Today, I finally bothered to look at the raw data after having been annoyed for quite some time by the links in the FreeBSD Security Advisories RSS feed not working. I do not recall when it stopped working, or if the links in this feed ever worked. The problem with the SA RSS feed (https://www.freebsd.org/security/rss.xml) is that the protocol is missing from every link, i.e. //security.FreeBSD.org/advisories/FreeBSD-SA-16:35.openssl.asc instead of https://security.FreeBSD.org/advisories/FreeBSD-SA-16:35.openssl.asc Other feeds, e.g. FreeBSD New Flash (https://www.freebsd.org/news/rss.xml), contain valid links which include the protocol.
It appears this problem has been corrected. I do not know when a change was made, only that the links to yesterday's SAs are valid.
It was probably due to r49712 that fixed the issue: https://svnweb.freebsd.org/doc?view=revision&revision=49712 PR closed. Thanks for filing it.