It's for an alleged "conspiracy to commit computer intrusion" with Chelsea Manning where the US claims he helped Manning get access to a passphrase so they could get some more documents[1]. We found out -- though it was criminally under-reported -- in 2018 that a case had been filed by the DOJ[2], but it's now public.

[1]: https://www.smh.com.au/world/europe/wikileaks-founder-julian... [2]: https://www.theguardian.com/media/2018/nov/16/julian-assange...