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Wannakey

WARNING

This software has only been tested and known to work under Windows XP. In order to work, your computer must not have been rebooted after being infected.

Please also note that you need some luck for this to work (see below), and so it might not work in every cases!

Introduction

This software allows to recover the prime numbers of the RSA private key that are used by Wanacry.

It does so by searching for them in the wcry.exe process. This is the process that generates the RSA private key. The main issue is that the CryptDestroyKey and CryptReleaseContext does not erase the prime numbers from memory before freeing the associated memory.

This is not really a mistake from the ransomware authors, as they properly use the Windows Crypto API. Indeed, for what I've tested, under Windows 10, CryptReleaseContext does cleanup the memory (and so this recovery technique won't work). It can work under Windows XP because, in this version, CryptReleaseContext does not do the cleanup. Moreover, MSDN states this, for this function : "After this function is called, the released CSP handle is no longer valid. This function does not destroy key containers or key pairs.". So, it seems that there are no clean and cross-platform ways under Windows to clean this memory.

If you are lucky (that is the associated memory hasn't been reallocated and erased), these prime numbers might still be in memory.

That's what this software tries to achieve.

Usage

You can use the binary in the bin/ folder. You first need to find the PID of the wcry.exe process using the Task Manager, and locate the 00000000.pky file.

Once you've got this, launch using cmd.exe:

> search_primes.exe PID path\to\00000000.pky

If a valid prime is found in memory, the priv.key file will be generated in the current directory.

You can then use https://github.com/odzhan/wanafork/ to decrypt your files!

WARNING: wanafork does not work directly for now directly under Windows XP. This should be fixed soon (hopefully)!

Compile from source

You can use Visual Studio 2015 express to compile the associated project. Be sure to select the compatible Windows XP toolchain in the project properties!

Credits

• @wiskitki who spotted the CryptReleaseContext issue with Windows 10 (which actually wipe the primes in memory).
• @hackerfantastic for releasing the sample I used
• Miasm (https://github.com/cea-sec/miasm) for its help extracting the DLL and reversing the whole thing
• Wine sources for the Windows RSA private key format.