CWE-36: Absolute Path Traversal


The product uses external input to construct a pathname that should be within a restricted directory, but it does not properly neutralize absolute path sequences such as "/abs/path" that can resolve to a location that is outside of that directory.

Submission Date :

July 19, 2006, midnight

Modification Date :

2023-06-29 00:00:00+00:00

Organization :

Extended Description

This allows attackers to traverse the file system to access files or directories that are outside of the restricted directory.

Example Vulnerable Codes

Example - 1

In the example below, the path to a dictionary file is read from a system property and used to initialize a File object.

String filename = System.getProperty("com.domain.application.dictionaryFile");File dictionaryFile = new File(filename);

However, the path is not validated or modified to prevent it from containing relative or absolute path sequences before creating the File object. This allows anyone who can control the system property to determine what file is used. Ideally, the path should be resolved relative to some kind of application or user home directory.

Example - 2

This script intends to read a user-supplied file from the current directory. The user inputs the relative path to the file and the script uses Python's os.path.join() function to combine the path to the current working directory with the provided path to the specified file. This results in an absolute path to the desired file. If the file does not exist when the script attempts to read it, an error is printed to the user.

file_data = open(path, 'r') as f:

print("Error - file not found")filename = sys.argv[1]path = os.path.join(os.getcwd(), filename)try:except FileNotFoundError as e:import osimport sysdef main():main()

However, if the user supplies an absolute path, the os.path.join() function will discard the path to the current working directory and use only the absolute path provided. For example, if the current working directory is /home/user/documents, but the user inputs /etc/passwd, os.path.join() will use only /etc/passwd, as it is considered an absolute path. In the above scenario, this would cause the script to access and read the /etc/passwd file.

file_data = open(path, 'r') as f:

print("Error - file not found")filename = sys.argv[1]path = os.path.normpath(f"{os.getcwd()}{os.sep}{filename}")try:except FileNotFoundError as e:import osimport sysdef main():main()

The constructed path string uses os.sep to add the appropriate separation character for the given operating system (e.g. '\' or '/') and the call to os.path.normpath() removes any additional slashes that may have been entered - this may occur particularly when using a Windows path. By putting the pieces of the path string together in this fashion, the script avoids a call to os.path.join() and any potential issues that might arise if an absolute path is entered. With this version of the script, if the current working directory is /home/user/documents, and the user inputs /etc/passwd, the resulting path will be /home/user/documents/etc/passwd. The user is therefore contained within the current working directory as intended.

Related Weaknesses

This table shows the weaknesses and high level categories that are related to this weakness. These relationships are defined to give an overview of the different insight to similar items that may exist at higher and lower levels of abstraction.

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