CWE-377: Insecure Temporary File

What is Insecure Temporary File?

• Overview: Insecure Temporary File (CWE-377) refers to the creation and use of temporary files in a way that exposes application and system data to potential attacks. These files are often used to store data temporarily during program execution but, if not handled securely, can be accessed or manipulated by unauthorized users.

• Exploitation Methods:

• Attackers can exploit this vulnerability by predicting or accessing the file name/location and inserting malicious data or code.
• Common attack patterns include race conditions, where attackers create a file with the same name before the legitimate application, and symlink attacks, where attackers replace the temporary file with a symbolic link to a sensitive file.

• Security Impact:

• Direct consequences of successful exploitation include unauthorized read/write access to temporary files, leading to data leaks or integrity issues.
• Potential cascading effects could involve privilege escalation or execution of arbitrary code.
• Business impact may include data breaches, loss of customer trust, or legal/regulatory repercussions.

• Prevention Guidelines:

• Specific code-level fixes include using secure functions for creating temporary files that ensure unique file names (e.g., mkstemp() in C/C++).
• Security best practices involve setting appropriate file permissions and using secure directories with restricted access.
• Recommended tools and frameworks include libraries or APIs that manage temporary files securely, such as Java's Files.createTempFile() or Python's tempfile module.

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Technical Details

Likelihood of Exploit: Not specified

Affected Languages: Not Language-Specific

Affected Technologies: Not specified

Vulnerable Code Example

Certainly! Let's improve the code examples and ensure they clearly demonstrate the vulnerability and the fix, while adhering to best practices and proper formatting.

import os
import tempfile

def create_temp_file(data):
    # Vulnerable usage of mktemp() which only generates a temporary file name
    # without creating or securing the file, leading to a race condition vulnerability.
    temp_file_name = tempfile.mktemp()  # Generates a name but doesn't create the file
    with open(temp_file_name, 'w') as f:  # Opens the file by name, vulnerable to race conditions
        f.write(data)
    return temp_file_name

How to fix Insecure Temporary File?

The vulnerability in the code arises from using the mktemp() function from the tempfile module, which generates a temporary file name without actually creating the file. This creates a security vulnerability known as a race condition, where an attacker could potentially create a file with the same name before the program does, leading to unauthorized access or data manipulation.

To fix this issue, we should use NamedTemporaryFile or TemporaryFile from the tempfile module, which safely creates and opens a temporary file, ensuring that the file is securely handled. This approach prevents race conditions and unauthorized access by creating the file and securing its name in one atomic operation.

Fixed Code Example

import os
import tempfile

def create_temp_file(data):
    # Securely creating a temporary file using NamedTemporaryFile
    # This safely creates and opens the file, preventing race conditions.
    with tempfile.NamedTemporaryFile(delete=False, mode='w') as temp_file:  # Securely creates and opens the file
        temp_file.write(data)  # Writes data to the securely opened file
        temp_file_name = temp_file.name  # Retrieves the name of the temporary file
    return temp_file_name  # Returns the secure file name

In the fixed code example, NamedTemporaryFile is used, which securely creates and opens a temporary file in one step. The file is opened with 'w' mode for writing, and delete=False ensures that the file is not immediately deleted after being closed, in case it needs to be accessed later in the program. This approach mitigates the risk of race conditions and ensures that the temporary file is handled securely.