#psychoPATH – a #blind #webroot #file #upload & #LFI #detection #tool

What is psychoPATH?

This tool is a customizable payload generator, initially designed to automate blind detection of web file upload implementations allowing to write files into the webroot (aka document root). The “blind” aspect is the key here and is inherent to dynamic testing usually conducted with no access to the source code or the filesystem.

Shortly after implementation it turned out the tool can also be very handy in hunting Local File Inclusion aka arbitrary file reading issues involving path traversal.

This tool helps to discover several kinds of vulnerabilities not detected by most scanners/payload sets:

file upload vulnerable to path traversal with the upload directory located inside the document root
file upload vulnerable to path traversal with the upload directory outside the document root
file upload not vulnerable to path traversal, but having the upload directory is inside of the document root, with no direct links to the uploaded file exposed by the application
local file inclusion/arbitrary file read vulnerable to path traversal with non-recurrent filters involved
Inisght into the file upload scenarios

At this point, controlling the uploaded file contents/extension is not the focus. One thing at a time, first we just want to detect if we can upload a legitimate file anywhere in the webroot.

1) Path traversal + upload dir outside the webroot

Let’s consider the following vulnerable Java servlet:

private static final String SAVE_DIR = “/tmp”;
protected void doPost(HttpServletRequest request,
HttpServletResponse response) throws ServletException, IOException {
String appPath = request.getServletContext().getRealPath(“”);
String savePath = SAVE_DIR;
File fileSaveDir = new File(savePath);
if (!fileSaveDir.exists()) {
fileSaveDir.mkdir();
}
String full_name=SAVE_DIR;
for (Part part : request.getParts()) {
String fileName = extractFileName(part);
part.write(savePath + File.separator + fileName);
full_name=savePath + File.separator + fileName;
}
The servlet is using user-supplied values as file names (e.g. a.jpg). It stores uploaded files in an upload directory /tmp located outside the document root (let’s assume the document root here is /var/lib/tomcat8/webapps/uploadbare_traversal_outside_webroot).

In order to get our file saved in the document root instead of the default upload directory, the user-supplied value, instead of benign a.jpg, would have to look more like ./../../../../../../../../../../var/lib/tomcat8/webapps/uploadbare_traversal_outside_webroot/a.jpg.

Assuming we cannot see any detailed error messages from the application and we have no access to the file system – all we know is that it is running on Apache Tomcat and that the URL is http://example.org/uploadbare_traversal_outside_webroot. In order to come up with this particular payload, we would have to try several possible document root variants, like:

/opt/tomcat8/example
/opt/tomcat5/webapps/example.org
/var/lib/tomcat6/webapps/uploadbare_traversal_outside_webroot
/var/lib/tomcat7/webapps/uploadbare_traversal_outside_webroot
/var/lib/tomcat8/webapps/uploadbare_traversal_outside_webroot
/opt/tomcat7/webapps/example
/opt/tomcat8/webapps/example.org
/opt/tomcat6/webapps/uploadbare_traversal_outside_webroot
/opt/tomcat7/webapps/example
/opt/tomcat8/webapps/example.org
/opt/tomcat5/webapps/uploadbare_traversal_outside_webroot […]
and so on. The remote webroot can depend on the platform, version, OS type, OS version as well as on internal standards within an organisation.

Based on well-known server-specific webroot paths + target hostname + user-specified variables, psychoPATH attempts to generate a comprehensive list of all potentially valid paths to use while blindly searching for vulnerable file upload. This approach was directly inspired by the technique used in –os-shell mode in sqlmap.

2) Path traversal + upload dir inside of the webroot

Let’s go to our next scenario then, which will be a slightly modified version of the previous example. The code remains the same, traversal-vulnerable. The only difference is the configured upload directory. Instead of /tmp/, we use /var/lib/tomcat8/webapps/uploadbase_traversal_inside_webroot/nowaytofindme/tmp.

The nowaytofindme/tmp directory is not explicitly linked by the application and there is no way to guess it with a dictionary-based enumeration approach.

Luckily for us, the application is still vulnerable to path traversal. We do not even need to guess the webroot. The payload to do the trick (put the file to /var/lib/tomcat8/webapps/uploadbase_traversal_inside_webroot, so it can be accessed by requesting http://example.org/uploadbase_traversal_inside_webroot/a.jpg is simply ../../a.jpg.

3) No path traversal + upload dir inside the webroot

The third example is not vulnerable to path traversal. The upload directory is located in the webroot (logs/tmp -> /var/lib/tomcat8/webapps/uploadbase_traversal_inside_webroot/logs/tmp). We already know it exists, supposedly among multiple other directories, like javascript/, css/ or images/ – but we would not normally suspect any of these could be the actual upload directory (no directory listing + the uploaded file is not explicitly linked by the application).

