diff --git a/src/subdomains/generic/user/models/auth/__tests__/auth-lightning-signature.spec.ts b/src/subdomains/generic/user/models/auth/__tests__/auth-lightning-signature.spec.ts
new file mode 100644
index 0000000000..b53c53a488
--- /dev/null
+++ b/src/subdomains/generic/user/models/auth/__tests__/auth-lightning-signature.spec.ts
@@ -0,0 +1,54 @@
+import { ConfigService } from 'src/config/config';
+import { AuthService } from '../auth.service';
+
+// Regression guard for the custodial-Lightning sign-in bypass: an empty stored signature must never
+// authenticate. verifySignature is private, so we exercise it via bracket access with a bare instance
+// (the Lightning branch only uses getSignMessages + the static CryptoService.getBlockchainsBasedOn).
+describe('AuthService custodial Lightning signature check', () => {
+  let service: AuthService;
+
+  // LNNID + 66 alnum → recognised as a Lightning address by CryptoService
+  const lightningAddress = `LNNID${'A'.repeat(66)}`;
+  // 140 lowercase alnum → matches the custodial-Lightning signature shape
+  const validShapeSignature = 'a'.repeat(140);
+
+  const verify = (signature: string, dbSignature: string | undefined, isSignUp = false): Promise<boolean> =>
+    (
+      service as unknown as {
+        verifySignature: (
+          address: string,
+          signature: string,
+          isCustodial: boolean,
+          key: string | undefined,
+          dbSignature: string | undefined,
+          blockchain: undefined,
+          isSignUp: boolean,
+        ) => Promise<boolean>;
+      }
+    ).verifySignature(lightningAddress, signature, false, undefined, dbSignature, undefined, isSignUp);
+
+  beforeAll(() => {
+    new ConfigService();
+  });
+
+  beforeEach(() => {
+    service = Object.create(AuthService.prototype);
+  });
+
+  it('rejects sign-in when the stored signature is empty (account takeover guard)', async () => {
+    await expect(verify(validShapeSignature, '')).resolves.toBe(false);
+    await expect(verify(validShapeSignature, undefined)).resolves.toBe(false);
+  });
+
+  it('rejects sign-in when the signature does not match the stored one', async () => {
+    await expect(verify(validShapeSignature, 'b'.repeat(140))).resolves.toBe(false);
+  });
+
+  it('accepts sign-in when the signature matches a non-empty stored signature', async () => {
+    await expect(verify(validShapeSignature, validShapeSignature)).resolves.toBe(true);
+  });
+
+  it('accepts sign-up (establishes the first signature) even without a stored signature', async () => {
+    await expect(verify(validShapeSignature, undefined, true)).resolves.toBe(true);
+  });
+});
diff --git a/src/subdomains/generic/user/models/auth/auth.service.ts b/src/subdomains/generic/user/models/auth/auth.service.ts
index 69189c7154..cd9f471678 100644
--- a/src/subdomains/generic/user/models/auth/auth.service.ts
+++ b/src/subdomains/generic/user/models/auth/auth.service.ts
@@ -152,7 +152,7 @@ export class AuthService {
     const custodyProvider = await this.custodyProviderService.getWithMasterKey(dto.signature).catch(() => undefined);
     if (
       !custodyProvider &&
-      !(await this.verifySignature(dto.address, dto.signature, isCustodial, dto.key, undefined, dto.blockchain))
+      !(await this.verifySignature(dto.address, dto.signature, isCustodial, dto.key, undefined, dto.blockchain, true))
     ) {
       throw new BadRequestException('Invalid signature');
     }
@@ -475,14 +475,18 @@ export class AuthService {
     key?: string,
     dbSignature?: string,
     blockchain?: Blockchain,
+    isSignUp = false,
   ): Promise<boolean> {
     const { defaultMessage, fallbackMessage } = this.getSignMessages(address);
 
     const blockchains = CryptoService.getBlockchainsBasedOn(address);
 
     if (blockchains.includes(Blockchain.LIGHTNING) && (isCustodial || /^[a-z0-9]{140,146}$/.test(signature))) {
-      // custodial Lightning wallet, only comparison check
-      return !dbSignature || signature === dbSignature;
+      // custodial Lightning wallet: no cryptographic check is possible, so the signature acts as a
+      // shared secret. On sign-up nothing is stored yet, so the first signature establishes it; on
+      // sign-in it must match a NON-EMPTY stored signature. An empty stored signature must never
+      // authenticate — otherwise any signature passes for an account whose credential was never set.
+      return isSignUp || (!!dbSignature && signature === dbSignature);
     }
 
     if (blockchains.includes(Blockchain.DEFICHAIN)) {