diff --git a/Auto Run Docs/SpecKit-web-header-analyzer-Phase-08-Security-Operations.md b/Auto Run Docs/SpecKit-web-header-analyzer-Phase-08-Security-Operations.md
index 37f6884..d5ba1c3 100644
--- a/Auto Run Docs/SpecKit-web-header-analyzer-Phase-08-Security-Operations.md	
+++ b/Auto Run Docs/SpecKit-web-header-analyzer-Phase-08-Security-Operations.md	
@@ -45,6 +45,6 @@ This phase protects the analysis service from abuse with per-IP rate limiting an
 - [x] `GET /api/health` returns `{status, version, uptime, scannerCount}`
 - [x] All routers and CORS middleware are registered in `main.py`
 - [x] Application starts statelessly — no database, no session management. Verified `backend/app/main.py` registers only CORS + rate limiter middleware and does not initialize any DB/session services.
-- [ ] CAPTCHA modal is keyboard accessible (Tab, Enter, Escape to close)
+- [x] CAPTCHA modal is keyboard accessible (Tab, Enter, Escape to close)
 - [ ] Linting passes on both sides
 - [ ] Run `/speckit.analyze` to verify consistency
diff --git a/frontend/src/__tests__/CaptchaChallenge.test.tsx b/frontend/src/__tests__/CaptchaChallenge.test.tsx
index f8b1f1c..5bcbfc2 100644
--- a/frontend/src/__tests__/CaptchaChallenge.test.tsx
+++ b/frontend/src/__tests__/CaptchaChallenge.test.tsx
@@ -194,4 +194,40 @@ describe("CaptchaChallenge", () => {
 
     expect(onClose).toHaveBeenCalled();
   });
+
+  it("keeps focus trapped within the modal when tabbing", () => {
+    const { container } = render(
+      <CaptchaChallenge
+        isOpen
+        challenge={sampleChallenge}
+        onVerify={vi.fn()}
+        onSuccess={vi.fn()}
+        onRetry={vi.fn()}
+        onClose={vi.fn()}
+      />,
+    );
+
+    const closeButton = getByTestId(container, "captcha-close") as HTMLButtonElement;
+    const submit = getByTestId(container, "captcha-submit") as HTMLButtonElement;
+
+    submit.focus();
+    expect(document.activeElement).toBe(submit);
+
+    act(() => {
+      submit.dispatchEvent(new KeyboardEvent("keydown", { key: "Tab", bubbles: true }));
+    });
+
+    expect(document.activeElement).toBe(closeButton);
+
+    closeButton.focus();
+    expect(document.activeElement).toBe(closeButton);
+
+    act(() => {
+      closeButton.dispatchEvent(
+        new KeyboardEvent("keydown", { key: "Tab", shiftKey: true, bubbles: true }),
+      );
+    });
+
+    expect(document.activeElement).toBe(submit);
+  });
 });
diff --git a/frontend/src/components/CaptchaChallenge.tsx b/frontend/src/components/CaptchaChallenge.tsx
index 11348f6..e478edd 100644
--- a/frontend/src/components/CaptchaChallenge.tsx
+++ b/frontend/src/components/CaptchaChallenge.tsx
@@ -44,6 +44,7 @@ export default function CaptchaChallenge({
   const titleId = useId();
   const inputRef = useRef<HTMLInputElement | null>(null);
   const answerRef = useRef("");
+  const modalRef = useRef<HTMLDivElement | null>(null);
 
   useLayoutEffect(() => {
     if (!isOpen) {
@@ -55,6 +56,13 @@ export default function CaptchaChallenge({
     answerRef.current = "";
   }, [isOpen, challenge?.challengeToken]);
 
+  useLayoutEffect(() => {
+    if (!isOpen) {
+      return;
+    }
+    inputRef.current?.focus();
+  }, [isOpen, challenge?.challengeToken]);
+
   const commitAnswer = useCallback((value: string) => {
     answerRef.current = value;
     setAnswer(value);
@@ -88,6 +96,43 @@ export default function CaptchaChallenge({
       if (event.key === "Escape") {
         event.preventDefault();
         onClose();
+        return;
+      }
+
+      if (event.key !== "Tab") {
+        return;
+      }
+      const container = modalRef.current;
+      if (!container) {
+        return;
+      }
+      const focusableElements = Array.from(
+        container.querySelectorAll<HTMLElement>(
+          'button, [href], input, select, textarea, [tabindex]:not([tabindex="-1"])',
+        ),
+      ).filter(
+        (element) =>
+          !element.hasAttribute("disabled") &&
+          element.getAttribute("aria-hidden") !== "true",
+      );
+
+      if (focusableElements.length === 0) {
+        return;
+      }
+
+      const firstElement = focusableElements[0];
+      const lastElement = focusableElements[focusableElements.length - 1];
+      const activeElement = document.activeElement;
+
+      if (event.shiftKey && activeElement === firstElement) {
+        event.preventDefault();
+        lastElement.focus();
+        return;
+      }
+
+      if (!event.shiftKey && activeElement === lastElement) {
+        event.preventDefault();
+        firstElement.focus();
       }
     },
     [onClose],
@@ -117,6 +162,7 @@ export default function CaptchaChallenge({
         data-testid="captcha-challenge"
         onKeyDown={handleKeyDown}
         tabIndex={-1}
+        ref={modalRef}
       >
         <div className="flex items-center justify-between gap-4">
           <div className="flex items-center gap-3">