nicholasserra · Oct 16, 2026 · Oct 14, 2026 · Oct 14, 2026
diff --git a/CHANGES.md b/CHANGES.md
@@ -2,6 +2,7 @@
 
 ## python-markdown2 2.5.6 (not yet released)
 
+- [pull #739] Fix `header-ids` extra putting sanitized HTML into header ids in safe mode, which allowed attribute injection (#649)
 - [pull #732] Fix excessive CPU use in the inline HTML tokenizer on repeated unclosed tag fragments (#707)
 - [pull #736] Clarify how to enable file variables in the Python API and command-line help (#269, #365).
 - [pull #730] Fix `tables` extra splitting multi-backtick code spans at a leading pipe and merging cells after a code span containing literal backticks.

diff --git a/lib/markdown2.py b/lib/markdown2.py
@@ -1743,8 +1743,11 @@ def _h_sub(self, match: re.Match[str]) -> str:
             n = min(n + demote_headers, 6)
         header_id_attr = ""
         if "header-ids" in self.extras:
+            # in safe mode, inline HTML is hashed by now. Unhash it so the
+            # sanitized HTML doesn't get swapped back into the id attribute
+            header_text = self._unhash_html_spans(header_group)
             header_id = self.header_id_from_text(
-                self._unescape_special_chars(header_group),
+                self._unescape_special_chars(header_text),
                 self.extras["header-ids"].get('prefix'), n)
             if header_id:
                 header_id_attr = ' id="%s"' % header_id

diff --git a/test/tm-cases/header_ids_safe_mode.html b/test/tm-cases/header_ids_safe_mode.html
@@ -0,0 +1,7 @@
+<h1 id="hello-bworldb">Hello &lt;b&gt;world&lt;/b&gt;</h1>
+
+<h1 id="sometext">&lt;sometext&gt;</h1>
+
+<h1 id="setext-emheaderem">Setext &lt;em&gt;header&lt;/em&gt;</h1>
+
+<h1 id="hi-b-xa-onmouseoveralert1thereb">Hi &lt;b x="a" onmouseover="alert(1)"&gt;there&lt;/b&gt;</h1>
diff --git a/test/tm-cases/header_ids_safe_mode.opts b/test/tm-cases/header_ids_safe_mode.opts
@@ -0,0 +1 @@
+{'safe_mode': 'escape', 'extras': ['header-ids']}
diff --git a/test/tm-cases/header_ids_safe_mode.text b/test/tm-cases/header_ids_safe_mode.text
@@ -0,0 +1,8 @@
+# Hello <b>world</b>
+
+# <sometext>
+
+Setext <em>header</em>
+======================
+
+# Hi <b x="a" onmouseover="alert(1)">there</b>
diff --git a/test/tm-cases/header_ids_safe_mode_replace.html b/test/tm-cases/header_ids_safe_mode_replace.html
@@ -0,0 +1,7 @@
+<h1 id="hello-htmlworldhtml">Hello [HTML_REMOVED]world[HTML_REMOVED]</h1>
+
+<h1 id="html">[HTML_REMOVED]</h1>
+
+<h1 id="setext-htmlheaderhtml">Setext [HTML_REMOVED]header[HTML_REMOVED]</h1>
+
+<h1 id="hi-htmltherehtml">Hi [HTML_REMOVED]there[HTML_REMOVED]</h1>
diff --git a/test/tm-cases/header_ids_safe_mode_replace.opts b/test/tm-cases/header_ids_safe_mode_replace.opts
@@ -0,0 +1 @@
+{'safe_mode': 'replace', 'extras': ['header-ids']}
diff --git a/test/tm-cases/header_ids_safe_mode_replace.text b/test/tm-cases/header_ids_safe_mode_replace.text
@@ -0,0 +1,8 @@
+# Hello <b>world</b>
+
+# <sometext>
+
+Setext <em>header</em>
+======================
+
+# Hi <b x="a" onmouseover="alert(1)">there</b>
Original file line number	Diff line number	Diff line change
		@@ -0,0 +1 @@
		{'safe_mode': 'escape', 'extras': ['header-ids']}
Original file line number	Diff line number	Diff line change
		@@ -0,0 +1 @@
		{'safe_mode': 'replace', 'extras': ['header-ids']}