fix: strict token format validation and TOCTOU race in secret store

- Reject session tokens with more or fewer than exactly 2 dot-separated
  segments in SessionCredentialService.verify()
- Add semaphore to ServerSecretStore.getOrCreateRandom() to prevent
  concurrent read-then-write race condition

Author: cursoragent

apps/server/src/auth/Layers/ServerSecretStore.ts

@@ -1,6 +1,6 @@
 import * as Crypto from "node:crypto";
 
-import { Effect, FileSystem, Layer, Path } from "effect";
+import { Effect, FileSystem, Layer, Path, Semaphore } from "effect";
 import * as PlatformError from "effect/PlatformError";
 
 import { ServerConfig } from "../../config.ts";
@@ -60,6 +60,8 @@ export const makeServerSecretStore = Effect.gen(function* () {
     );
   };
 
+  const mutex = yield* Semaphore.make(1);
+
   const getOrCreateRandom: ServerSecretStoreShape["getOrCreateRandom"] = (name, bytes) =>
     get(name).pipe(
       Effect.flatMap((existing) => {
@@ -70,6 +72,7 @@ export const makeServerSecretStore = Effect.gen(function* () {
         const generated = Crypto.randomBytes(bytes);
         return set(name, generated).pipe(Effect.as(Uint8Array.from(generated)));
       }),
+      mutex.withPermits(1),
     );
 
   const remove: ServerSecretStoreShape["remove"] = (name) =>

apps/server/src/auth/Layers/SessionCredentialService.ts

@@ -56,7 +56,13 @@ export const makeSessionCredentialService = Effect.gen(function* () {
   });
 
   const verify: SessionCredentialServiceShape["verify"] = Effect.fn("verify")(function* (token) {
-    const [encodedPayload, signature] = token.split(".");
+    const parts = token.split(".");
+    if (parts.length !== 2) {
+      return yield* new SessionCredentialError({
+        message: "Malformed session token.",
+      });
+    }
+    const [encodedPayload, signature] = parts;
     if (!encodedPayload || !signature) {
       return yield* new SessionCredentialError({
         message: "Malformed session token.",