Summary In "@astrojs/rss", the "source.title" and "enclosure.type" item fields are interpolated directly into XML template strings without XML-character escaping before being parsed by "fast-xml-parser". An attacker who controls these field values can inject arbitrary XML elements into the generated RSS feed. Details Two fields in "packages/astro-rss/src/index.ts" are affected: "source.title" item.source = parser.parse( "<source url="${result.source.url}">${result.source.title}</source>", ).source; "source.title" is validated only as "z.string()", with no restriction on XML special characters. A value containing "</source>" followed by arbitrary XML is parsed as real XML elements, merging injected nodes into the RSS item. "enclosure.type" item.enclosure = parser.parse( "<enclosure url="${enclosureURL}" length="${result.enclosure.length}" type="${result.enclosure.type}"/>", ).enclosure; "enclosure.type" is also "z.string()" and is interpolated into an XML attribute without escaping. A value containing """ followed by additional XML can break out of the attribute and inject extra elements. Proof of Concept "source.title" injection: source: { url: 'https://legit.example.com', title: '</source><item><title>INJECTED</title><link>https://evil.com</link></item><source>', } // Result: RSS feed contains an injected <item> element with an evil.com link "enclosure.type" injection: enclosure: { url: 'https://example.com/a.mp3', length: 0, type: 'audio/mpeg" /><link>https://evil.example.com</link><enclosure fake="', } // Result: RSS feed contains an injected <link> element Both injections were confirmed with "fast-xml-parser": the injected ""link": "https://evil.com"" appears in the parsed output. Impact An attacker who can control "source.title" or "enclosure.type" values (e.g., via a CMS, database, or user-submitted content that populates "RSSFeedItem") can inject arbitrary XML into the generated RSS feed. This corrupts feed structure, injects false metadata (e.g., a fake "<link>" pointing to a malicious URL), and can cause feed readers to misparse or display attacker-controlled content. In SSR mode ("output: 'server'"), the poisoned feed is served on every request to all subscribers. Patches Fixed in "@astrojs/rss@4.0.19".