HAProxy before 2.8.2 accepts # as part of the URI component, which might allow remote attackers to obtain sensitive information or have unspecified other impact upon misinterpretation of a path_end rule, such as routing index.html#.png to a static server.
https://www.mail-archive.com/haproxy%40formilux.org/msg43861.html
https://lists.w3.org/Archives/Public/ietf-http-wg/2023JulSep/0070.html
https://lists.debian.org/debian-lts-announce/2023/12/msg00010.html