BSides Zurich – Nail in the JKS coffin

On Saturday I was happy to speak at the fabulous BSides Zurich about the Java Key Store topic. You can find my slides “Nail in the JKS coffin” as a PDF here. It was my second time at a BSides format and I really like the idea of having a short talk and then some more time to discuss the topic with interested people. I also included the “after the presentation” slides we used for roughly 50% of the discussion time. I hope you enjoyed the talk and I’m looking forward to hear some feedback. Although it was sold out, you should definitely come next year, it was one of my favorite public conferences.

cheers,
floyd

Java Key Store (JKS) format is weak and insecure (CVE-2017-10356)

While preparing my talk for the marvelous BSides Zurich I noticed again how nearly nobody on the Internet warns you that Java’s JKS file format is weak and insecure. While users only need to use very strong passwords and keep the Key Store file secret to be on the safe side (for now!), I think it is important to tell people when a technology is weak. People should stop using JKS now, as I predict a very long phase-out period. JKS was around and the default since Java had its first Key Store. Your security relies on a single SHA-1 calculation here.

Please note that I’m not talking about any other Key Store type (BKS, PKCS#12, etc.), but see the cryptosense website for articles about them.

I don’t want to go into the details “why” JKS is insecure, you can read all about it here:

I wrote an email to the Oracle security team, as I think assigning a CVE number would help people to refer to this issue and raise awareness for developers. My original email sent on September, 18 2017:

I would like to ask Oracle to assign a CVE Number for Java’s weak
encryption in JKS files for secure storage of private keys (Java Key
Store files). JKS uses a weak encryption scheme based on SHA1.

I think it is important to raise awareness that JKS is weak by assigning
a CVE number, even when it is going to be replaced in Java 1.9 with PKCS#12.

The details of the weakness are published on the following URLs:

– As an article in the POC||GTFO 0x15 magazine, I attached it to this
email, the full magazine can also be found on
https://www.alchemistowl.org/pocorgtfo/pocorgtfo15.pdf
– https://cryptosense.com/mighty-aphrodite-dark-secrets-of-the-java-keystore/
– https://github.com/floyd-fuh/JKS-private-key-cracker-hashcat

As the article states, no documentation anywhere in the Java world
mentions that JKS is a weak storage format. I would like to change this,
raise awareness and a CVE assignment would help people refer to this issue.

The timeline so far:

  • September, 18 2017: Notified Oracle security team via email
  • September, 18 2017: Generic response that my email was forwarded to the Oracle team that investigates these issues
  • September, 20 2017: Oracle assigned a tracking number (S0918336)
  • September, 25 2017: Automated email status report: Under investigation / Being fixed in main codeline
  • October, 10 2017: Requested an update and asked if they could assign a CVE number
  • October, 11 2017: Response, they are still investigating.
  • October, 13 2017: Oracle writes “We have confirmed the issue and will be addressing it in a future release”. In an automated email I get Oracle states “The following issue reported by you is fixed in the upcoming Critical Patch Update, due to be released at 1:00 PM, U.S. Pacific Time, on October 17, 2017.”.
  • October 17, 2017: Oracle assigned a CVE in their Oracle Critical Patch Update Advisory – October 2017: CVE-2017-10356. The guys from Cryptosense got credited too it seems. However, the documentation of Oracle so far didn’t change anywhere I could see it.
  • November 16, 2017: I asked again to clarify what the countermeasures are and what they are planning to do with JKS. They seem to be mixing my CVE and the JKS issues with other issue in other Key Store types.
  • November 17, 2017: Oracle replied (again, mixing-in issues of other Key Store types): “In JDK 9 the default keystore format is PKCS#11 which doesn’t have the limits of the JKS format — and we’ve put in some migration capability also. For all versions we have increased the iteration counts [sic!] used significantly so that even though the algorithms are weak, a brute-force search will take a lot longer. For older versions we will be backporting the missing bits of PKCS#11 so that it can be used as the keystore type.”. That was the good part of the answer, even though JKS has no iteration count. The second part where I asked if they could add some links to their Critical Path Update Advisory was: “In order to prevent undue risks to our customers, Oracle will not provide additional information about the specifics of vulnerabilities beyond what is provided in the Critical Patch Update (or Security Alert) advisory and pre-release note, the pre-installation notes, the readme files, and FAQs.”.

That’s it for me for now. I’m too tired to start arguing about keeping technical details secret. So basically I have to hope that everyone finds this blog posts when searching for CVE-2017-10356.

Cracking Java’s weak encryption – Nail in the JKS coffin

POC||GTFO journal edition 0x15 came out a while ago and I’m happy to have contributed the article “Nail in the JKS coffin”. You should really read the article, I’m not going to repeat myself here. I’ve also made the code available on my “JKS private key cracker hashcat” github repository.

For those who really need a TL;DR, the developed cracking technique relies on three main issues with the JKS format:

  1. Due the unusual design of JKS the key store password can be ignored and the private key password cracked directly.
  2. By exploiting a weakness of the Password Based Encryption scheme for the private key in JKS described by cryptosense, the effort to try a password is minimal (one SHA-1 calculation).
  3. As public keys are not encrypted in the JKS file format, we can determine the algorithm and key size of the public key to know the PKCS#8 encoded fingerprint we have to expect in step 2.

For a practical TL;DR, see the github repository on how JksPrivkPrepare.jar can be used together with the hashcat password cracking tool to crack passwords.

Not affected of the described issues are other key store file formats such as JCEKS, PKCS12 or BKS. It is recommended to use the PKCS12 format to store private keys and to store the files in a secure location. For example it is recommended to store Android app release JKS files somewhere else than a repository such as git.