CWE-23: Relative Path Traversal

Assume all input is malicious. Use an "accept known good" input validation strategy, i.e., use a list of acceptable inputs that strictly conform to specifications. Reject any input that does not strictly conform to specifications, or transform it into something that does.

When performing input validation, consider all potentially relevant properties, including length, type of input, the full range of acceptable values, missing or extra inputs, syntax, consistency across related fields, and conformance to business rules. As an example of business rule logic, "boat" may be syntactically valid because it only contains alphanumeric characters, but it is not valid if the input is only expected to contain colors such as "red" or "blue."

Do not rely exclusively on looking for malicious or malformed inputs. This is likely to miss at least one undesirable input, especially if the code's environment changes. This can give attackers enough room to bypass the intended validation. However, denylists can be useful for detecting potential attacks or determining which inputs are so malformed that they should be rejected outright.

When validating filenames, use stringent allowlists that limit the character set to be used. If feasible, only allow a single "." character in the filename to avoid weaknesses such as CWE-23 , and exclude directory separators such as "/" to avoid CWE-36 . Use a list of allowable file extensions, which will help to avoid CWE-434 .

Do not rely exclusively on a filtering mechanism that removes potentially dangerous characters. This is equivalent to a denylist, which may be incomplete ( CWE-184 ). For example, filtering "/" is insufficient protection if the filesystem also supports the use of "\" as a directory separator. Another possible error could occur when the filtering is applied in a way that still produces dangerous data ( CWE-182 ). For example, if "../" sequences are removed from the ".../...//" string in a sequential fashion, two instances of "../" would be removed from the original string, but the remaining characters would still form the "../" string.

Strategy: Input Validation

Inputs should be decoded and canonicalized to the application's current internal representation before being validated ( CWE-180 ). Make sure that the application does not decode the same input twice ( CWE-174 ). Such errors could be used to bypass allowlist validation schemes by introducing dangerous inputs after they have been checked.

Use a built-in path canonicalization function (such as realpath() in C) that produces the canonical version of the pathname, which effectively removes ".." sequences and symbolic links ( CWE-23 , CWE-59 ). This includes:

getCanonicalPath() in Java

GetFullPath() in ASP.NET

realpath() or abs_path() in Perl

Effectiveness: Moderate

Class: Not Language-Specific (Undetermined Prevalence)

Class: Not Technology-Specific (Undetermined Prevalence)

Class: Web Based (Often Prevalent)

AI/ML (Often Prevalent)

The following URLs are vulnerable to this attack:

A simple way to execute this attack is like this:

The following code could be for a social networking application in which each user's profile information is stored in a separate file. All files are stored in a single directory.

While the programmer intends to access files such as "/users/cwe/profiles/alice" or "/users/cwe/profiles/bob", there is no verification of the incoming user parameter. An attacker could provide a string such as:

The program would generate a profile pathname like this:

When the file is opened, the operating system resolves the "../" during path canonicalization and actually accesses this file:

As a result, the attacker could read the entire text of the password file.

Notice how this code also contains an error message information leak ( CWE-209 ) if the user parameter does not produce a file that exists: the full pathname is provided. Because of the lack of output encoding of the file that is retrieved, there might also be a cross-site scripting problem ( CWE-79 ) if profile contains any HTML, but other code would need to be examined.

The following code demonstrates the unrestricted upload of a file with a Java servlet and a path traversal vulnerability. The action attribute of an HTML form is sending the upload file request to the Java servlet.

When submitted the Java servlet's doPost method will receive the request, extract the name of the file from the Http request header, read the file contents from the request and output the file to the local upload directory.

This code does not perform a check on the type of the file being uploaded ( CWE-434 ). This could allow an attacker to upload any executable file or other file with malicious code.

Additionally, the creation of the BufferedWriter object is subject to relative path traversal ( CWE-23 ). Since the code does not check the filename that is provided in the header, an attacker can use "../" sequences to write to files outside of the intended directory. Depending on the executing environment, the attacker may be able to specify arbitrary files to write to, leading to a wide variety of consequences, from code execution, XSS ( CWE-79 ), or system crash.

Note: this is a curated list of examples for users to understand the variety of ways in which this weakness can be introduced. It is not a complete list of all CVEs that are related to this CWE entry.