java - What's the best way to defend against a path traversal attack?

Question

I have a Java server implementation (TFTP if it matters to you) and I'd like to ensure that it's not susceptible to path traversal attacks allowing access to files and locations that shouldn't be available.

My best attempt at defending so far is to reject any entries that match File.isAbsolute() and then rely on File.getCanonicalPath() to resolve any ../ and ./ components out of the path. Finally I ensure that the resulting path is still within the required root directory of my server:

public String sanitize(final File dir, final String entry) throws IOException {
    if (entry.length() == 0) {
        throw new PathTraversalException(entry);
    }

    if (new File(entry).isAbsolute()) {
        throw new PathTraversalException(entry);
    }

    final String canonicalDirPath = dir.getCanonicalPath() + File.separator;
    final String canonicalEntryPath = new File(dir, entry).getCanonicalPath();

    if (!canonicalEntryPath.startsWith(canonicalDirPath)) {
        throw new PathTraversalException(entry);
    }

    return canonicalEntryPath.substring(canonicalDirPath.length());
}

Are there security issues that this misses? Are there better / faster to achieve the same result reliably?

The code needs to work consistently across Windows and Linux.

Answer 1

The following may help. It compares the canonical and absolute paths, and if they differ, then it'll fail. Only tested on a mac/linux system (ie no windows).

This is for the case where you want to allow the user to supply a relative path, not an absolute path, and you don't allow any parent directory references.

public void failIfDirectoryTraversal(String relativePath)
{
    File file = new File(relativePath);

    if (file.isAbsolute())
    {
        throw new RuntimeException("Directory traversal attempt - absolute path not allowed");
    }

    String pathUsingCanonical;
    String pathUsingAbsolute;
    try
    {
        pathUsingCanonical = file.getCanonicalPath();
        pathUsingAbsolute = file.getAbsolutePath();
    }
    catch (IOException e)
    {
        throw new RuntimeException("Directory traversal attempt?", e);
    }


    // Require the absolute path and canonicalized path match.
    // This is done to avoid directory traversal 
    // attacks, e.g. "1/../2/" 
    if (! pathUsingCanonical.equals(pathUsingAbsolute))
    {
        throw new RuntimeException("Directory traversal attempt?");
    }
}