diff --git a/apps/api/src/modules/auth/auth.service.spec.ts b/apps/api/src/modules/auth/auth.service.spec.ts
index e58d604..cd268b8 100644
--- a/apps/api/src/modules/auth/auth.service.spec.ts
+++ b/apps/api/src/modules/auth/auth.service.spec.ts
@@ -9,7 +9,18 @@ import * as bcrypt from 'bcryptjs';
  */
 jest.mock('@nestjs/jwt', () => ({ JwtService: class JwtService {} }));
 
-import { AuthService } from './auth.service';
+/**
+ * The real bcryptjs, with `compare` wrapped so a test can assert it was never
+ * reached. It calls straight through, so every other test still runs against
+ * the real implementation. (A namespace import's properties cannot be
+ * re-defined, so jest.spyOn is not an option here.)
+ */
+jest.mock('bcryptjs', () => {
+  const actual = jest.requireActual('bcryptjs');
+  return { ...actual, compare: jest.fn(actual.compare) };
+});
+
+import { AuthService, compareLocalHash, OTP_ONLY_PASSWORD_HASH } from './auth.service';
 import type { AdminSessionService } from '../sessions/admin-session.service';
 import {
   IamInvalidCredentialsError,
@@ -258,6 +269,58 @@ describe('AuthService login', () => {
     );
   });
 
+  /* ─────────────── accounts with no usable password ─────────────── */
+
+  describe('accounts with no usable password', () => {
+    const noPassword = ['', OTP_ONLY_PASSWORD_HASH];
+
+    it.each(noPassword)('refuses login for hash %j, whatever is typed', async (hash) => {
+      const { service } = build(disabledIam(), { ...adminRow(), passwordHash: hash });
+
+      // Including the empty string and the sentinel itself: neither may open it
+      for (const typed of ['', 'x', OTP_ONLY_PASSWORD_HASH]) {
+        await expect(
+          service.login({ phone: '09120000000', password: typed }),
+        ).rejects.toThrow('نام کاربری یا رمز عبور اشتباه است');
+      }
+    });
+
+    it.each(noPassword)('refuses hash %j on the IAM fallback path too', async (hash) => {
+      const verifyCredentials = jest.fn().mockRejectedValue(new IamUnavailableError('down'));
+      const { service } = build(
+        { enabled: true, allowsLocalFallback: true, verifyCredentials },
+        { ...adminRow(), passwordHash: hash },
+      );
+
+      await expect(
+        service.login({ phone: '09120000000', password: '' }),
+      ).rejects.toBeInstanceOf(UnauthorizedException);
+    });
+
+    it('never hands a malformed hash to bcrypt', async () => {
+      const compare = bcrypt.compare as unknown as jest.Mock;
+      compare.mockClear();
+
+      expect(await compareLocalHash('x', OTP_ONLY_PASSWORD_HASH)).toBe(false);
+      expect(await compareLocalHash('x', '')).toBe(false);
+      expect(await compareLocalHash('x', null)).toBe(false);
+      expect(compare).not.toHaveBeenCalled();
+
+      // …while a real hash still goes through it
+      await compareLocalHash(PASSWORD, passwordHash);
+      expect(compare).toHaveBeenCalledTimes(1);
+    });
+
+    it("still accepts the legacy server's PHP $2y$ hashes", async () => {
+      // PHP's $2y$ is the same algorithm as $2b$; the legacy passwords
+      // (customers and the sales experts moving to AdminUser) use it.
+      const legacy = '$2y$' + (await bcrypt.hash(PASSWORD, 4)).slice(4);
+
+      expect(await compareLocalHash(PASSWORD, legacy)).toBe(true);
+      expect(await compareLocalHash('wrong', legacy)).toBe(false);
+    });
+  });
+
   /* ─────────────── two-factor ─────────────── */
 
   it('explains that a two-factor account cannot log in through this path', async () => {
diff --git a/apps/api/src/modules/auth/auth.service.ts b/apps/api/src/modules/auth/auth.service.ts
index 7614430..5006a4f 100644
--- a/apps/api/src/modules/auth/auth.service.ts
+++ b/apps/api/src/modules/auth/auth.service.ts
@@ -11,6 +11,30 @@ import {
 import { AdminSessionService } from '../sessions/admin-session.service';
 import type { LoginDto } from './dto/login.dto';
 
+/**
+ * Value written into `passwordHash` for accounts that must never log in with a
+ * password — the ~1,590 legacy customers who only ever used OTP
+ * (docs/DATA-MIGRATION-MAP.md §10.4). The `!` prefix is the Django/Rails
+ * "unusable password" convention: readable by anyone looking at the row, and
+ * matched by no password.
+ */
+export const OTP_ONLY_PASSWORD_HASH = '!OTP_ONLY_NO_PASSWORD_LOGIN';
+
+/**
+ * A real bcrypt hash always starts with `$2` ($2a$, $2b$, or PHP's $2y$ from
+ * the legacy server — all three verify with bcryptjs). Anything else is not a
+ * password: '' on accounts created without one, or OTP_ONLY_PASSWORD_HASH.
+ *
+ * Such hashes are refused here, BEFORE bcrypt is called, so the outcome never
+ * depends on how a particular bcrypt build treats a malformed hash. bcryptjs 3
+ * happens to return false today; a future version or a native binding might
+ * throw and turn a wrong password into a 500 (owner ruling 2026-10-07).
+ */
+export async function compareLocalHash(password: string, passwordHash: string | null | undefined) {
+  if (typeof passwordHash !== 'string' || !passwordHash.startsWith('$2')) return false;
+  return bcrypt.compare(password, passwordHash);
+}
+
 @Injectable()
 export class AuthService {
   private readonly logger = new Logger(AuthService.name);
@@ -100,7 +124,7 @@ export class AuthService {
     clientIp?: string,
   ): Promise<{ ok: boolean; identity?: IamIdentity }> {
     if (!this.iam.enabled) {
-      return { ok: await bcrypt.compare(dto.password, passwordHash) };
+      return { ok: await compareLocalHash(dto.password, passwordHash) };
     }
 
     try {
@@ -132,7 +156,7 @@ export class AuthService {
       }
 
       // fallback mode: the account may not have been migrated to the IAM yet
-      return { ok: await bcrypt.compare(dto.password, passwordHash) };
+      return { ok: await compareLocalHash(dto.password, passwordHash) };
     }
   }