jueves, 25 de enero de 2024

Stop Using MD-5, Now!

TL;DR: Don't use MD-5 to identify malware samples. Believe me, it is a bad idea. Use SHA-256 or a stronger hash function.

This post is dedicated to all malware researchers, still using MD-5 to identify malware samples.

Before deep-diving into the details, let me explain my view on this topic. Whenever you want to identify a malware, it is only OK to publish the MD-5 hash of the malware if you post at least the SHA-256 hash of the malware as well. Publishing only the MD-5 hash is unprofessional. If you want to understand why, please continue reading. If you know about the problem, but want to help me spread the word, please link to my site www.stopusingmd5now.com.

By writing articles/posts/etc. and publishing the MD-5 hash only, it is the lesser problem that you show people your incompetency about hash functions, but you also teach other people to use MD-5. And it spreads like a disease... Last but not least, if I find a sample on your blog post, and you use MD-5 only, I can't be sure we have the same sample.

Here is a list to name a few bad examples (order is in Google search rank order):


Introduction to (cryptographic) hash functions

A long time ago (according to some sources since 1970) people started designing hash functions, for an awful lot of different reasons. It can be used for file integrity verification, password verification, pseudo-random generation, etc. But one of the most important properties of a cryptographic hash function is that it can "uniquely" identify a block of data with a small, fixed bit string. E.g., malware can be identified by using only the hash itself, so everybody who has the same malware sample will have the same hash; thus they can refer to the malware by the hash itself.

It is easy to conclude that there will always be collisions, where a different block of data has the same result hashes. The domain (block of data) is infinite, while the codomain (possible hash values) is finite. The question is how easy it is to find two different blocks of data, having the same hash. Mathematicians call this property "collision resistance." Proper cryptographic hash functions are collision-resistant, meaning it is impractical or impossible to find two different blocks of data, which have the same hash.

In 1989 Ronald Rivest (the first letter in the abbreviation of the RSA algorithm) designed the MD-2 hashing algorithm. Since 1997 there are publications about that this hashing algorithm is far from perfect.

In 1990 Ronald Rivest designed the MD-4 algorithm, which is considered as broken at least from 1991. But MD-4 is still in use from Windows XP until Windows 8 in the password protocol (NTLM). Unfortunately, there are more significant problems with NTLM besides using MD-4, but this can be the topic of a different blog post.

In 1991 (you might guess who) designed yet another hashing algorithm called MD-5, to replace MD-4  (because of the known weaknesses). But again, in from 1993 it has been shown many times that MD-5 is broken as well. According to Wikipedia, "On 18 March 2006, Klima published an algorithm [17] that can find a collision within one minute on a single notebook computer, using a method he calls tunneling". This means, that with the 8 years old computing power of a single notebook one can create two different files having the same MD-5 hash. But the algorithms to generate collisions have been improved since, and "a 2013 attack by Xie Tao, Fanbao Liu, and Dengguo Feng breaks MD-5 collision resistance in 2^18 time. This attack runs in less than a second on a regular computer." The key takeaway here is that it is pretty damn hard to design a secure cryptographic hash function, which is fast, but still safe. I bet that if I would develop a hash function, Ron would be able to hack it in minutes.

Now, dear malware researcher, consider the following scenario. You as, a malware analyst, find a new binary sample. You calculate the MD-5 hash of the malware, and Google for that hash. You see this hash value on other malware researchers or on a sandbox/vendor's site. This site concludes that this sample does this or that, and is either malicious or not. Either because the site is also relying solely on MD-5 or because you have only checked the MD-5 and the researcher or sandbox has a good reputation, you move on and forget this binary. But in reality, it is possible that your binary is totally different than the one analyzed by others. The results of this mistake can scale from nothing to catastrophic.

If you don't believe me, just check the hello.exe and erase.exe on this site from Peter Sellinger. Same MD-5, different binaries; a harmless and a (fake) malicious one... And you can do the same easily at home. No supercomputers,  no NSA magic needed.

On a side-note, it is important to mention that even today it can be hard to find a block of data (in generic), if only the MD-5 hash is known ("pre image resistance"). I have heard people arguing this when I told them using MD-5 as a password hash function is a bad idea. The main problem with MD-5 as a password hash is not the weaknesses in MD-5 itself, but the lack of salt, lack of iterations, and lack of memory hardness. But still, I don't see any reason why you should use MD-5 as a building block for anything, which has anything to do with security. Would you use a car to drive your children to the school, which car has not been maintained in the last 23 year? If your answer is yes, you should neither have children nor a job in IT SEC.

Conclusion

If you are a malware researcher, and used MD-5 only to identify malware samples in the past, I suggest to write it down 1000 times: "I promise I won't use MD-5 to identify malware in the future."

I even made a website dedicated to this problem, www.stopusingmd5now.com . The next time you see a post/article/whatever where malware is identified by the MD-5 hash only, please link to this blog post or website, and the world will be a better and more professional place.


PS: If you are a forensics investigator, or software developer developing software used in forensics, the same applies to you.
PS 2: If you find this post too provocative and harsh, there is a reason for this ...

