CVE-2023-32001 - libcurl's Vulnerability to TOCTOU Exploitation via Cookie, HSTS, and alt-svc Data Saving

In recent years, numerous vulnerabilities have surfaced affecting even widely-used libraries, such as libcurl. One such vulnerability, CVE-2023-32001, enables attackers to exploit libcurl's Time of Check to Time of Use (TOCTOU) race condition when saving cookie, HSTS, or alt-svc data to files. By taking advantage of this flaw, attackers gain the ability to create or overwrite protected files, potentially leading to severe security consequences. This article delves into the exploitative aspects of the vulnerability, referencing original sources, providing code snippets to demonstrate the issue, and offering possible remediation methods.

Link to Original Reference: https://curl.se/docs/CVE-2023-32001.html

The Vulnerability

CVE-2023-32001 concerns libcurl's stat() and fopen() functions and the manner in which they are vulnerable to a TOCTOU race condition. When libcurl saves cookie, HSTS, or alt-svc data to files, it calls these functions, inadvertently opening the door for exploitation. A diligent attacker can manipulate these calls such that the victim is tricked into creating or overwriting protected data files unintentionally.

Code Snippet

The following code snippet demonstrates the problematic usage of stat() and fopen() functions, leading to the TOCTOU race condition vulnerability:

struct_stat file_stat;
FILE *file;

if(stat(file_name, &file_stat) == -1) {
    // Error handling
} else {
    if(S_ISREG(file_stat.st_mode)) {
        file = fopen(file_name, "wb");
        if(file == NULL) {
            // Error handling
        }
    }
}

As seen in the code snippet, the stat() function checks if the file exists and, subsequently, the fopen() function attempts to open it. However, the time gap between these two function calls is enough for an attacker to manipulate conditions, thus exploiting the race condition.

Exploit Details

Upon successful exploitation of CVE-2023-32001, an attacker can trick a victim into overwriting or creating protected files in unintended ways. This can be done by strategically controlling the resources provided to the environment where libcurl is used (e.g., file systems or file permissions). For instance, an attacker could disguise a symbolic link as a legitimate data file, leading the victim to write sensitive information into an unintended file.

Possible Remediation

To mitigate the impact of CVE-2023-32001, developers and administrators should update libcurl to version 7.76. or later as soon as possible. In these updated versions, libcurl utilizes the fstat() function alongside a file descriptor to determine the existing file's state, thus eliminating the time gap and the possibility of a race condition.

Code Snippet (Fixed)

The following code snippet demonstrates the remediated usage of fstat() function to eliminate the aforementioned vulnerability:

struct_stat file_stat;
FILE *file;

file = fopen(file_name, "wb");
if(file == NULL) {
    // Error handling
} else {
    if(fstat(fileno(file), &file_stat) == -1) {
        // Error handling
    } else {
        if(S_ISREG(file_stat.st_mode)) {
            // Proceed with data writing
        }
    }
}

As seen in the fixed code snippet, the updated version of libcurl ensures that the fstat() function is called immediately after opening the file, effectively eliminating any window for an attacker to exploit the race condition vulnerability.

By proactively addressing issues such as CVE-2023-32001, developers and administrators can greatly enhance the overall security of their applications that depend on libraries like libcurl. Regular audits and updates can prevent many security vulnerabilities before they have a chance to cause harm. Remember, it's always better to be proactive than reactive when it comes to cybersecurity!

Timeline

Published on: 07/26/2023 21:15:00 UTC
Last modified on: 08/03/2023 15:11:00 UTC