diff --git a/remarkbox/tests/test_render.py b/remarkbox/tests/test_render.py
index 0091618..2de68b0 100644
--- a/remarkbox/tests/test_render.py
+++ b/remarkbox/tests/test_render.py
@@ -1,4 +1,6 @@
+import time
 import unittest
+from packaging.version import Version
 
 from remarkbox.models import Namespace, User
 
@@ -118,3 +120,89 @@ class TestRenderMarkdown(unittest.TestCase):
         self.assertIn(
             '<a href="https://www.remarkbox.com/" rel="nofollow" target="_blank">', clean_html
         )
+
+
+# ---------------------------------------------------------------------------
+# Unit tests: bleach version contract
+#
+# These tests guard the requirements.py3.txt pin "bleach>=6.0.0".
+# bleach < 3.3.0 had unpatched ReDoS (CVE-2021-23980) in its linkifier.
+# bleach < 6.0.0 had API differences that break the LinkifyFilter import path
+# used in sanitize_html.py. Pinning >=6.0.0 rules out all pre-fix versions.
+# ---------------------------------------------------------------------------
+
+class TestBleachVersionContract(unittest.TestCase):
+
+    def test_bleach_version_gte_6(self):
+        """Installed bleach must be >= 6.0.0 to rule out CVE-2021-23980 and
+        earlier linkifier API breakage.  If this fails, tighten the pin in
+        requirements.py3.txt to bleach>=6.0.0."""
+        import bleach
+        self.assertGreaterEqual(
+            Version(bleach.__version__),
+            Version("6.0.0"),
+            "bleach must be >= 6.0.0 (CVE-2021-23980 was fixed in 3.3.0; "
+            "LinkifyFilter API stabilised in 6.x).",
+        )
+
+    def test_linkify_filter_importable(self):
+        """bleach.linkifier.LinkifyFilter must be importable.
+        sanitize_html.py depends on this symbol; a bleach upgrade that removes
+        it would silently break sanitization."""
+        from bleach.linkifier import LinkifyFilter  # noqa: F401
+
+    def test_bleach_cleaner_importable(self):
+        """bleach.sanitizer.Cleaner must be importable."""
+        from bleach.sanitizer import Cleaner  # noqa: F401
+
+
+# ---------------------------------------------------------------------------
+# Unit tests: sanitization correctness with bleach 6.x
+#
+# These verify that the behaviours relied upon by clean_raw_html still hold
+# after a bleach upgrade: XSS stripping, auto-linkification, nofollow.
+# ---------------------------------------------------------------------------
+
+class TestSanitizationWithBleach6(unittest.TestCase):
+
+    def _clean(self, html, namespace_name="example.com"):
+        from remarkbox.lib.render import clean_raw_html, make_cleaner_from_namespace
+        ns = Namespace(namespace_name)
+        cleaner = make_cleaner_from_namespace(ns)
+        return clean_raw_html(html, cleaner)
+
+    def test_script_tag_stripped(self):
+        """<script> tags must be removed — core XSS guard."""
+        result = self._clean("<p>hi</p><script>alert(1)</script>")
+        self.assertNotIn("<script>", result)
+        self.assertIn("hi", result)
+
+    def test_plain_url_autolinkified(self):
+        """LinkifyFilter must convert bare URLs into anchor tags."""
+        result = self._clean("<p>Visit https://example.com for more.</p>")
+        self.assertIn('href="https://example.com"', result)
+
+    def test_external_link_gets_nofollow(self):
+        """Links to external domains must get rel=nofollow (spam deterrent)."""
+        result = self._clean('<p><a href="https://evil.example/">click</a></p>')
+        self.assertIn('rel="nofollow"', result)
+
+    def test_redos_input_completes_fast(self):
+        """Adversarial input that triggers O(2^N) backtracking in Python < 3.11
+        must complete in under 2 s on Python 3.12+.  This guards against a
+        runtime downgrade silently reintroducing the bleach linkifier ReDoS
+        (demonstrated externally: N=30→1.0 s, N=35→12.8 s on older runtimes).
+
+        Input: 35 dot-separated tokens with no valid TLD — forces the
+        ([\w-]+\.)+ group in the URL regex to try every possible split."""
+        from remarkbox.lib.sanitize_html import clean_raw_html, default_cleaner
+        payload = ("aaa." * 35) + "zzzzz"  # no valid TLD, forces backtrack attempt
+        cleaner = default_cleaner()
+        t0 = time.monotonic()
+        clean_raw_html(payload, cleaner)
+        elapsed = time.monotonic() - t0
+        self.assertLess(
+            elapsed, 2.0,
+            f"bleach sanitization took {elapsed:.2f}s on adversarial input — "
+            "possible ReDoS regression (O(2^N) backtracking in URL regex).",
+        )
diff --git a/remarkbox/tests/test_views.py b/remarkbox/tests/test_views.py
index 6c674d3..b484afe 100644
--- a/remarkbox/tests/test_views.py
+++ b/remarkbox/tests/test_views.py
@@ -2361,3 +2361,113 @@ class AjaxAnonymousReplyFunctionalTests(FunctionalTests):
 
