From ba2cab6fc41ced55cc4630ebcc65fb6154d9dca6 Mon Sep 17 00:00:00 2001
From: Waylan Limberg <waylan.limberg@icloud.com>
Date: Wed, 18 Jun 2025 09:55:49 -0400
Subject: [PATCH 1/2] Ensure incomplete markup declaration in raw HTML doesn't
 crash parser.

See Python bug report at gh-77057 for details. Until we drop support for
Python < 3.13 (where this was fixed upstream), we need to avoid the
unwanted error by checking for it explicitly. Fixes #1534.
---
 docs/changelog.md                            | 1 +
 markdown/extensions/md_in_html.py            | 4 ++++
 markdown/htmlparser.py                       | 4 ++++
 tests/test_syntax/blocks/test_html_blocks.py | 7 +++++++
 4 files changed, 16 insertions(+)

diff --git a/docs/changelog.md b/docs/changelog.md
index 9f972a35..0b00d9df 100644
--- a/docs/changelog.md
+++ b/docs/changelog.md
@@ -13,6 +13,7 @@ See the [Contributing Guide](contributing.md) for details.
 
 ### Fixed
 
+* Ensure incomplete markup declaration in raw HTML doesn't crash parser (#1534).
 * Fixed dropped content in `md_in_html` (#1526).
 * Fixed HTML handling corner case that prevented some content from not being rendered (#1528).
 
diff --git a/markdown/extensions/md_in_html.py b/markdown/extensions/md_in_html.py
index ba73c942..c8c02081 100644
--- a/markdown/extensions/md_in_html.py
+++ b/markdown/extensions/md_in_html.py
@@ -280,6 +280,10 @@ def parse_pi(self, i: int) -> int:
 
     def parse_html_declaration(self, i: int) -> int:
         if self.at_line_start() or self.intail or self.mdstack:
+            if self.rawdata[i:i+3] == '<![' and not self.rawdata[i:i+9] == '<![CDATA[':
+                # We have encountered the bug in #1534 (Python bug gh-77057).
+                # Provide an override until we drop support for Python < 3.13.
+                return self.parse_bogus_comment(i)
             # The same override exists in `HTMLExtractor` without the check
             # for `mdstack`. Therefore, use parent of `HTMLExtractor` instead.
             return super(HTMLExtractor, self).parse_html_declaration(i)
diff --git a/markdown/htmlparser.py b/markdown/htmlparser.py
index 1528325c..86462782 100644
--- a/markdown/htmlparser.py
+++ b/markdown/htmlparser.py
@@ -275,6 +275,10 @@ def parse_pi(self, i: int) -> int:
 
     def parse_html_declaration(self, i: int) -> int:
         if self.at_line_start() or self.intail:
+            if self.rawdata[i:i+3] == '<![' and not self.rawdata[i:i+9] == '<![CDATA[':
+                # We have encountered the bug in #1534 (Python bug gh-77057).
+                # Provide an override until we drop support for Python < 3.13.
+                return self.parse_bogus_comment(i)
             return super().parse_html_declaration(i)
         # This is not the beginning of a raw block so treat as plain data
         # and avoid consuming any tags which may follow (see #1066).
diff --git a/tests/test_syntax/blocks/test_html_blocks.py b/tests/test_syntax/blocks/test_html_blocks.py
index 8ea3904c..18dc1145 100644
--- a/tests/test_syntax/blocks/test_html_blocks.py
+++ b/tests/test_syntax/blocks/test_html_blocks.py
@@ -1275,6 +1275,13 @@ def test_raw_cdata_indented(self):
             )
         )
 
+    def test_not_actually_cdata(self):
+        # Ensure bug reported in #1534 is avoided.
+        self.assertMarkdownRenders(
+            '<![',
+            '<p>&lt;![</p>'
+        )
+
     def test_raw_cdata_code_span(self):
         self.assertMarkdownRenders(
             self.dedent(

From 653890c9d9fcece6c5a4a7e601eabbd8dd733173 Mon Sep 17 00:00:00 2001
From: Waylan Limberg <waylan.limberg@icloud.com>
Date: Wed, 18 Jun 2025 10:07:16 -0400
Subject: [PATCH 2/2] cleanup

---
 markdown/extensions/md_in_html.py | 2 +-
 markdown/htmlparser.py            | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/markdown/extensions/md_in_html.py b/markdown/extensions/md_in_html.py
index c8c02081..5256e904 100644
--- a/markdown/extensions/md_in_html.py
+++ b/markdown/extensions/md_in_html.py
@@ -281,7 +281,7 @@ def parse_pi(self, i: int) -> int:
     def parse_html_declaration(self, i: int) -> int:
         if self.at_line_start() or self.intail or self.mdstack:
             if self.rawdata[i:i+3] == '<![' and not self.rawdata[i:i+9] == '<![CDATA[':
-                # We have encountered the bug in #1534 (Python bug gh-77057).
+                # We have encountered the bug in #1534 (Python bug `gh-77057`).
                 # Provide an override until we drop support for Python < 3.13.
                 return self.parse_bogus_comment(i)
             # The same override exists in `HTMLExtractor` without the check
diff --git a/markdown/htmlparser.py b/markdown/htmlparser.py
index 86462782..b357e708 100644
--- a/markdown/htmlparser.py
+++ b/markdown/htmlparser.py
@@ -276,7 +276,7 @@ def parse_pi(self, i: int) -> int:
     def parse_html_declaration(self, i: int) -> int:
         if self.at_line_start() or self.intail:
             if self.rawdata[i:i+3] == '<![' and not self.rawdata[i:i+9] == '<![CDATA[':
-                # We have encountered the bug in #1534 (Python bug gh-77057).
+                # We have encountered the bug in #1534 (Python bug `gh-77057`).
                 # Provide an override until we drop support for Python < 3.13.
                 return self.parse_bogus_comment(i)
             return super().parse_html_declaration(i)