GHSA-9v4j-7g44-qcqw Algernon: Auto-refresh SSE event server binds to all interfaces with Access-Control-Allow-Origin: * and no authentication

• Go github.com/xyproto/algernon: < 1.17.7 → fixed in 1.17.7

Summary When auto-refresh is enabled, Algernon spins up an SSE handler that streams a `data:` line for every filesystem event under the watched directory. The handler performs no authentication of any kind — no shared token, no cookie check against the `permissions2` userstate, no IP allow-list, no path-prefix permission. Any client that can complete a TCP connection to the listener address receives the stream. This advisory covers the authentication gap in isolation. The cross-origin browser-reach (advisory #2b) and the network-reach (advisory #2c) amplify the impact, but each is independently fixable; this finding addresses the case where a same-origin or LAN-local client connects directly to the SSE port and reads the stream without proving anything about its identity. Details Root cause — the SSE handler does not consult `permissions2` or any other auth ```go // vendor/github.com/xyproto/recwatch/eventserver.go:100-144 (1.17.6) func GenFileChangeEvents(events TimeEventMap, mut *sync.Mutex, maxAge time.Duration, allowed string) http.HandlerFunc { return func(w http.ResponseWriter, _ *http.Request) { w.Header().Set("Content-Type", "text/event-stream;charset=utf-8") w.Header().Set("Cache-Control", "no-cache") w.Header().Set("Connection", "keep-alive") w.Header().Set("Access-Control-Allow-Origin", allowed) // ... loop emits one SSE record per filename touched ... } } ``` Note the handler signature: `func(w http.ResponseWriter, _ *http.Request)`. The request is discarded — no `Cookie`, `Authorization`, query-string, or remote-IP check is performed before the stream begins. In 1.17.6 the listener was placed on its own `http.ServeMux` (recwatch/eventserver.go:200-215), wholly outside the `perm.Rejected` middleware chain that gates Algernon's main HTTP listener. Even an operator who had configured admin/user path prefixes via `perm.AddAdminPath`, set a `cookieSecret`, and forced authentication on every URL of the main server had no way to gate this listener — it was unreachable from the `mux` argument the perm middleware uses. Why authentication matters for this listener The stream contents are not public data. They reveal: • Which files the developer is actively editing, with sub-second timing precision. • The existence of files inside the watched root (including files the operator may have meant to keep private — `.env.local`, `secrets.lua`, in-progress draft files). • By inference, the directory layout of the project. A client that can connect to the listener obtains a low-rate continuous information disclosure for the lifetime of the connection. The handler is an infinite `for {}` loop — there is no natural session boundary or expiry. Source-level evidence ```text $ rg -n 'GenFileChangeEvents|EventServer\(' vendor/github.com/xyproto/recwatch/ vendor/github.com/xyproto/recwatch/eventserver.go:101:func GenFileChangeEvents(events TimeEventMap, mut *sync.Mutex, maxAge time.Duration, allowed string) http.HandlerFunc { vendor/github.com/xyproto/recwatch/eventserver.go:177:func EventServer(path, allowed, eventAddr, eventPath string, refreshDuration time.Duration) { $ rg -n 'Cookie|Authorization|Token|state\.User' vendor/github.com/xyproto/recwatch/eventserver.go zero matches — no authentication primitive is referenced anywhere in the file ``` PoC (against 1.17.6) ```bash 1. Operator runs algernon with auto-refresh on a project directory: algernon -a /path/to/project # spins up :5553 on Linux/macOS, localhost:5553 on Windows 2. Any client that can reach the listener connects without credentials: curl -sN http://:5553/sse => id: 0 data: /path/to/project/secret-notes.md id: 1 data: /path/to/project/.env.local ``` No `Cookie`, no `Authorization`, no `X-Token`, no preflight, no challenge. The connection succeeds and the stream is delivered for as long as the client keeps the socket open. Impact • Confidentiality: medium. Continuous information disclosure of filenames and edit timing to anyone who can connect. • Integrity: none. • Availability: low. Each connection consumes a goroutine indefinitely; many simultaneous connections can exhaust descriptors. Suggestions to fix Primary fix — require a shared secret on the SSE endpoint. The auto-refresh feature already injects a script into served HTML (engine/sse.go:118-165); that script knows the SSE URL. Add a per-startup token, embed it in the injected JS, and require it on the SSE request: ```go // engine/sse.go -- in InsertAutoRefresh tmplData.SessionToken = ac.sseToken // generated once at startup, e.g. crypto/rand 32 bytes // JS: // var source = new EventSource('...?token={{.SessionToken}}'); // recwatch handler: // if subtle.ConstantTimeCompare([]byte(r.URL.Query().Get("token")), // []byte(serverToken)) != 1 { // http.Error(w, "forbidden", http.StatusForbidden); return // } ``` Cookie-bearing requests work too if `recwatch.EventServer` is moved behind `perm.Rejected` (see "Defence in depth"). The token approach is the smaller change. Defence in depth — mount the SSE handler on the main mux. Moving `recwatch.EventServerHandler` onto the main `http.ServeMux` automatically places the SSE handler behind whatever middleware the operator has configured — `perm.Rejected`, `tollbooth`, custom auth wrappers. This closes the same-origin half of the gap without a per-token implementation. Any dedicated-port path bypasses `perm.Rejected` because it uses its own `http.ServeMux`, and that path needs the token fix from "Primary fix" above. Live verification ``` $ ./algernon.exe --nodb --httponly --server -a --addr 127.0.0.1:18781 --quiet poc2/site $ ( curl -sN --max-time 4 http://127.0.0.1:5553/sse > stream.txt & sleep 1 echo "edit-1" >> poc2/site/secret-notes.md echo "edit-2" >> poc2/site/.env.local wait ) $ cat stream.txt id: 0 data: C:\Users\xbox\Desktop\VulnTesting\algernon-main\poc-test\poc2\site\secret-notes.md id: 1 data: C:\Users\xbox\Desktop\VulnTesting\algernon-main\poc-test\poc2\site\.env.local ``` No `Cookie`, no `Authorization` header. Stream delivered.