Relative Path Traversal

The software 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.


Description

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

Demonstrations

The following examples help to illustrate the nature of this weakness and describe methods or techniques which can be used to mitigate the risk.

Note that the examples here are by no means exhaustive and any given weakness may have many subtle varieties, each of which may require different detection methods or runtime controls.

Example One

The following URLs are vulnerable to this attack:

http://example.com.br/get-files.jsp?file=report.pdf
http://example.com.br/get-page.php?home=aaa.html
http://example.com.br/some-page.asp?page=index.html

A simple way to execute this attack is like this:

http://example.com.br/get-files?file=../../../../somedir/somefile
http://example.com.br/../../../../etc/shadow
http://example.com.br/get-files?file=../../../../etc/passwd

Example Two

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.

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 "<li>$_</li>\n";
}
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:

../../../etc/passwd

The program would generate a profile pathname like this:

/users/cwe/profiles/../../../etc/passwd

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

/etc/passwd

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 Three

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.

public class FileUploadServlet extends HttpServlet {


  ...

  protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {


    response.setContentType("text/html");
    PrintWriter out = response.getWriter();
    String contentType = request.getContentType();

    // the starting position of the boundary header
    int 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 data
    if (contentType != null && contentType.indexOf("multipart/form-data") != -1) {


      // extract the filename from the Http header
      BufferedReader 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 directory
      try {

        BufferedWriter bw = new BufferedWriter(new FileWriter(uploadLocation+filename, true));
        for (String line; (line=br.readLine())!=null; ) {
          if (line.indexOf(boundary) == -1) {
            bw.write(line);
            bw.newLine();
            bw.flush();
          }
        } //end of for loop
        bw.close();



      } catch (IOException ex) {...}
      // output successful upload response HTML page

    }
    // output unsuccessful upload response HTML page
    else
    {...}

  }
    ...


}

As with the previous example this code does not perform a check on the type of the file being uploaded. 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-22, CWE-23). 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.

See Also

File Handling Issues

Weaknesses in this category are related to the handling of files within a software system. Files, directories, and folders are so central to information technology tha...

SFP Secondary Cluster: Path Traversal

This category identifies Software Fault Patterns (SFPs) within the Path Traversal cluster (SFP16).

Comprehensive CWE Dictionary

This view (slice) covers all the elements in CWE.

CWE Cross-section

This view contains a selection of weaknesses that represent the variety of weaknesses that are captured in CWE, at a level of abstraction that is likely to be useful t...

Weaknesses Introduced During Implementation

This view (slice) lists weaknesses that can be introduced during implementation.


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