So, the payload is simply a.jpg. The file (or its copy) is put under logs/tmp/a.jpg – but no sane person would search for the file they just uploaded in a directory/subdirectory called logs/tmp. psychoPATH is not a sane person.

Other possible issues with traversal-vulnerable cases

There might be another vulnerable, hard to discover variant of a file upload function prone to path traversal. Both traversal cases described above would not get detected if there was no +w permission on the webroot (/var/lib/tomcat8/webapps/uploadbare_traversal_outside_webroot and /var/lib/tomcat8/webapps/uploadbare_traversal_inside_webroot, respectively). The only spark of hope here is that any of the subdirectories, like images/, has such permission enabled. This is why all the directories inside the webroot are also worth shooting at explicitly, leading to payloads like ./../../../../../../../../var/lib/tomcat8/uploadbare_traversal_inside_webroot/images/a.jpg or ./../images/a.jpg.

Are any of these upload scenarios even likely?

Not very (although all met in the wild, they are just quite rare), this is why it appeared sensible to automate the entire check.

Speaking of path traversal, most of today’s frameworks and languages are path traversal-proof. Still, sometimes they are not used properly. When it comes to languages, they usually (e.g. PHP) strip any path sequences from the standard variable holding the POST-ed file (Content-Disposition: form-data; name=”file”; filename=”test.jpg”), still it is quite frequent the application is using another user-supplied variable to rename the file after upload (and this is our likely-successful injection point).

When it comes to uploading files to hidden/not explicitly linked directories in the document root (those explicitly linked like Your file is here: uploads/a.jpg are trivial to find, no tools needed), it is usually a result of weird development practices and/or some forgotten ‘temporary’ code put together for testing – and never removed. Still, it happens and it is being missed, as, again, no sane person would think of e.g. searching for the file they just uploaded in a webroot subdirectory called logs/tmp.

The algorithm

The following is a general algorithm we employ to perform blind detection of any of the cases mentioned above:

upload a legitimate file that is accepted by the application (a benign filename, extension, format and size) – we want to avoid false negatives due to any additional validation checks performed by the application
estimate the possible potentially valid webroots, including their variants with webroot-located subdirectories and path traversal payloads (both relative, like ../../a.jpg and absolute, like ./../../../var/lib/tomcat6/webapps/servletname/a.jpg, ./../../../var/lib/tomcat6/webapps/servletname/img/a.jpg and so on) – this step is automated and customizable
attempt to upload the file using all the payloads from the step above, placing a unique string in each file we attempt to upload (this step is automated and customizable as well)
search for the uploaded file by attempting GETs to all known directories in the webroot, e.g. http://example.org/a.jpg, http://example.org/img/a.jpg and so on (this step is automated as well)
if we find the file in any of the locations, we look into its contents to identify the unique string (payload mark), so we can track down the successful payload
The evasive payloads

The basic traversal payload is ../, or more generally (a holder-based approach will become handy once Windows support + encodings are involved).

The following are the potential bypassable filter scenarios:

removing only ../
removing only ./
removing ../ and then ./
removing ./ and then ../
This applies both to file uploading and file reading (LFI), please see the psychoPATH usage – LFI hunting section for more details. A filter removing all occurrences of .. or / does not seem to be bypassable (please let me know if I am wrong).

….//….//….//….//….//….//….//….// -> rm ../-> ../../../../../../../../ OK

…//…//…//…//…//…//…//…//…// -> rm ./ -> ../../../../../../../../../ OK

…..///…..///…..///…..///…..///…..///…../// -> rm ../ -> …//…//…//…//…//…//…// -> rm ./ -> ../../../../../../../ OK

…..///…..///…..///…..///…..///…..///…../// -> rm ./ -> ….//….//….//….//….//….//….// -> rm ../ -> ../../../../../../../ OK

So, we only need three evasive payloads:

….//
…//
…..///
Webroot-guessing – optimization

To reduce the eventual number of payloads, by default the tool does not prepend the same document root with traversal strings which differ only in the number of the traversal sequences they consist of (e.g. ../ vs ../../ vs ../../../) – as these are redundant when used with absolute paths. Instead, only the longest variant is used, e.g. …..///…..///…..///…..///…..///…..///…..///var/lib/tomcat8/webapps/upload. This significantly reduces the number of payloads sent. It might, however, be an issue – if the variable we are injecting into is somehow limited on its length, e.g. application rejects any values longer than 45 characters and the upload directory is /tmp – in that case …..///var/lib/tomcat8/webapps/upload would do the trick instead. If you are worried about the payload length and you care less about the number of payloads, turn optimization off.

psychoPATH usage – hunting uploads in the dark

The extension interface consists of several lists of elements used to build permutations of all potentially valid payloads: The {TARGET} holder is automatically replaced with elements from the Targets list, by default containing the Host header of the target. If the hostname is a subdomain, e.g. foo.bar.example.org, it will automatically be propagated into several possible target values, like foo, bar, example and foo.bar.example.org.

