However as I don't have log4j on the outside it's fine (it gets redirected to https, and then asked for an x509 certificate, which it doesn't provide, so gets dropped)
Minecraft logs every message from other players it receives, so all you have to do is type a message and everyone on the server is exploited.
Yes this is still possible if you log any user-modifiable value. One example would be logging out a user agent header - if an attacker spoofs this to include a JDNI URI then the vulnerability can be exploited.
This is why this CVE is so scary - I would imagine the majority of applications using log4j will log out a user-supplied value at some point.
How about feeding the magic string via Host header in your requests and then cutting off? You wouldn't even need to establish the full TLS handshake, SNI is sent in the clear, and you would get to hit every single load-balancer and middle box - and everything they send their logs to.
Are you ever logging strings that come from outside your software, be it things users type in, HTTP headers, ...? Then you don't control if you are "using any of the jni special Uris".
This vulnerability is equivalent using printf(userData) when you should have done printf("%s", userData). From the perspective of the library developer, the feature is only available in the format string.
The log4j maintainers seem to have realized that this (the "%m" in a PatternLayout doing lookups) is a bad idea around version 2.10 (released in 2017) or even version 2.7[2] (released in 2016). These versions both included changes that allowed you to disable this behavior. Unfortunately, the Java compatibility mindset meant that they didn't take the further logical step of making the behavior that disables lookups the default.
I think this vulnerability should be used as a lesson against the vagaries of the classic Java API design issues that we're now finally starting to turn away from. Having an extensible formatting mechanism is not necessary a bad idea, but the problem with this and so many other "magic" features provided by Java libraries is that they are:
* Opt out, instead of opt-in
* Hard to discover - if you don't read the ENTIRE log4j documentation (which is pretty large!), it's hard know that this stuff is happening.
* Too inclusive - adding JNDI was a bad idea, but even allowing things like environment variables or JMX Beans to be looked-up wholesale from a non-sanitized message is a bad idea.
The problem is much deeper than log4j really. In hindsight, features like JNDI, RMI, and most of all Java Serialization should have never been part of Java in the first place.
If you log any sort of input from outside, then attackers get to supply their own URLs.
This will trigger even if you're doing it yourself because log4j also keeps recursively expanding macros in the log string until no more are found. So parameterizing your log statements isn't a defense against this the way parameterizing your SQL statements would be for SQL injection.