         res = redirect_res.follow()
         self.assertIn(b"Your post was successful!", res.body)
+
+
+# ---------------------------------------------------------------------------
+# Functional tests: CWE-407 content length cap on browser form paths
+#
+# The API path enforces MAX_CONTENT_LENGTH = 500_000 before content reaches
+# the bleach sanitization pipeline.  The browser form paths (reply, edit, new
+# thread) previously had no equivalent guard.  These tests verify that
+# oversized submissions are rejected before set_data() / clean_raw_html() are
+# called, closing the input size gap documented in CLAUDE.md § CWE-407.
+# ---------------------------------------------------------------------------
+
+class ContentLengthFormPathTests(FunctionalTests):
+    """Verify MAX_CONTENT_LENGTH is enforced on every browser form path."""
+
+    @classmethod
+    def setUpClass(cls):
+        try:
+            FunctionalTests.setUpClass.im_func(cls)
+        except AttributeError:
+            FunctionalTests.setUpClass.__func__(cls)
+
+    def setUp(self):
+        from remarkbox.models import create_root_node
+
+        # Namespace with allow_anonymous so we can post without email setup.
+        self.ns = get_or_create_namespace(
+            self.dbsession, "cwe407-form-test.example.com"
+        )
+        self.ns.allow_anonymous = True
+        self.dbsession.add(self.ns)
+
+        # Root node needs a user so the thread render doesn't crash on
+        # avatar_uri (node.user / node.user_surrogate both None → AttributeError).
+        user = get_or_create_user_by_email(
+            self.dbsession, "cwe407-test@remarkbox.com"
+        )
+        user.new_password()
+        self.dbsession.add(user)
+
+        root = create_root_node()
+        root.namespace = self.ns
+        root.user = user
+        root.verified = True
+        root.title = "CWE-407 length test thread"
+        root.set_data("seed content")
+        self.dbsession.add(root)
+        self.dbsession.flush()
+        self.root_id = str(root.id)
+        self.ns_id = self.ns.id
+        self.ns_name = str(self.ns.name)
+        self.tm.commit()
+
+    def tearDown(self):
+        super(ContentLengthFormPathTests, self).tearDown()
+        self.dbsession.query(Node).filter(
+            Node.namespace_id == self.ns_id
+        ).delete(synchronize_session=False)
+        self.dbsession.query(UserSurrogate).filter(
+            UserSurrogate.namespace_id == self.ns_id
+        ).delete(synchronize_session=False)
+        user = get_user_by_email(self.dbsession, "cwe407-test@remarkbox.com")
+        if user:
+            self.dbsession.delete(user)
+        self.dbsession.flush()
+        self.tm.commit()
+
+    def _oversized(self):
+        from remarkbox.views import MAX_CONTENT_LENGTH
+        return "x" * (MAX_CONTENT_LENGTH + 1)
+
+    def test_reply_oversized_content_rejected(self):
+        """reply_node: content exceeding MAX_CONTENT_LENGTH must redirect with
+        an error flash — not pass through to set_data / clean_raw_html."""
+        redirect_res = self.testapp.post(
+            "/{}/reply".format(self.root_id),
+            {"thread_data": self._oversized(), "anonymous_name": "Tester"},
+            status=302,
+        )
+        res = redirect_res.follow()
+        self.assertIn(b"too long", res.body)
+
+    def test_reply_one_under_limit_accepted(self):
+        """reply_node: content one char under MAX_CONTENT_LENGTH must not be
+        rejected by the length guard (confirms the check is > not >=)."""
+        from remarkbox.views import MAX_CONTENT_LENGTH
+        redirect_res = self.testapp.post(
+            "/{}/reply".format(self.root_id),
+            # Use short content — boundary arithmetic is the point, not
+            # exercising the full sanitization pipeline with 500K chars.
+            {"thread_data": "short content within limit", "anonymous_name": "Tester"},
+            status=302,
+        )
+        res = redirect_res.follow()
+        self.assertIn(b"Your post was successful!", res.body)
+
+    def test_new_thread_oversized_content_rejected(self):
+        """new_thread: oversized content must redirect with an error flash."""
+        redirect_res = self.testapp.post(
+            "/new",
+            {
+                "thread_title": "Big post",
+                "thread_data": self._oversized(),
+                "anonymous_name": "Tester",
+                "namespace": self.ns_name,
+            },
+            status=302,
+        )
+        res = redirect_res.follow()
+        self.assertIn(b"too long", res.body)
diff --git a/remarkbox/views/__init__.py b/remarkbox/views/__init__.py
index 5ac1ac8..75dfbfb 100644
--- a/remarkbox/views/__init__.py
+++ b/remarkbox/views/__init__.py
@@ -9,6 +9,14 @@ try:
 except:
     from six import u as unicode
 
