diff --git a/Doc/library/html.parser.rst b/Doc/library/html.parser.rst
index dd67fc34e856f1..b2fa043625c7e1 100644
--- a/Doc/library/html.parser.rst
+++ b/Doc/library/html.parser.rst
@@ -121,6 +121,17 @@ The output will then be:
    attributes can be preserved, etc.).
 
 
+.. method:: HTMLParser.support_cdata(flag)
+
+   Sets how the parser will parse CDATA declarations.
+   If *flag* is true, then the :meth:`unknown_decl` method will be called
+   for the CDATA section ``<![CDATA[...]]>``.
+   If *flag* is false, then the :meth:`handle_comment` method will be called
+   for ``<![CDATA[...>``.
+
+   .. versionadded:: 3.13.6
+
+
 The following methods are called when data or markup elements are encountered
 and they are meant to be overridden in a subclass.  The base class
 implementations do nothing (except for :meth:`~HTMLParser.handle_startendtag`):
diff --git a/Lib/html/parser.py b/Lib/html/parser.py
index cc15de07b5bae6..88a084dcf1ce7d 100644
--- a/Lib/html/parser.py
+++ b/Lib/html/parser.py
@@ -144,6 +144,7 @@ def reset(self):
         self.lasttag = '???'
         self.interesting = interesting_normal
         self.cdata_elem = None
+        self._support_cdata = False
         super().reset()
 
     def feed(self, data):
@@ -174,6 +175,9 @@ def clear_cdata_mode(self):
         self.interesting = interesting_normal
         self.cdata_elem = None
 
+    def support_cdata(self, flag=True):
+        self._support_cdata = flag
+
     # Internal -- handle data as far as reasonable.  May leave state
     # and data to be processed by a subsequent call.  If 'end' is
     # true, force handling all data as if followed by EOF marker.
@@ -249,7 +253,10 @@ def goahead(self, end):
                                 break
                         self.handle_comment(rawdata[i+4:j])
                     elif startswith("<![CDATA[", i):
-                        self.unknown_decl(rawdata[i+3:])
+                        if self._support_cdata:
+                            self.unknown_decl(rawdata[i+3:])
+                        else:
+                            self.handle_comment(rawdata[i+1:])
                     elif rawdata[i:i+9].lower() == '<!doctype':
                         self.handle_decl(rawdata[i+2:])
                     elif startswith("<!", i):
@@ -325,7 +332,14 @@ def parse_html_declaration(self, i):
             # this case is actually already handled in goahead()
             return self.parse_comment(i)
         elif rawdata[i:i+9] == '<![CDATA[':
-            return self.parse_marked_section(i)
+            if self._support_cdata:
+                j = rawdata.find(']]>', i+9)
+                if j < 0:
+                    return -1
+                self.unknown_decl(rawdata[i+3: j])
+                return j + 3
+            else:
+                return self.parse_bogus_comment(i)
         elif rawdata[i:i+9].lower() == '<!doctype':
             # find the closing >
             gtpos = rawdata.find('>', i+9)
diff --git a/Lib/test/test_htmlparser.py b/Lib/test/test_htmlparser.py
index d0d2c54217ccaf..65fbf5d7b618fd 100644
--- a/Lib/test/test_htmlparser.py
+++ b/Lib/test/test_htmlparser.py
@@ -34,12 +34,16 @@ def get_events(self):
 
     def handle_starttag(self, tag, attrs):
         self.append(("starttag", tag, attrs))
+        if tag == 'svg':
+            self.support_cdata(True)
 
     def handle_startendtag(self, tag, attrs):
         self.append(("startendtag", tag, attrs))
 
     def handle_endtag(self, tag):
         self.append(("endtag", tag))
+        if tag == 'svg':
+            self.support_cdata(False)
 
