In haml versions prior to version 5.0.0.beta.2, when using user input to perform tasks on the server, characters like < > " ' must be escaped properly. In this case, the ' character was missed. An attacker can manipulate the input to introduce additional attributes, potentially executing code.
https://security.gentoo.org/glsa/202007-27
https://lists.debian.org/debian-lts-announce/2021/12/msg00028.html
https://lists.debian.org/debian-lts-announce/2019/11/msg00007.html
https://github.com/haml/haml/commit/18576ae6e9bdcb4303fdbe6b3199869d289d67c2