CWE-243: Creation of chroot Jail Without Changing Working Directory

What is Creation of chroot Jail Without Changing Working Directory?

• Overview: Creation of chroot Jail Without Changing Working Directory is a security vulnerability where the chroot() system call is used to create a restricted environment (jail) but fails to change the current working directory, allowing access to files outside of the jail.

• Exploitation Methods:

• Attackers can escape the chroot jail by navigating to file system resources outside of the jail using relative paths.
• Common attack patterns include manipulating the current working directory or using symbolic links to access restricted files.

• Security Impact:

• Direct consequences include unauthorized access to sensitive files outside of the jail.
• Potential cascading effects include privilege escalation and further compromise of the system.
• Business impact could involve data breaches, loss of customer trust, and legal ramifications.

• Prevention Guidelines:

• Specific code-level fixes include calling chdir("/") immediately after chroot() to change the working directory to the root of the jail.
• Security best practices involve thoroughly validating and sanitizing file paths and ensuring minimal permissions are granted.
• Recommended tools and frameworks include static analysis tools to detect improper chroot() usage and employing sandboxing alternatives for enhanced security.

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Technical Details

Likelihood of Exploit: High

Affected Languages: C, C++

Affected Technologies: Not specified

The chroot() system call allows a process to change its perception of the root directory of the file system. After properly invoking chroot(), a process cannot access any files outside the directory tree defined by the new root directory. Such an environment is called a chroot jail and is commonly used to prevent the possibility that a processes could be subverted and used to access unauthorized files. For instance, many FTP servers run in chroot jails to prevent an attacker who discovers a new vulnerability in the server from being able to download the password file or other sensitive files on the system.

Vulnerable Code Example

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

void setup_chroot_jail(const char *new_root) {
    // Vulnerable code: chroot is called but the working directory is not changed
    if (chroot(new_root) != 0) {
        perror("chroot failed");
        exit(EXIT_FAILURE);
    }

    // Missing chdir("/") to change working directory after chroot
    // This allows accessing files outside the jail if the current working directory is not within the new root
    // A malicious user could potentially exploit this oversight to escape the chroot jail
}

How to fix Creation of chroot Jail Without Changing Working Directory?

To properly secure a chroot jail, it is essential to change the current working directory to a safe location inside the chroot environment. This is because the chroot system call changes the root directory for the current process but does not automatically change the current working directory. If the current working directory is not within the new root, the process might still have access to files outside the jail.

Steps to fix the vulnerability:

1. After calling chroot(), immediately change the current working directory using chdir("/"). This ensures that the process's working directory is within the confines of the new root directory.
2. Verify the success of both chroot() and chdir() calls, and handle errors appropriately to prevent the program from continuing in an insecure state.

Fixed Code Example

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

void setup_chroot_jail(const char *new_root) {
    // Secure code: chroot is called and working directory is changed
    if (chroot(new_root) != 0) {
        perror("chroot failed");
        exit(EXIT_FAILURE);
    }

    // Change the working directory to the root of the new chroot jail
    if (chdir("/") != 0) {
        perror("chdir failed");
        exit(EXIT_FAILURE);
    }
    
    // Now, the process is safely contained within the chroot jail
}

In this fixed example, we ensure the process's working directory is safely set inside the chroot environment by using chdir("/") immediately after the chroot() call. This prevents any unintended access to files outside of the jail, effectively containing the process within the new root directory. The code now correctly handles errors for both chroot() and chdir(), ensuring the program exits securely if any step fails.