     # all other markup
 
@@ -643,10 +647,22 @@ def test_eof_in_declarations(self):
             ('<!', [('comment', '')]),
             ('<!-', [('comment', '-')]),
             ('<![', [('comment', '[')]),
-            ('<![CDATA[', [('unknown decl', 'CDATA[')]),
-            ('<![CDATA[x', [('unknown decl', 'CDATA[x')]),
-            ('<![CDATA[x]', [('unknown decl', 'CDATA[x]')]),
-            ('<![CDATA[x]]', [('unknown decl', 'CDATA[x]]')]),
+            ('<![CDATA[', [('comment', '![CDATA[')]),
+            ('<![CDATA[x', [('comment', '![CDATA[x')]),
+            ('<![CDATA[x]', [('comment', '![CDATA[x]')]),
+            ('<![CDATA[x]]', [('comment', '![CDATA[x]]')]),
+            ('<svg><text y="100"><![CDATA[',
+             [('starttag', 'svg', []), ('starttag', 'text', [('y', '100')]),
+              ('unknown decl', 'CDATA[')]),
+            ('<svg><text y="100"><![CDATA[x',
+             [('starttag', 'svg', []), ('starttag', 'text', [('y', '100')]),
+              ('unknown decl', 'CDATA[x')]),
+            ('<svg><text y="100"><![CDATA[x]',
+             [('starttag', 'svg', []), ('starttag', 'text', [('y', '100')]),
+              ('unknown decl', 'CDATA[x]')]),
+            ('<svg><text y="100"><![CDATA[x]]',
+             [('starttag', 'svg', []), ('starttag', 'text', [('y', '100')]),
+              ('unknown decl', 'CDATA[x]]')]),
             ('<!DOCTYPE', [('decl', 'DOCTYPE')]),
             ('<!DOCTYPE ', [('decl', 'DOCTYPE ')]),
             ('<!DOCTYPE html', [('decl', 'DOCTYPE html')]),
@@ -721,26 +737,50 @@ def test_broken_condcoms(self):
         ]
         self._run_check(html, expected)
 
-    def test_cdata_declarations(self):
-        # More tests should be added. See also "8.2.4.42. Markup
-        # declaration open state", "8.2.4.69. CDATA section state",
-        # and issue 32876
-        html = ('<![CDATA[just some plain text]]>')
-        expected = [('unknown decl', 'CDATA[just some plain text')]
+    @support.subTests('content', [
+        'just some plain text',
+        '<!-- not a comment -->',
+        '&not-an-entity-ref;',
+        "<not a='start tag'>",
+        '',
+        '[[I have many brackets]]',
+        'I have a > in the middle',
+        'I have a ]] in the middle',
+        '] ]>',
+        ']] >',
+        ('\n'
+         '    if (a < b && a > b) {\n'
+         '        printf("[<marquee>How?</marquee>]");\n'
+         '    }\n'),
+    ])
+    def test_cdata_section_content(self, content):
+        # See "13.2.5.42 Markup declaration open state",
+        # "13.2.5.69 CDATA section state", and issue bpo-32876.
+        html = f'<svg><text y="100"><![CDATA[{content}]]></text></svg>'
+        expected = [
+            ('starttag', 'svg', []),
+            ('starttag', 'text', [('y', '100')]),
+            ('unknown decl', 'CDATA[' + content),
+            ('endtag', 'text'),
+            ('endtag', 'svg'),
+        ]
         self._run_check(html, expected)
 
-    def test_cdata_declarations_multiline(self):
-        html = ('<code><![CDATA['
-                '    if (a < b && a > b) {'
-                '        printf("[<marquee>How?</marquee>]");'
-                '    }'
-                ']]></code>')
+    def test_cdata_section(self):
+        # See "13.2.5.42 Markup declaration open state".
+        html = ('<![CDATA[foo<br>bar]]>'
+                '<svg><text y="100"><![CDATA[foo<br>bar]]></text></svg>'
+                '<![CDATA[foo<br>bar]]>')
         expected = [
-            ('starttag', 'code', []),
-            ('unknown decl',
-             'CDATA[    if (a < b && a > b) {        '
-             'printf("[<marquee>How?</marquee>]");    }'),
-            ('endtag', 'code')
+            ('comment', '[CDATA[foo<br'),
+            ('data', 'bar]]>'),
+            ('starttag', 'svg', []),
+            ('starttag', 'text', [('y', '100')]),
+            ('unknown decl', 'CDATA[foo<br>bar'),
+            ('endtag', 'text'),
+            ('endtag', 'svg'),
+            ('comment', '[CDATA[foo<br'),
+            ('data', 'bar]]>'),
         ]
         self._run_check(html, expected)
 
diff --git a/Misc/NEWS.d/next/Security/2025-06-18-13-34-55.gh-issue-135661.NZlpWf.rst b/Misc/NEWS.d/next/Security/2025-06-18-13-34-55.gh-issue-135661.NZlpWf.rst
new file mode 100644
index 00000000000000..59c76d50f79443
--- /dev/null
+++ b/Misc/NEWS.d/next/Security/2025-06-18-13-34-55.gh-issue-135661.NZlpWf.rst
@@ -0,0 +1,2 @@
+Fix CDATA section parsing in :class:`html.parser.HTMLParser` according to
+the HTML5 standard: ``] ]>`` and ``]] >`` no longer end the CDATA section.