Add detailed vulnerability report for lowdb async adapter

SECURITY.md

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+<!-- GITHUB SECURITY ADVISORY SUBMISSION -->
+<!-- Title:              Silent Data Loss via Write Coalescing in steno (lowdb dependency) -->
+<!-- Severity:           Medium -->
+<!-- Ecosystem:          npm -->
+<!-- Package name:       lowdb -->
+<!-- Affected versions:  <= 7.0.1 -->
+<!-- Patched versions:   (leave blank) -->
+<!-- CWE:                CWE-362 -->
+
+## Summary
+
+lowdb v7.0.1's async file adapter depends on steno v4.0.2, whose `Writer` silently drops intermediate writes when a write is in progress. All `write()` promises resolve successfully, but only the last queued write is persisted to disk. This is a data integrity violation — callers receive a false durability guarantee, leading to silent data loss under concurrent load.
+
+## Details
+
+When a steno `Writer` is locked (write in progress), subsequent `write()` calls are handled by `#add()` (`steno/lib/index.js`, lines 35-45):
+
+```javascript
+#add(data) {
+    this.#nextData = data;  // OVERWRITES any previously queued data
+    this.#nextPromise ||= new Promise((resolve, reject) => {
+        this.#next = [resolve, reject];
+    });
+    return new Promise((resolve, reject) => {
+        this.#nextPromise?.then(resolve).catch(reject);
+    });
+}
+```
+
+If writes A, B, C are queued while write #1 is in progress:
+1. `#add(A)` → `#nextData = A`
+2. `#add(B)` → `#nextData = B` (A silently dropped)
+3. `#add(C)` → `#nextData = C` (B silently dropped)
+4. Write #1 completes → only C is written
+5. **All three promises resolve** — callers of `write(A)` and `write(B)` believe their data was persisted
+
+## PoC
+
+```javascript
+import { Writer } from 'steno';
+const writer = new Writer('test.json');
+
+// Fire 10 concurrent writes
+const promises = [];
+for (let i = 0; i < 10; i++) {
+    promises.push(writer.write(JSON.stringify({ id: i })));
+}
+await Promise.all(promises);
+// All 10 promises resolved ✓
+
+// Read back from disk:
+import fs from 'node:fs';
+const persisted = JSON.parse(fs.readFileSync('test.json', 'utf-8'));
+console.log(persisted); // { id: 9 } — only the LAST write survived
+
+// Writes 0-8 (90%) silently lost despite successful promise resolution
+```
+
+**Tested and confirmed:** 9 out of 10 writes (90%) silently dropped. All 10 `write()` promises resolved successfully, giving callers a false durability guarantee.
+
+## Impact
+
+Any web application using lowdb's async adapter that handles concurrent requests will silently lose user data:
+
+- API returns **200 OK** to the user, but their data is **never persisted**
+- No errors, no warnings — completely silent data loss
+- Under typical web server load (multiple concurrent requests), the **majority** of writes are lost
+- Affects audit logs, user data, configuration changes — anything stored in lowdb
+
+## Remediation
+
+1. Replace write coalescing with a sequential write queue that persists every write
+2. At minimum, reject or error on coalesced writes instead of silently dropping them
+3. Document the write-coalescing behavior prominently so developers avoid relying on write durability
+
+**Note:** This vulnerability originates in the `steno` package (same maintainer). A separate advisory may be appropriate for `steno` itself.