Critical Description
http://www.tenable.com/security/research/tra-2016-12
Summary
There exists a Java Object in the Apache Commons FileUpload library that can be manipulated in such a way that when it is deserialized, it can write or copy files to disk in arbitrary locations. Furthermore, while the Object can be used alone, this new vector can be integrated with ysoserial to upload and execute binaries in a single deserialization call. This may or may not work depending on an application's implementation of the FileUpload library.
Background
In late 2015 FoxGlove Security released a write up on using Chris Frohoff’s yososerial tool to gain remote code execution on a variety of commercial products, based on a presentation at AppSec Cali in January, 2015. The ysoserial tool uses “gadgets” in Apache Commons Collections, Groovy, and Spring that do “unexpected” things during deserialization. Specifically, the ysoserial payloads eventually execute Runtime.getRuntime().exec() allowing for remote Java code execution.
The Apache Commons project maintains a library called “FileUpload” to make “it easy to add robust, high-performance, file upload capability to your servlets and web applications.” One of the classes in the FileUpload library is called “DiskFileItem”. A DiskFileItem is used to handle file uploads. Interestingly, DiskFileItem is serializable and implements custom writeObject() and readObject() functions.
DiskFileItem’s readObject Implementation
Here is the implementation that currently exists at the projects repository tip (as of 1/28/16):
632 private void readObject(ObjectInputStream in)
633 throws IOException, ClassNotFoundException {
634 // read values
635 in.defaultReadObject();
636
637 /* One expected use of serialization is to migrate HTTP sessions
638 * containing a DiskFileItem between JVMs. Particularly if the JVMs are
639 * on different machines It is possible that the repository location is
640 * not valid so validate it.
641 */
642 if (repository != null) {
643 if (repository.isDirectory()) {
644 // Check path for nulls
645 if (repository.getPath().contains("\0"))
650 } else
{ 651 throw new IOException(format( 652 "The repository [%s] is not a directory", 653 repository.getAbsolutePath())); 654 }655 }
656
657 OutputStream output = getOutputStream();
658 if (cachedContent != null)
else
{ 661 FileInputStream input = new FileInputStream(dfosFile); 662 IOUtils.copy(input, output); 663 IOUtils.closeQuietly(input); 664 dfosFile.delete(); 665 dfosFile = null; 666 }667 output.close();
668
669 cachedContent = null;
670 }
This is interesting due to the apparent creation of files. However, after analyzing the state of DiskFileItem after serialization it became clear that arbitrary file creation was not supposed to be intended:
dfos (a type of OutputStream) is transient and therefore it is not serialized. dfos is regenerated by the getOutputStream() call above (which also generates the new File to write out to).
The “repository” (or directory that the file is written to) has to be valid at the time of serialization in order for successful deserialization to occur.
If there is no “cachedContent” then readObject() tries to read in the file from disk.
That filename is always generated via getOutputStream.
Serialized Object Modification
The rules listed above do not take into account that someone might modify the serialized data before it is deserialized. Three important elements get serialized that we can modify:
The repository path (aka the directory that the file is read/written from).
If there is cachedContent (i.e. data that didn’t get written to the file) then that gets serialized
If there is no cachedContent (i.e. all data was written to disk) the full path to the output file exists.
The threshold value that controls if “cachedContent” is written to disk or not.
Modifying these three elements in the serialized object gives us the ability to:
Create files wherever we have permission on the system. The caveat here is that we only have control of the file path and not the final filename.
Copy the contents of files from one file on the system to a location we specify (again we only control the directory path and not the filename). This will also attempt to delete the file we copy from.. so be careful.