fix(selfhost): log blob-store path-traversal gets distinctly from misses

src/selfhost/blob-store.ts

@@ -19,10 +19,26 @@ export function createFsBlobStore(baseDir: string): R2Bucket {
     return full;
   };
   const store = {
-    /** Stream a stored object's bytes, or null on a miss (ENOENT / unreadable). The serve route reads `.body`. */
+    /** Stream a stored object's bytes, or null on a miss (ENOENT / unreadable). The serve route reads `.body`.
+     *  A path-traversal key still returns null (safe miss for `/loopover/shot`), but is logged distinctly from
+     *  an ordinary miss so probes are visible (#6283) — put/delete keep throwing the same check unguarded. */
     async get(key: string): Promise<R2ObjectBody | null> {
+      let path: string;
       try {
-        const bytes = await readFile(pathFor(key));
+        path = pathFor(key);
+      } catch (error) {
+        console.warn(
+          JSON.stringify({
+            level: "warn",
+            event: "selfhost_blob_key_escapes_base_dir",
+            key: typeof key === "string" ? key.slice(0, 200) : String(key).slice(0, 200),
+            message: error instanceof Error ? error.message.slice(0, 200) : String(error).slice(0, 200),
+          }),
+        );
+        return null;
+      }
+      try {
+        const bytes = await readFile(path);
         return { body: new Response(bytes).body } as unknown as R2ObjectBody;
       } catch {
         return null;

test/unit/selfhost-blob-store.test.ts

@@ -1,7 +1,7 @@
 import { mkdtempSync, rmSync } from "node:fs";
 import { tmpdir } from "node:os";
 import { join } from "node:path";
-import { afterEach, beforeEach, describe, expect, it } from "vitest";
+import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
 import { createFsBlobStore } from "../../src/selfhost/blob-store";
 
 describe("createFsBlobStore (#10 — self-host visual screenshot persistence)", () => {
@@ -36,8 +36,34 @@ describe("createFsBlobStore (#10 — self-host visual screenshot persistence)",
 
   it("rejects a key that escapes the base dir — put throws, get is a safe miss (no traversal)", async () => {
     const store = createFsBlobStore(dir);
-    await expect(store.put("../escape.png", new Uint8Array([1]))).rejects.toThrow(/escapes base dir/);
-    expect(await store.get("../../etc/passwd")).toBeNull(); // the pathFor throw is caught inside get → safe miss
+    const warn = vi.spyOn(console, "warn").mockImplementation(() => {});
+    try {
+      await expect(store.put("../escape.png", new Uint8Array([1]))).rejects.toThrow(/escapes base dir/);
+      expect(await store.get("../../etc/passwd")).toBeNull(); // the pathFor throw is caught inside get → safe miss
+    } finally {
+      warn.mockRestore();
+    }
+  });
+
+  it("logs a path-traversal get distinctly from an ordinary miss (#6283)", async () => {
+    const store = createFsBlobStore(dir);
+    const warn = vi.spyOn(console, "warn").mockImplementation(() => {});
+    try {
+      expect(await store.get("gittensory/shots/missing.png")).toBeNull();
+      expect(warn).not.toHaveBeenCalled();
+
+      expect(await store.get("../../etc/passwd")).toBeNull();
+      expect(warn).toHaveBeenCalledTimes(1);
+      const payload = JSON.parse(String(warn.mock.calls[0]?.[0]));
+      expect(payload).toMatchObject({
+        level: "warn",
+        event: "selfhost_blob_key_escapes_base_dir",
+        key: "../../etc/passwd",
+        message: expect.stringMatching(/escapes base dir/i),
+      });
+    } finally {
+      warn.mockRestore();
+    }
   });
 
   it("delete removes a stored object — a subsequent get is a miss", async () => {