fix(security): require /etc/shells for shell env fallback

src/infra/shell-env.test.ts

@@ -28,6 +28,7 @@ describe("shell env fallback", () => {
   }
 
   function runShellEnvFallbackForShell(shell: string) {
+    resetShellPathCacheForTests();
     const env: NodeJS.ProcessEnv = { SHELL: shell };
     const exec = vi.fn(() => Buffer.from("OPENAI_API_KEY=from-shell\0"));
     const res = loadShellEnvFallback({
@@ -170,18 +171,44 @@ describe("shell env fallback", () => {
     expect(exec).toHaveBeenCalledWith("/bin/sh", ["-l", "-c", "env -0"], expect.any(Object));
   });
 
-  it("uses trusted absolute SHELL path when executable on posix-style paths", () => {
-    const accessSyncSpy = vi.spyOn(fs, "accessSync").mockImplementation(() => undefined);
+  it("falls back to /bin/sh when SHELL is absolute but not registered in /etc/shells", () => {
+    const readFileSyncSpy = vi
+      .spyOn(fs, "readFileSync")
+      .mockImplementation((filePath, encoding) => {
+        if (filePath === "/etc/shells" && encoding === "utf8") {
+          return "/bin/sh\n/bin/bash\n/bin/zsh\n";
+        }
+        throw new Error(`Unexpected readFileSync(${String(filePath)}) in test`);
+      });
     try {
-      const trustedShell = "/usr/bin/zsh-trusted";
-      const { res, exec } = runShellEnvFallbackForShell(trustedShell);
-      const expectedShell = process.platform === "win32" ? "/bin/sh" : trustedShell;
+      const { res, exec } = runShellEnvFallbackForShell("/opt/homebrew/bin/evil-shell");
 
       expect(res.ok).toBe(true);
       expect(exec).toHaveBeenCalledTimes(1);
-      expect(exec).toHaveBeenCalledWith(expectedShell, ["-l", "-c", "env -0"], expect.any(Object));
+      expect(exec).toHaveBeenCalledWith("/bin/sh", ["-l", "-c", "env -0"], expect.any(Object));
     } finally {
-      accessSyncSpy.mockRestore();
+      readFileSyncSpy.mockRestore();
+    }
+  });
+
+  it("uses SHELL when it is explicitly registered in /etc/shells", () => {
+    const readFileSyncSpy = vi
+      .spyOn(fs, "readFileSync")
+      .mockImplementation((filePath, encoding) => {
+        if (filePath === "/etc/shells" && encoding === "utf8") {
+          return "/bin/sh\n/usr/bin/zsh-trusted\n";
+        }
+        throw new Error(`Unexpected readFileSync(${String(filePath)}) in test`);
+      });
+    try {
+      const trustedShell = "/usr/bin/zsh-trusted";
+      const { res, exec } = runShellEnvFallbackForShell(trustedShell);
+
+      expect(res.ok).toBe(true);
+      expect(exec).toHaveBeenCalledTimes(1);
+      expect(exec).toHaveBeenCalledWith(trustedShell, ["-l", "-c", "env -0"], expect.any(Object));
+    } finally {
+      readFileSyncSpy.mockRestore();
     }
   });
 

src/infra/shell-env.ts

@@ -8,13 +8,6 @@ import { sanitizeHostExecEnv } from "./host-env-security.js";
 const DEFAULT_TIMEOUT_MS = 15_000;
 const DEFAULT_MAX_BUFFER_BYTES = 2 * 1024 * 1024;
 const DEFAULT_SHELL = "/bin/sh";
-const TRUSTED_SHELL_PREFIXES = [
-  "/bin/",
-  "/usr/bin/",
-  "/usr/local/bin/",
-  "/opt/homebrew/bin/",
-  "/run/current-system/sw/bin/",
-];
 let lastAppliedKeys: string[] = [];
 let cachedShellPath: string | null | undefined;
 let cachedEtcShells: Set<string> | null | undefined;
@@ -70,21 +63,7 @@ function isTrustedShellPath(shell: string): boolean {
 
   // Primary trust anchor: shell registered in /etc/shells.
   const registeredShells = readEtcShells();
-  if (registeredShells?.has(shell)) {
-    return true;
-  }
-
-  // Fallback for environments where /etc/shells is incomplete/unavailable.
-  if (!TRUSTED_SHELL_PREFIXES.some((prefix) => shell.startsWith(prefix))) {
-    return false;
-  }
-
-  try {
-    fs.accessSync(shell, fs.constants.X_OK);
-    return true;
-  } catch {
-    return false;
-  }
+  return registeredShells?.has(shell) === true;
 }
 
 function resolveShell(env: NodeJS.ProcessEnv): string {