Update: I have modified two malware (Citadel, Atrax) with the help of HashClash, and now those have the same MD-5. Many thanks for Marc Stevens for his research, publishing his code, and help given during the collision finding.
More info

  1. Pentest Tools For Ubuntu
  2. Hack Tools Github
  3. Hacking Tools For Games
  4. Hack Tools For Ubuntu
  5. Hack Tools
  6. New Hack Tools
  7. Pentest Tools Windows
  8. Hacker Tools Hardware
  9. Pentest Tools For Ubuntu
  10. Hacking Tools For Mac
  11. Github Hacking Tools
  12. Pentest Tools Alternative
  13. Hack Tools For Pc
  14. Pentest Tools Android
  15. Hack Tools Pc
  16. Game Hacking
  17. Hack Tools For Mac
  18. Pentest Automation Tools
  19. Pentest Tools Url Fuzzer
  20. Top Pentest Tools
  21. Pentest Tools
  22. Hack Tools For Ubuntu
  23. Pentest Tools Url Fuzzer
  24. Best Hacking Tools 2020
  25. Hack Tools Mac
  26. Hak5 Tools
  27. Hacking Tools 2019
  28. Hacking Tools Pc
  29. Hacker Tools Mac
  30. Best Pentesting Tools 2018
  31. Pentest Tools List
  32. Hacking Tools Windows
  33. Pentest Tools Bluekeep
  34. Hack Tools For Mac
  35. Hacking Tools For Beginners
  36. Hacking Tools And Software
  37. Hacking Tools Online
  38. Hackrf Tools
  39. Pentest Tools Port Scanner
  40. Pentest Box Tools Download
  41. Pentest Tools For Windows
  42. Hack Tools Github
  43. Hacking Tools
  44. Kik Hack Tools
  45. Hacker Tools For Windows
  46. New Hacker Tools
  47. Hack Tools For Pc
  48. Hacker Tools
  49. Pentest Tools Online
  50. Tools 4 Hack
  51. Hacking Tools For Mac
  52. Pentest Tools For Android
  53. Hacking Tools Software
  54. Bluetooth Hacking Tools Kali
  55. Install Pentest Tools Ubuntu
  56. Hack Tools Download
  57. Hacking Tools For Windows 7
  58. What Is Hacking Tools
  59. Pentest Tools
  60. Beginner Hacker Tools
  61. Hacking Tools Name
  62. Wifi Hacker Tools For Windows
  63. Nsa Hack Tools
  64. Nsa Hacker Tools
  65. Hacker Tools Online
  66. Hacker Tools Apk Download
  67. Hack Tools For Pc
  68. Best Hacking Tools 2020
  69. Hacking Tools Windows 10
  70. How To Make Hacking Tools
  71. Hack Tools For Ubuntu
  72. Hacker Tools For Pc
  73. Hacking Tools For Games
  74. New Hacker Tools
  75. Best Hacking Tools 2020
  76. Hacker Tools For Windows
  77. Ethical Hacker Tools
  78. Hack Tools
  79. Hacking Tools For Windows 7
  80. Hack Rom Tools
  81. Computer Hacker
  82. Hacking Tools Usb
  83. Pentest Box Tools Download
  84. Pentest Tools Tcp Port Scanner
  85. Game Hacking
  86. Pentest Tools Framework
  87. Hacker Tools Linux
  88. What Is Hacking Tools
  89. Hacker
  90. Pentest Tools Android
  91. Hacker Tools Free Download
  92. Hacking Tools
  93. Hack Rom Tools
  94. Pentest Tools Github
  95. Hacker Tools 2020
  96. Hacker Techniques Tools And Incident Handling
  97. Hack Rom Tools
  98. Hack Tools For Ubuntu
  99. Hacker Tools For Windows
  100. Hack Tool Apk
  101. How To Install Pentest Tools In Ubuntu
  102. Hacker Techniques Tools And Incident Handling
  103. Hacker Tools For Ios
  104. Hacker Tools For Mac
  105. Pentest Tools Github
  106. How To Make Hacking Tools
  107. Hacking Tools For Pc
  108. Hacking Tools For Beginners
  109. Hacking Tools
  110. Best Hacking Tools 2019
  111. Physical Pentest Tools
  112. Hacker Tools For Ios
  113. Hacker Hardware Tools
  114. Hak5 Tools
  115. Hacker Tools Linux
  116. Pentest Tools Online
  117. Hacker Tools Github
  118. Pentest Tools For Windows
  119. Blackhat Hacker Tools
  120. Hack Tools Download
  121. Hacking Tools Windows 10
  122. Hack Tool Apk No Root
  123. Hack Rom Tools
  124. Pentest Tools Open Source
  125. Hacking Tools For Windows 7
  126. Android Hack Tools Github
  127. Hacker Tools 2019
  128. Termux Hacking Tools 2019
  129. Pentest Tools Kali Linux
  130. Hack Tools 2019
  131. Hacking Tools For Pc
  132. Pentest Tools For Windows
  133. Pentest Tools Website
  134. Hack Tools For Windows
  135. Hack Rom Tools
  136. Ethical Hacker Tools
  137. Hacker Security Tools
  138. Hacking Tools 2020
  139. Pentest Tools Bluekeep
  140. Hacking Tools Download
  141. Wifi Hacker Tools For Windows
  142. Hack Tools Pc
  143. Hack Tools For Games
  144. Android Hack Tools Github

No hay comentarios: