Discovering an OSSEC/Wazuh Encryption Issue

I’m trying to get the Wazuh agent (a fork of OSSEC, one of the most popular open source security tools, used for intrusion detection) to talk to our custom backend (namely, our LogSentinel SIEM Collector) to allow us to reuse the powerful Wazuh/OSSEC functionalities for customers that want to install an agent on each endpoint rather than just one collector that “agentlessly” reaches out to multiple sources.

But even though there’s a good documentation on the message format and encryption, I couldn’t get to successfully decrypt the messages. (I’ll refer to both Wazuh and OSSEC, as the functionality is almost identical in both, with the distinction that Wazuh added AES support in addition to blowfish)

That lead me to a two-day investigation on possible reasons. The first side-discovery was the undocumented OpenSSL auto-padding of keys and IVs described in my previous article. Then it lead me to actually writing C code (an copying the relevant Wazuh/OSSEC pieces) in order to debug the issue. With Wazuh/OSSEC I was generating one ciphertext and with Java and openssl CLI – a different one.

I made sure the key, key size, IV and mode (CBC) are identical. That they are equally padded and that OpenSSL’s EVP API is correctly used. All of that was confirmed and yet there was a mismatch, and therefore I could not decrypt the Wazuh/OSSEC message on the other end.

After discovering the 0-padding, I also discovered a mistake in the documentation, which used a static IV of FEDCA9876543210 rather than the one found in the code, where the 0 preceded 9 – FEDCA0987654321. But that didn’t fix the issue either, only got me one step closer.

A side-note here on IVs – Wazuh/OSSEC is using a static IV, which is a bad practice. The issue is reported 5 years ago, but is minor, because they are using some additional randomness per message that remediates the use of a static IV; it’s just not idiomatic to do it that way and may have unexpected side-effects.

So, after debugging the C code, I got to a simple code that could be used to reproduce the issue and asked a question on Stackoverflow. 5 minutes after posting the question I found another, related question that had the answer – using hex strings like that in C doesn’t work. Instead, they should be encoded: char *iv = (char *)"\xFE\xDC\xBA\x09\x87\x65\x43\x21\x00\x00\x00\x00\x00\x00\x00\x00";. So, the value is not the bytes corresponding to the hex string, but the ASCII codes of each character in the hex string. I validated that in the receiving Java end with this code:

This has an implication on the documentation, as well as on the whole scheme as well. Because the Wazuh/OSSEC AES key is: MD5(password) + MD5(MD5(agentName) + MD5(agentID)){0, 15}, the 2nd part is practically discarded, because the MD5(password) is 32 characters (= 32 ASCII codes/bytes), which is the length of the AES key. This makes the key derived from a significantly smaller pool of options – 16 bytes to choose from, rather than 256.

I raised an issue with Wazuh. Although this can be seen as a vulnerability (due to the reduced key space), it’s rather minor from security point of view, and as communication is mostly happening within the corporate network, I don’t think it has to be privately reported and fixed immediately.

Yet, I made a recommendation for introducing an additional configuration option to allow to transition to the updated protocol without causing backward compatibility issues. In fact, I’d go further and recommend using TLS/DTLS rather than a home-grown, AES-based scheme. Mutual authentication can be achieved through TLS mutual authentication rather than through a shared secret.

It’s satisfying to discover issues in popular software, especially when they are not written in your “native” programming language. And as a rule of thumb – encodings often cause problems, so we should be extra careful with them.

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