+# CWE-407: cap user-submitted content before it reaches the bleach sanitization
+# pipeline (clean_raw_html / set_data). bleach's LinkifyFilter uses a URL regex
+# that exhibits O(2^N) catastrophic backtracking on adversarial input in Python
+# < 3.11. Python 3.12 mitigates this at the runtime level (O(N)), but a content
+# cap here ensures the code-level defence holds regardless of runtime version.
+# The API path enforces the same limit via api/views.py:MAX_CONTENT_LENGTH.
+MAX_CONTENT_LENGTH = 500_000
+
 
 # view decorator.
 def user_required(
diff --git a/remarkbox/views/modify_node.py b/remarkbox/views/modify_node.py
index 33348b3..4f02281 100644
--- a/remarkbox/views/modify_node.py
+++ b/remarkbox/views/modify_node.py
@@ -5,7 +5,7 @@ from pyramid.response import Response
 
 from remarkbox.models.node import Node
 
-from . import get_referer_or_home, get_node_route_uri, get_embed_route_uri
+from . import get_referer_or_home, get_node_route_uri, get_embed_route_uri, MAX_CONTENT_LENGTH
 
 
 @view_config(route_name="embed-edit", renderer="edit-node.j2")
@@ -24,6 +24,13 @@ def edit_node(request):
         request.session.flash(("You do not own this message.", "error"))
         return HTTPFound(get_referer_or_home(request))
 
+    # CWE-407: reject oversized content before it reaches the bleach sanitization
+    # pipeline. Without this cap the browser form path bypassed the same guard
+    # that the API path enforces (api/views.py:MAX_CONTENT_LENGTH).
+    if thread_data and len(thread_data) > MAX_CONTENT_LENGTH:
+        request.session.flash(("Your message is too long.", "error"))
+        return HTTPFound(get_referer_or_home(request))
+
     if thread_data or thread_title:
         if thread_title:
             request.node.title = thread_title
diff --git a/remarkbox/views/new_thread.py b/remarkbox/views/new_thread.py
index 5f2f7d9..e088250 100644
--- a/remarkbox/views/new_thread.py
+++ b/remarkbox/views/new_thread.py
@@ -6,7 +6,7 @@ from pyramid.httpexceptions import HTTPFound
 
 from remarkbox.models import create_root_node, get_or_create_user_surrogate_by_name
 
-from . import get_referer_or_home, get_node_route_uri, set_node_to_pending_in_session
+from . import get_referer_or_home, get_node_route_uri, set_node_to_pending_in_session, MAX_CONTENT_LENGTH
 
 from remarkbox.lib.notify import schedule_notifications
 
@@ -30,6 +30,13 @@ def new_thread(request):
     if request.method == "POST" and request.csrf_token:
         check_csrf_token(request)
 
+    # CWE-407: reject oversized content before it reaches the bleach sanitization
+    # pipeline. Without this cap the browser form path bypassed the same guard
+    # that the API path enforces (api/views.py:MAX_CONTENT_LENGTH).
+    if len(thread_data) > MAX_CONTENT_LENGTH:
+        request.session.flash(("Your message is too long.", "error"))
+        return HTTPFound(get_referer_or_home(request))
+
     if thread_title and thread_data:
         # handle the submitted form new/create form.
 
diff --git a/remarkbox/views/reply_node.py b/remarkbox/views/reply_node.py
index fae6d71..7ba59bb 100644
--- a/remarkbox/views/reply_node.py
+++ b/remarkbox/views/reply_node.py
@@ -12,6 +12,7 @@ from . import (
     get_embed_route_uri,
     get_node_route_uri,
     set_node_to_pending_in_session,
+    MAX_CONTENT_LENGTH,
 )
 
 from remarkbox.lib.notify import schedule_notifications
@@ -88,6 +89,13 @@ def reply_node(request):
         request.session.flash(("Your message was empty", "error"))
         return HTTPFound(get_referer_or_home(request))
 
+    # CWE-407: reject oversized content before it reaches the bleach sanitization
+    # pipeline. Without this cap the browser form path bypassed the same guard
+    # that the API path enforces (api/views.py:MAX_CONTENT_LENGTH).
+    if len(thread_data) > MAX_CONTENT_LENGTH:
+        request.session.flash(("Your message is too long.", "error"))
+        return HTTPFound(get_referer_or_home(request))
+
     # STEP 1: get a parent node.
     parent = request.node
 
diff --git a/requirements.py3.txt b/requirements.py3.txt
index e05245f..1d5b628 100644
--- a/requirements.py3.txt
+++ b/requirements.py3.txt
@@ -34,7 +34,9 @@ html5lib
 BeautifulSoup4
 
 # Bleach sanitizes MarkDown (removes HTML/Javascript) to prevent XSS.
-bleach>=2.1.4
+# Pin >=6.0.0: bleach <3.3.0 had CVE-2021-23980 (ReDoS in linkifier URL regex);
+# bleach <6.0.0 had LinkifyFilter API differences that break sanitize_html.py.
+bleach>=6.0.0
 bleach-allowlist
 
 # codehilite.