CVE-2024-4741 - Use-After-Free Vulnerability in OpenSSL’s `SSL_free

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Overview

CVE-2024-4741 exposes a rare but potentially severe security flaw in OpenSSL. The bug stems from unsafe handling in the SSL_free_buffers function, which can cause a use-after-free situation under certain timing conditions. While the misuse risk is limited—this function is seldom used directly—it still warrants attention for application developers and security teams.

This write-up breaks down the vulnerability in plain language, includes example code, explains the risks, and provides references for further reading.

The Issue in Simple Terms

A use-after-free bug happens when a program tries to use memory it already gave back to the system—think of grabbing a book you just returned to the library. With OpenSSL's SSL_free_buffers function, under specific networking conditions, this can happen:

- First Scenario: OpenSSL processed some of a network record (the "header"), but not the new data ("body"). If you call SSL_free_buffers now, it frees the buffer, but OpenSSL is still trying to use it.
- Second Scenario: A full data record is in the buffer, but your app has read only part of it. If you prematurely free the buffer, OpenSSL continues to operate on a now-freed memory zone.

Either situation opens the door to data corruption, crashes, or (in worst cases) remote code execution—if a clever attacker engineers just the right situation.

Key point: Only code that directly calls SSL_free_buffers is vulnerable. If your app or dependencies don't call it, you're not at risk.

Vulnerable Code Example

Below is a minimal, illustrative C snippet to show how improper use of SSL_free_buffers triggers the bug. Don't use this pattern!

#include <openssl/ssl.h>

// Assume ssl is a properly set up, connected SSL* object

// Hypothetical broken sequence
void insecure_free_buffers(SSL *ssl) {
    // (1) Read some data, only the header arrives so far
    int n = SSL_read(ssl, buffer, HEADER_SIZE);

    // (2) Now, attacker causes delayed/partial delivery

    // (3) App prematurely frees buffer before reading full record
    SSL_free_buffers(ssl);  // DANGEROUS! Use-after-free triggers here

    // (4) Subsequent reads may crash or worse
    n = SSL_read(ssl, buffer, BODY_SIZE);  // Use-after-free occurs
}

This is an illustrative race—your real code would likely be more complex, but this demonstrates how the vulnerability arises.

Exploitability

- Who is at risk? Only applications that *explicitly call* SSL_free_buffers. Most apps (including mainstream servers and clients) *never* call this directly.
- Attack Vector: An attacker on the network could potentially manipulate when data arrives to maximize the odds that your app frees an in-use buffer.
- Impact: Data corruption, application crash, or—if an attacker crafts their attack just right—potential remote code execution.

There are currently no known active exploits in the wild. This likely reflects how rarely the vulnerable API is used outside of specific, low-level or custom OpenSSL integrations.

Recommended Actions

- Audit your code: Search for any use of SSL_free_buffers. If you find it, review usage and update OpenSSL to a non-vulnerable version as soon as possible.
- Update OpenSSL: Patch to the latest version with the fix applied (see official OpenSSL advisory).

If using a Linux distribution’s packages, apply their security updates as soon as they are released.

References

- OpenSSL CVE-2024-4741 Security Advisory
- OpenSSL Source Code Repository
- NIST NVD Entry for CVE-2024-4741

Final Thoughts

While this vulnerability does not affect most mainstream applications, if your project or a library you depend on uses SSL_free_buffers, this is a serious issue. The race condition lurking here could—if found and exploited—lead to classic, devastating security outcomes.

As always, keeping dependencies up to date and auditing any unusual API usage is the best defense!

*This analysis is exclusive and intended to clarify CVE-2024-4741 for developers and security teams. Always consult original advisories for authoritative details.*

Timeline

Published on: 11/13/2024 11:15:04 UTC
Last modified on: 11/13/2024 15:35:12 UTC