Cross-origin browser requests via unescaped Link fields

Output-escaping flaw in cow_link in Nine Nines Cowlib 2.9.0 and later lets attackers smuggle Link directives via crafted fields. The cow_link:do_link/1 serializer writes target URIs and rel values into a Link header without validation and emits attribute keys verbatim, so >, ", , whitespace, and = break header grammar and inject new directives. Reachability depends on an application feeding attacker-influenced parsed Link data back into cow_link:link/1; when that pattern exists, victim browsers can be driven to preconnect, preload, or prerender attacker-controlled origins.