diff --git a/backend/src/controllers/stream.controller.ts b/backend/src/controllers/stream.controller.ts
index 5476fb6e..13462554 100644
--- a/backend/src/controllers/stream.controller.ts
+++ b/backend/src/controllers/stream.controller.ts
@@ -66,8 +66,34 @@ function sumStringI128(values: string[]): string {
  */
 export const createStream = async (req: Request, res: Response) => {
   try {
+    const callerPublicKey = (req as AuthenticatedRequest).user?.publicKey;
+    if (!callerPublicKey) {
+      return res.status(401).json({ error: 'Unauthorized', message: 'Authentication required' });
+    }
+
     const { streamId, sender, recipient, tokenAddress, ratePerSecond, depositedAmount, startTime } = req.body;
 
+    // Issue #809: validate identity fields before any DB write.
+    if (typeof sender !== 'string' || sender.length === 0) {
+      return res.status(400).json({ error: 'Invalid sender: must be a non-empty string' });
+    }
+    if (typeof recipient !== 'string' || recipient.length === 0) {
+      return res.status(400).json({ error: 'Invalid recipient: must be a non-empty string' });
+    }
+    if (typeof tokenAddress !== 'string' || tokenAddress.length === 0) {
+      return res.status(400).json({ error: 'Invalid tokenAddress: must be a non-empty string' });
+    }
+
+    // Issue #809: the authenticated wallet may only create/modify streams it owns.
+    // Without this, any logged-in wallet could POST an arbitrary `sender` and have
+    // it persisted, or flip another owner's cancelled stream back to active.
+    if (sender !== callerPublicKey) {
+      return res.status(403).json({
+        error: 'Forbidden',
+        message: 'sender must match the authenticated wallet',
+      });
+    }
+
     const parsedStreamId = Number.parseInt(streamId, 10);
     const parsedStartTime = Number.parseInt(startTime, 10);
     const parsedRatePerSecond = BigInt(ratePerSecond);
@@ -91,6 +117,18 @@ export const createStream = async (req: Request, res: Response) => {
 
     const endTime = parsedStartTime + Number(parsedDepositedAmount / parsedRatePerSecond);
 
+    // Issue #809: never let the upsert update branch touch a stream owned by a
+    // different wallet. The caller is already proven to equal `sender` above, so
+    // reject any existing row whose sender differs — this blocks reactivating or
+    // overwriting someone else's (e.g. cancelled) stream.
+    const existing = await prisma.stream.findUnique({ where: { streamId: parsedStreamId } });
+    if (existing && existing.sender !== callerPublicKey) {
+      return res.status(403).json({
+        error: 'Forbidden',
+        message: 'Cannot modify a stream owned by another wallet',
+      });
+    }
+
     const stream = await prisma.stream.upsert({
       where: { streamId: parsedStreamId },
       update: {
diff --git a/backend/tests/stream.controller.test.ts b/backend/tests/stream.controller.test.ts
index ef37fe1b..b58b2335 100644
--- a/backend/tests/stream.controller.test.ts
+++ b/backend/tests/stream.controller.test.ts
@@ -62,6 +62,8 @@ describe('Stream Controller', () => {
       query: {},
       params: {},
     };
+    // Authenticated caller matches body.sender by default (Issue #809).
+    (req as any).user = { publicKey: 'GSENDER' };
     res = {
       status: vi.fn().mockReturnThis(),
       json: vi.fn().mockReturnThis(),
@@ -70,6 +72,7 @@ describe('Stream Controller', () => {
 
   describe('createStream', () => {
     it('should create a stream successfully', async () => {
+      (prisma.stream.findUnique as any).mockResolvedValue(null);
       (prisma.stream.upsert as any).mockResolvedValue({ streamId: 123 });
 
       await createStream(req as Request, res as Response);
@@ -78,6 +81,52 @@ describe('Stream Controller', () => {
       expect(prisma.stream.upsert).toHaveBeenCalled();
     });
 
+    it('should return 401 when the request is unauthenticated', async () => {
+      (req as any).user = undefined;
+
+      await createStream(req as Request, res as Response);
+
+      expect(res.status).toHaveBeenCalledWith(401);
+      expect(prisma.stream.upsert).not.toHaveBeenCalled();
+    });
+
+    it('should return 403 when the caller is not the body sender (Issue #809)', async () => {
+      (req as any).user = { publicKey: 'GATTACKER' };
+
+      await createStream(req as Request, res as Response);
+
+      expect(res.status).toHaveBeenCalledWith(403);
+      expect(prisma.stream.upsert).not.toHaveBeenCalled();
+    });
+
+    it('should reject 400 when sender is missing (Issue #809)', async () => {
+      delete req.body.sender;
+      (req as any).user = { publicKey: 'GSENDER' };
+
+      await createStream(req as Request, res as Response);
+
+      expect(res.status).toHaveBeenCalledWith(400);
+      expect(prisma.stream.upsert).not.toHaveBeenCalled();
+    });
+
+    it('should return 403 and not reactivate a cancelled stream owned by another wallet (Issue #809)', async () => {
+      // A victim previously created (and cancelled) this stream.
+      (prisma.stream.findUnique as any).mockResolvedValue({
+        streamId: 123,
+        sender: 'GVICTIM',
+        isActive: false,
+      });
+      // Attacker authenticates as themselves and sets sender to their own key,
+      // trying to hijack the victim's streamId.
+      req.body.sender = 'GATTACKER';
+      (req as any).user = { publicKey: 'GATTACKER' };
+
+      await createStream(req as Request, res as Response);
+
+      expect(res.status).toHaveBeenCalledWith(403);
+      expect(prisma.stream.upsert).not.toHaveBeenCalled();
+    });
+
     it('should return 400 for invalid streamId', async () => {
       req.body.streamId = 'abc';
       await createStream(req as Request, res as Response);