All the lists can be manually adjusted by using relevant Paste, Load, Remove and Add buttons.

The Doc roots lists offer several sub-groups:

By default, all sub-groups are included as basic payload-building units.

First, we perform a legitimate upload request to the target application. Then we right-click on the relevant request and click Propagate to psychoPATH context menu button:

This is required to let the plugin know about the target host and protocol, so it can adjust the payloads with the relevant information from the site map (like the host or the directories from the site map) – please note the changed list of Suffixes and Targets after propagation:

Now we send the request to Intruder. We set the attack type to Pitchfork. Then we need to select two payload holders. One is the file name value we are about to inject into. The other is just a section in the uploaded file content – this is where the unique payload mark will be put. Currently payload marks have fixed length of 7 characters. When injecting into image formats, the safest way is to mark exactly seven characters of a string – e.g. a piece of exif data. This way we won’t encounter false negatives if the application is checking the validity of the image file before putting it into the upload directory of our interest:

Then we move to the Payloads tab. For the first payload, we change the type from “Simple list” to “Extension generated”. We choose the “Path traversal” extension generator. We UNCHECK the the default “URL-encode these characters” box:

Then we proceed to the second payload set (the payload mark). For the first payload, we change the type from “Simple list” to “Extension generated”. We choose the “Payload marker” extension generator:

We hit the “Start attack” button and watch the results.

It might be handy to sort the responses by the status code/any other property of the server’s response:

According to the sorted output, the application responded differently to several payloads which either refered to var/lib/tomcat8/webapps directory (which was the valid base document root in this case) or did not use any traversal combinations at all. Still, sometimes the response we receive might not give us any hints (entirely blind scanario).

We keep the Intruder attack window open and proceed to the verification phase (searching the entire site map for the file we uploaded).

In order to do this, we simply take a valid, preferably authenticated GET request to the application and send it to Intruder. We select the URI section as the only payload holder:

In the Payloads section, again we change from “Simple” to “Extension generated”. This time we choose “Directory checker” as the payload generator. We UNCHECK the the default “URL-encode these characters” box:

We hit the “Start attack” button and watch the results (now we are explicitly interested in getting “200 OK” response). Normally there would be a bit more, like ten or few dozens of site map-derived directories queried, in the example below it’s just one (uploadbare_traversal_outside_webroot):

As we can see, the file has been created under uploadbare_traversal_outside_webroot/a.jpg. By looking at the payload marker spot, we can identify 585 as the number of the golden payload. We look back to the Intruder attack and search for the request with Payload 2 equal to 585:

Now we know the golden payload to reach the document root was ./../../../../../../..//var/lib/tomcat8/webapps//uploadbare_traversal_outside_webroot/a.jpg.

For other two examples, the results for the payloads that have worked, would look as follows, respectively: /uploadbare_traversal_inside_webroot:

/uploadbare_no_traversal_inside_webroot:

psychoPATH usage – hunting LFI

The Path traversal generator can be easily used for hunting Local File Inclusion/arbitrary file reading issues as well – and it’s much simpler than hunting uploads. The test_cases/LFI directory contains three vulnerable PHP scripts, reflecting the non-recurrent filter cases broken down in the “evasive payloads” section.

Below is a short presentation on how all three can be quickly detected with the payloads provided by psychoPATH. First screenshot shows us the response of the script when a benign string foo is provided under the file variable. No content is returned:

We send the request to Intruder and mark the injection point: We choose Extension generated Path traversal payload type. Please not unchecking the URL-encode these characters – as a matter of fact the most reliable approach is to test each input like this twice – with and without URL-encoding:

Then we move to the psychoPATH configuration panel. We choose the file name, for instance /etc/passwd. We clear the web roots, targets and suffixes list, as we are nog going to need them to perform this attack: We simply run “Start attack” and watch how each of the evasive techniques works on its corresponding vulnerable case:

The perl script

Initially this tool was developed as a perl script – which is still available, although no longer maintained at the moment.

TODO

test on different resolution, make sure the project is easily runnable/importable
separate apache-like suffixes from the main list, they are there by default and do not go away once other than all/apache webroot set is picked
more examples of test cases
test evasive techniques involving the windows \ backslash in different environments
Nice-to-haves:
implement windows support
add a “Copy to clipboard” button for generated payloads, so the output payloads can be used with other tools
add support for ZIP traversals
extend the tool with extension control mode (defeating the filters in order to upload an executable – different tricks depending on the platform)??
in case of a positive result, automatically track back the payload that did the trick (instead of the dir check mode?)

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