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.