OAMBuster – Multithreaded exploit for CVE-2018-2879

Oracle OAM is a widely used component that handles authentication for many web applications. Any request to a protected resource on the web application redirects to OAM, acting as middleware to perform user authentication.

Last year, a vulnerability was found in the login process which contains a serious cryptographic flaw. It can be exploited to:

  • Steal a session token
  • Create a session token for any user

Basically it allows to login to the website and impersonate any user. Bad stuff.

Technical Details

During login, Oracle OAM will send a value called encquery as part of the URL. This parameter contains some values like a salt and a timestamp, and is encrypted with AES-CBC. However:

  • There is no HMAC attached, to verify that the encrypted string wasn’t tampered with
  • The website responds with an error message when there is a padding error during decryption

This makes it vulnerable to a classic Padding Oracle Attack.

With AES-CBC, blocks of ciphertext are chained and dependent on each other. By adding blocks at the end and tampering with the second-to-last block, we can learn if the plain text is valid.

However, there is one extra challenge: We can not just add blocks of data to the end of the string because the decrypted value contains a validation hash at the end. When adding a block (that turns into gibberish when decrypted), the hash won’t be valid and we will not be able to use the oracle – it simply will always provide an error.

The fix to this problem is simple: we first have to search for the character that turns into a space (0x20) when decrypted. Now the hash will be untouched, and the gibberish we place behind it is interpreted as a new value.

For more in depth information, please read these excellent blog posts by SEC Consult:

Exploit

There have been exploits in the wild for some time, but they are single threaded. Because this attack requires to guess byte values one by one, it can take about 4 hours to complete (without rate limiting).

With RedTimmy, we have developed a multithreaded version of the exploit called OAMBuster. Source code is available on GitHub.

The first two stages of the attack, showing the verification of a target being vulnerable to CVE-2018-2879.

OAMBuster is able to perform the following actions:

  • Verify if the target is vulnerable to the attack (<30 seconds)
  • Decrypt the encquery string, using POA
  • Decrypt any given string, using POA
  • Encrypt any given string, using POA

The final two functions can be used for example to decrypt the OAMAuthnCookie containing the session token, and then re-encrypt it.

Benefits of the multithreaded implementation

Because OAMBuster has multiple threads running, it can decrypt multiple blocks at the same time. So for example, when there are 16 blocks in total, the tool can run 16 threads on the last byte of each block. When a thread is finished, it continues to work on the second-to-last byte of the block, and so forth, working from back to front. Bytes within a block can not be parallelized, as they are dependent on each other.

Worthless to say, you can subscribe our blackhat course if you want to play with it more:

Links

https://github.com/redtimmy/OAMBuster

FlexPaper <= 2.3.6 RCE

Around one year ago we discovered a Remote Command Execution vulnerability on FlexPaper (https://www.flowpaper.com). The vendor was immediately contacted and a CVE registered (2018-11686). However the vulnerability itself has remained undisclosed until now, regardless the fact that a patch has been issued with the release 2.3.7 of the project.

What is Flexpaper

FlexPaper is an open source project, released under GPL license, quite widespread over the internet.  It provides document viewing functionalities to web clients, mobile and tablet devices. At least until 2014 the component has been actively used by WikiLeaks, when it was discovered to be affected by a XSS vulnerability subsequently patched. The remote command execution vulnerability hereby descripted has remained 0day until being reported to vendor in April 2018.

Setup.php

The “php/setup.php” script is used to initialize the Flexpaper configuration file in the “config/” folder. The function “pdf2swfEnabled()” passes the user input unsafely to “exec()” that leads straight to arbitrary command execution.

However, this entry point can be only reached in case Flexpaper has not been initialized (i.e. there is no configuration file in the “config/” folder) which is the main reason to have the software downloaded and installed.

Therefore, after the configuration process is completed, the “exec()” function cannot be hit with arbitrary user input.

File removal via change_config.php

FlexPaper <= 2.3.6 also suffers from an unrestricted file overwrite vulnerability in “php/change_config.php“. The component exposes a functionality to update the configuration file. However, access to this file is improperly guarded, as can be seen in the code snippet below

The “SAVE_CONFIG” is done before the “FLEXPAPER_AUTH” authorization check is performed. Therefore, a not authenticated user can send a POST request and have the configuration file updated. Even more interesting is the fact that after the configuration file is updated, the script will remove all files in the directory that is configured under “path.swf”. As this path was just updated by the attacker, he is in full control of the directory in which he wants to delete files. 

An example of a HTTP request, which results in deletion of the configuration file, is depicted below.

Back to Setup.php

Once the Flexpaper configuration file is deleted, the vulnerable entry point at “setup.php” becomes reachable again. Now the attacker is one GET request far from triggering RCE. The malicious payload is provided to the “PDF2SWF_PATH” parameter. The command is injected just prepending a semicolon “;” character as shown below

In the example above the server was forced to base64 decode a PHP web shell (see following image) and write that in to a file named “tiger_shell.php” in the webserver’s document root.

The web shell

The web shell adopted in this case takes as an input a key and an arbitrary base64 encoded command submitted via GET request. It base64 decode and execute the command only if the key matches with the hardcoded one. From now on, the attacker can launch any arbitrary command like this:

The command injected through “cmd” parameter, for this specific example, is the base64 encoded representation of “id;uname -a;pwd”. Needless to say it can be whatever command the attacker wants to run.

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Some additional notes

During last year we have reported this vulnerability to several broadcasting and media companies that were using the affected component. Some government websites as well.

Exploit code

The exploit has been published here.