CWE-23: Relative Path Traversal


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

Submission Date :

July 19, 2006, midnight

Modification Date :

2023-10-26 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

The following URLs are vulnerable to this attack:

A simple way to execute this attack is like this:

Example - 2

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.

print "<li>$_</li>\n";
my $dataPath = "/users/cwe/profiles";my $username = param("user");my $profilePath = $dataPath . "/" . $username;open(my $fh, "<", $profilePath) || ExitError("profile read error: $profilePath");print "<ul>\n";while (<$fh>) {}print "</ul>\n";

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.

Example - 3

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.

<form action="FileUploadServlet" method="post" enctype="multipart/form-data">Choose a file to upload:<input type="file" name="filename"/><br/><input type="submit" name="submit" value="Submit"/></form>

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.

bw.write(line);bw.newLine();bw.flush();if (line.indexOf(boundary) == -1) {}
BufferedWriter bw = new BufferedWriter(new FileWriter(uploadLocation+filename, true));for (String line; (line=br.readLine())!=null; ) {} //end of for loopbw.close();
// extract the filename from the Http headerBufferedReader br = new BufferedReader(new InputStreamReader(request.getInputStream()));...pLine = br.readLine();String filename = pLine.substring(pLine.lastIndexOf("\\"), pLine.lastIndexOf("\""));...// output the file to the local upload directorytry {} catch (IOException ex) {...}// output successful upload response HTML page
response.setContentType("text/html");PrintWriter out = response.getWriter();String contentType = request.getContentType();// the starting position of the boundary headerint ind = contentType.indexOf("boundary=");String boundary = contentType.substring(ind+9);String pLine = new String();String uploadLocation = new String(UPLOAD_DIRECTORY_STRING); //Constant value// verify that content type is multipart form dataif (contentType != null && contentType.indexOf("multipart/form-data") != -1) {}// output unsuccessful upload response HTML pageelse{...}......protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {}public class FileUploadServlet extends HttpServlet {}

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.

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