In this post, we will discuss a recently identified vulnerability, CVE-2023-27536, which affects libcurl versions prior to 8... This vulnerability has been discovered in the connection reuse feature, and it could potentially lead to unauthorized access to sensitive information during krb5/kerberos/negotiate/GSSAPI transfers.

The Vulnerability (CVE-2023-27536)

An authentication bypass vulnerability exists in libcurl versions prior to 8.., specifically within its connection reuse feature. Due to a failure in checking for changes in the CURLOPT_GSSAPI_DELEGATION option, it is possible for previously established connections to be reused with incorrect user permissions.

This particular vulnerability affects krb5/kerberos/negotiate/GSSAPI transfers and can result in unauthorized access to sensitive information. The connection reuse feature is meant to increase efficiency and speed up data transfers. However, when the CURLOPT_GSSAPI_DELEGATION option is changed without proper checks, it opens up the potential for this security flaw.

Here is a simple demonstration of using CURLOPT_GSSAPI_DELEGATION with libcurl in C

#include <curl/curl.h>

int main(void)
  CURL *curl;
  CURLcode res;

  curl = curl_easy_init();

  if(curl) {
    curl_easy_setopt(curl, CURLOPT_URL, "";);

    res = curl_easy_perform(curl);

    if(res != CURLE_OK)
      fprintf(stderr, "curl_easy_perform() failed: %s\n",


  return ;

In this example, connections might be reused with incorrect user permissions when CURLOPT_GSSAPI_DELEGATION is changed without properly checking for any alterations.

Exploit Details

An attacker could exploit this vulnerability by intercepting a connection that has been reused with incorrect user permissions. This would potentially allow unauthorized access to sensitive information being transferred using krb5/kerberos/negotiate/GSSAPI.

Here are some original references to learn more about this vulnerability

1. Official CVE-2023-27536 Information
2. libcurl Security Advisory
3. libcurl GitHub Repository


The safest option to mitigate this vulnerability is to avoid reusing connections if the CURLOPT_GSSAPI_DELEGATION option has been changed. Alternatively, users are encouraged to update their libcurl installation to version 8.. or later, which includes a patch for this issue.


The discovery of the CVE-2023-27536 vulnerability in libcurl's connection reuse feature serves as a reminder of the importance of secure coding practices and regularly updating software to the latest versions. By understanding and addressing security issues like this, developers can create more secure applications that are less likely to suffer unauthorized access and data breaches.

Remember to always keep your software up-to-date and follow best coding practices to minimize the risk of vulnerabilities like CVE-2023-27536.


Published on: 03/30/2023 20:15:00 UTC
Last modified on: 04/21/2023 23:15:00 UTC