A recent vulnerability (CVE-2024-7264) discovered in libcurl's ASN1 parser code has implications for those using the library and poses a potential risk. The GTime2str() function, used for parsing an ASN.1 Generalized Time field, can lead to unintended consequences if given a syntactically incorrect field. These issues could range from a simple crash to possibly returning heap contents to the application when CURLINFO_CERTINFO is used.

In this long read post, we'll first dive into the background of the issue and utilize code snippets to illustrate the problem at hand. Next, we'll outline the necessary steps to reproduce and exploit the vulnerability, and finally, we'll point to the solution offered by the developers as well as suggesting best practices for avoiding similar vulnerabilities in the future.

Background

The vulnerability stems from the GTime2str() function present in libcurl's ASN1 parser code. This function is responsible for parsing an ASN.1 Generalized Time field. In layman's terms, ASN.1 (Abstract Syntax Notation One) is a standardized notation for representing data structures in a compact and machine-readable format. Generalized Time, on the other hand, is a specific field used for specifying time in a human-readable format.

The issue arises when the parser is given a syntactically incorrect field. In this case, the parser uses -1 for the length of the *time fraction*, which can lead to several unwanted effects. This could cause a strlen() operation to be performed on a pointer to a heap buffer area that is not (purposely) null terminated. In the worst-case scenario, this flaw could allow heap contents to be exposed to the application when CURLINFO_CERTINFO is employed.

Here's a simplified code snippet illustrating the GTime2str() function

char *GTime2str(asn1_time *atime) {
  char *output_str = NULL;
  long length;
  
  /* ... */

  length = /* time fraction length calculation */;
  if (length == -1) {
    strlen(some_pointer);
  }

  /* ... */

  return output_str;
}

Reproducing and Exploiting the Vulnerability

To reproduce the vulnerability, an attacker needs to provide syntactically incorrect ASN.1 Generalized Time fields, which would lead to the -1 length value setting for the *time fraction* and subsequent issues.

It's worth noting that while the most common outcome may be a crash, there is still the possibility that sensitive heap data can be leaked via the CURLINFO_CERTINFO functionality to exploit. One way to carry out this exploit could be to develop a malicious website with a specifically crafted certificate that is syntactically incorrect in the time field and lures users to visit it using libcurl.

Solution and Best Practices

The libcurl developers have been quick to address the vulnerability, and it's important to update your library to the latest version (check official libcurl release notes). This fix helps eliminate the possibility of the wrong input leading to unintended consequences.

Additionally, users should always validate the input and inquiry about the authenticity of any certificate received over the network. These best practices can help limit the potential for future exploits in similar scenarios.

Conclusion

In this post, we covered CVE-2024-7264, a vulnerability in libcurl's ASN1 parser code, with an in-depth analysis of the problem and code snippets to illustrate the inner workings of the issue. We also provided steps to reproduce and exploit the vulnerability and pointed to the solution provided by the developers, drawing attention to best practices for avoiding similar pitfalls in the future.

Timeline

Published on: 07/31/2024 08:15:02 UTC
Last modified on: 08/12/2024 17:30:51 UTC