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CWE-36: Absolute Path Traversal
 DescriptionDescription Summary The software 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.
Extended Description This allows attackers to traverse the file system to access files or directories that are outside of the restricted directory. Time of Introduction
Applicable PlatformsLanguages All Common Consequences
Demonstrative ExamplesExample 1 In the example below, the path to a dictionary file is read from a system property and used to initialize a File object. (Bad Code) Example
Language: Java 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 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 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. (Good Code) Example
Language: HTML <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. (Bad Code) Example
Language: Java 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. Observed Examples
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Page Last Updated:
February 20, 2013
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