i2p before 2.3.0 (Java) allows de-anonymizing the public IPv4 and IPv6 addresses of i2p hidden services (aka eepsites) via a correlation attack across the IPv4 and IPv6 addresses that occurs when a tunneled, replayed message has a behavior discrepancy (it may be dropped, or may result in a Wrong Destination response). An attack would take days to complete.
https://xeiaso.net/blog/CVE-2023-36325
https://i2pgit.org/i2p-hackers/i2p.i2p/-/commit/82aa4e19fbb37ca1bd752ec1b836120beec0985f
https://geti2p.net/en/blog/post/2023/06/25/new_release_2.3.0