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 linkenclosure.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> elementBoth 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.
References
iaohkut-from-NightWolf-Team Reporter
Summary
In
@astrojs/rss, thesource.titleandenclosure.typeitem fields are interpolated directly into XML template strings without XML-character escaping before being parsed byfast-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.tsare affected:source.titlesource.titleis validated only asz.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.typeenclosure.typeis alsoz.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.titleinjection:enclosure.typeinjection: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.titleorenclosure.typevalues (e.g., via a CMS, database, or user-submitted content that populatesRSSFeedItem) 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.References