CWE-363: Race Condition Enabling Link Following

What is Race Condition Enabling Link Following?

• Overview: A race condition vulnerability where a program checks the status of a file or directory before accessing it, allowing an attacker to replace the file with a link in the interim, resulting in the program accessing an unintended file.

• Exploitation Methods:

• Attackers can exploit this vulnerability by quickly replacing the checked file with a symbolic link that points to another file between the time of check and the time of use.
• Common attack patterns include using symbolic links or hard links to redirect file access to sensitive or unauthorized files.

• Security Impact:

• Direct consequences include unauthorized access to sensitive files or directories.
• Potential cascading effects could involve data leakage, privilege escalation, or system compromise.
• Business impact may include loss of data integrity, legal consequences, and damage to reputation.

• Prevention Guidelines:

• Specific code-level fixes include using secure functions that perform atomic operations to check and use files, such as using open with O_NOFOLLOW flag in UNIX systems.
• Security best practices involve avoiding race conditions by minimizing time between check and use, and validating files after opening them.
• Recommended tools and frameworks include using static analysis tools to detect race conditions and employing secure libraries that handle file operations safely.

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Technical Details

Likelihood of Exploit: Not specified

Affected Languages: Not Language-Specific

Affected Technologies: Not specified

Vulnerable Code Example

import os

def process_file(file_path):
    # Check if the file exists and is a regular file
    if os.path.exists(file_path) and os.path.isfile(file_path):
        with open(file_path, 'r') as file:
            data = file.read()
            print("File data:", data)
    else:
        print("Invalid file path")

# This function is vulnerable to a race condition where the file could be replaced
# with a symbolic link after the existence and type check but before the file is opened.

How to fix Race Condition Enabling Link Following?

To fix this vulnerability, we should ensure that the file or directory is accessed safely without being subject to a race condition. In Python, one way to achieve this is by using os.open() with flags that ensure the file is opened in a safe manner. Specifically, use the os.O_NOFOLLOW flag to prevent the file from being opened if it is a symbolic link. Additionally, use file descriptors and low-level operations to maintain control over the file's identity throughout the access process.

Fixed Code Example

import os

def process_file(file_path):
    try:
        # Open the file safely using os.open with O_NOFOLLOW to prevent symlink following
        fd = os.open(file_path, os.O_RDONLY | os.O_NOFOLLOW)
        with os.fdopen(fd, 'r') as file:
            data = file.read()
            print("File data:", data)
    except FileNotFoundError:
        print("File not found")
    except IsADirectoryError:
        print("Specified path is a directory, not a file")
    except OSError as e:
        # Catch any OS-related errors, such as if the file is a symbolic link
        print("Error:", e)

# The os.open with O_NOFOLLOW ensures that the file cannot be a symbolic link,
# thus preventing the race condition from being exploited.

Explanation

The vulnerable code example demonstrates a classic race condition where the file's existence and type are checked before opening it. This leaves a window of opportunity for an attacker to replace the file with a symbolic link between these operations. The fixed code mitigates this by using os.open() with the os.O_NOFOLLOW flag, which ensures that the file opened is not a symbolic link, thus closing the race condition window by combining the checks and the file opening into a single atomic operation.