{"id":"CVE-2026-28499","aliases":["GHSA-6jj5-j4j8-8473"],"url":"https://o3.security/vulnerability/CVE-2026-28499","summary":"LeafKit's HTML escaping may be skipped for Collection values, enabling XSS","details":"### Summary\nLeafKit HTML-escaping is not working correctly when a template prints a collection (Array / Dictionary) via `#(value)`. This can result in XSS, allowing potentially untrusted input to be rendered unescaped.\n\n### Details\nLeafKit attempts to escape expressions during serialization, but due to [`LeafData.htmlEscaped()`](https://github.com/vapor/leaf-kit/blob/8ff06839d8b3ddf74032d2ade01e3453eb556d30/Sources/LeafKit/LeafData/LeafData.swift#L322)'s implementation, when the escaped type's conversion to `String` is marked as `.ambiguous` (as it is the case for Arrays and Dictionaries), an unescaped `self` is returned.\n\n> **Note: I recommend first looking at the POC, before taking a look at the details below, as it is simple.** In the detailed, verbose analysis below, I explored the functions involved in more detail, in hopes that it will help you understand and locate this issue.\n\n#### The issue's detailed analysis:\n1. Leaf expression serialization eventually reaches `LeafSerializer`'s `serialize` private function below. This is where the `leafData` is `.htmlEscaped()`, and then serialized.\n\nhttps://github.com/vapor/leaf-kit/blob/8ff06839d8b3ddf74032d2ade01e3453eb556d30/Sources/LeafKit/LeafSerialize/LeafSerializer.swift#L60-L66\n\n2. The `LeafData.htmlEscaped()` method uses the `LeafData.string` computed property to convert itself to a string. Then, it calls the `htmlEscaped()` method on it. However, if the string conversion fails, notice that an unescaped, unsafe `self` is returned (line 324 below):\n\nhttps://github.com/vapor/leaf-kit/blob/8ff06839d8b3ddf74032d2ade01e3453eb556d30/Sources/LeafKit/LeafData/LeafData.swift#L321-L328\n\n\n3. Regarding why `.string` may return nil, if the escaped value is not a string already, a convesion is attempted, which may fail.\n\nhttps://github.com/vapor/leaf-kit/blob/8ff06839d8b3ddf74032d2ade01e3453eb556d30/Sources/LeafKit/LeafData/LeafData.swift#L211-L216\n\nIn this specific case, the conversion fails at line 303 below, when `conversion.is >= level` is checked. The check fails because [`.array` and `.dictionary` conversions to `.string` are deemed `.ambiguous`](https://github.com/vapor/leaf-kit/blob/8ff06839d8b3ddf74032d2ade01e3453eb556d30/Sources/LeafKit/LeafData/LeafData.swift#L525-L535). If we forcefully allow ambiguous conversions, the vulnerability disappears, as the conversion is successful.\n\nhttps://github.com/vapor/leaf-kit/blob/8ff06839d8b3ddf74032d2ade01e3453eb556d30/Sources/LeafKit/LeafData/LeafData.swift#L295-L319\n\n5. Coming back to `LeafSerializer`'s `serialize` private method, we are now interested in finding out what happens after `LeafData.htmlEscaped()` returns self. Recall from `1.` that the output was then `.serialized()`. Thus, the unescaped `LeafData` follows the normal serialization path, as if it were HTML-escaped. More specifically, serialization is done [here](https://github.com/vapor/leaf-kit/blob/8ff06839d8b3ddf74032d2ade01e3453eb556d30/Sources/LeafKit/LeafData/LeafDataStorage.swift#L52-L63), where `.map` / `.mapValues` is called, unsafely serializing each element of the dictionary.\n\n### PoC\n\n\nIn a new Vapor project created with `vapor new poc -n --leaf`, use a simple leaf template like the following:\n```html\n\n\n \n

#(username)

\n

someDict:

\n

#(someDict)

\n \n\n```\n\nAnd the following `routes.swift`:\n```swift\nimport Vapor\n\nstruct User: Encodable {\n var username: String\n var someDict: [String: String]\n}\n\nfunc routes(_ app: Application) throws {\n app.get { req async throws in\n try await req.view.render(\"index\", User(\n username: \"Escaped XSS - \",\n someDict: [\"\":\"\"]\n ))\n }\n}\n\n```\n\nBy running and accessing the server in a browser, XSS should be triggered twice (with `alert(1337)` and `alert(31337)`). `var someDict: [String: String]` could also be replaced with an array / dictionary of a different type, such as another `Encodable` stuct.\n\nAlso note that, in a real concerning scenario, the array / dictionary would contain (i.e. reflect) data inputted by the user.\n\n### Impact\nThis is a cross-site scripting (XSS) vulnerability in rendered Leaf templates. Vapor/Leaf applications that render user-controlled data inside arrays or dictionaries using `#(value)` may be impacted.","published":"2026-03-18T01:19:36.122Z","modified":"2026-08-12T03:51:21.707176607Z","cvss":null,"epss":{"score":0.00265,"percentile":0.18592,"asOf":"2026-09-17"},"cisaKev":null,"exploitsKnown":0,"affectedPackages":[{"ecosystem":"SwiftURL","name":"github.com/vapor/leaf-kit","fixedVersion":"1.14.2"}],"fix":{"url":"https://github.com/vapor/leaf-kit/commit/6044b844caa858a0c5f2505ac166f5a057c990dc","label":"vapor/leaf-kit@6044b84"},"references":[{"type":"WEB","url":"https://github.com/vapor/leaf-kit/releases/tag/1.14.2"},{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/28xxx/CVE-2026-28499.json"},{"type":"ADVISORY","url":"https://github.com/vapor/leaf-kit/security/advisories/GHSA-6jj5-j4j8-8473"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-28499"},{"type":"FIX","url":"https://github.com/vapor/leaf-kit/commit/6044b844caa858a0c5f2505ac166f5a057c990dc"},{"type":"PACKAGE","url":"https://github.com/vapor/leaf-kit"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-08-12T03:51:21.707176607Z"}}