From 9a27d77bfa7a726d504b173aa9ce6b2219223369 Mon Sep 17 00:00:00 2001
From: Rupayon Haldar <80724680+rupayon123@users.noreply.github.com>
Date: Sun, 4 Oct 2026 08:50:41 -0400
Subject: [PATCH] fix: stop parsing when plugin rules do not advance

---
 markdown_it/parser_block.py   |  2 ++
 markdown_it/parser_inline.py  |  5 +++++
 tests/test_parser_progress.py | 31 +++++++++++++++++++++++++++++++
 3 files changed, 38 insertions(+)
 create mode 100644 tests/test_parser_progress.py

diff --git a/markdown_it/parser_block.py b/markdown_it/parser_block.py
index 50a7184c..c6de5c30 100644
--- a/markdown_it/parser_block.py
+++ b/markdown_it/parser_block.py
@@ -85,6 +85,8 @@ def tokenize(self, state: StateBlock, startLine: int, endLine: int) -> None:
             # - return True
             for rule in rules:
                 if rule(state, line, endLine, False):
+                    if state.line <= line:
+                        raise RuntimeError("block rule didn't increment state.line")
                     break
 
             # set state.tight if we had an empty line before current tag
diff --git a/markdown_it/parser_inline.py b/markdown_it/parser_inline.py
index af66a7fa..a7b07360 100644
--- a/markdown_it/parser_inline.py
+++ b/markdown_it/parser_inline.py
@@ -152,6 +152,8 @@ def skipToken(self, state: StateInline) -> None:
                 ok = rule(state, True)
                 state.level -= 1
                 if ok:
+                    if state.pos <= pos:
+                        raise RuntimeError("inline rule didn't increment state.pos")
                     break
         else:
             # Too much nesting, just skip until the end of the paragraph.
@@ -179,6 +181,7 @@ def tokenize(self, state: StateInline) -> None:
         maxNesting = state.md.options["maxNesting"]
 
         while state.pos < end:
+            pos = state.pos
             # Try all possible rules.
             # On success, rule should:
             #
@@ -190,6 +193,8 @@ def tokenize(self, state: StateInline) -> None:
                 for rule in rules:
                     ok = rule(state, False)
                     if ok:
+                        if state.pos <= pos:
+                            raise RuntimeError("inline rule didn't increment state.pos")
                         break
 
             if ok:
diff --git a/tests/test_parser_progress.py b/tests/test_parser_progress.py
new file mode 100644
index 00000000..df4b7bbb
--- /dev/null
+++ b/tests/test_parser_progress.py
@@ -0,0 +1,31 @@
+"""Regression tests for custom rules that claim a match without consuming input."""
+
+import pytest
+
+from markdown_it import MarkdownIt
+from markdown_it.rules_inline.state_inline import StateInline
+
+
+def test_block_rule_must_advance_line() -> None:
+    md = MarkdownIt("commonmark")
+    md.block.ruler.before("paragraph", "stuck", lambda state, start, end, silent: True)
+
+    with pytest.raises(RuntimeError, match=r"block rule didn't increment state\.line"):
+        md.parse("text\n")
+
+
+def test_inline_rule_must_advance_position() -> None:
+    md = MarkdownIt("commonmark")
+    md.inline.ruler.before("text", "stuck", lambda state, silent: True)
+
+    with pytest.raises(RuntimeError, match=r"inline rule didn't increment state\.pos"):
+        md.parse("text")
+
+
+def test_inline_validation_rule_must_advance_position() -> None:
+    md = MarkdownIt("commonmark")
+    md.inline.ruler.before("text", "stuck", lambda state, silent: True)
+    state = StateInline("text", md, {}, [])
+
+    with pytest.raises(RuntimeError, match=r"inline rule didn't increment state\.pos"):
+        md.inline.skipToken(state)