This algorithm is tricky, and uses phutil_safe_html() directly, which makes it potentially unsafe.
In particular, D8859 fixes a bug with it which caused it to produce non-utf8 output. This doesn't guarantee it's a security problem, but does make it suspicious.
I don't actually see a way to break it, but rewrite it so that it's absolutely bulletproof and does not need to call phutil_safe_html().