In Nim 1.2.4, the standard library browsers mishandles the URL argument to browsers.openDefaultBrowser. This argument can be a local file path that will be opened in the default explorer. An attacker can pass one argument to the underlying open command to execute arbitrary registered system commands.
An issue was discovered in Nim before 1.6.2. The RST module of the Nim language stdlib, as used in NimForum and other products, permits the javascript: URI scheme and thus can lead to XSS in some applications. (Nim versions 1.6.2 and later are fixed; there may be backports of the fix to some earlier versions. NimForum 2.2.0 is fixed.)