixed code

int utf8asn1str(const unsigned char* data, size_t len) {
    unsigned char buf[4];
    if (len > 4) {
        // NO free(buf);
        return -1;
    }
    // Safe parsing now
    return ;
}

Final Words

CVE-2024-6197 is a classic case of mistaken memory management, and while it may usually just crash your program, it’s a great reminder: never call free() on memory you didn’t explicitly malloc().

Keep your dependencies up to date, and check your code — especially those handling low-level parsing and memory.


*Exclusively for educational & security research use — let’s keep the internet safer!*

Timeline

Published on: 07/24/2024 08:15:03 UTC
Last modified on: 08/26/2024 15:25:59 UTC