diff --git a/libs/langchain/langchain_classic/agents/mrkl/output_parser.py b/libs/langchain/langchain_classic/agents/mrkl/output_parser.py
index 1c08b68338f..4323dc32d25 100644
--- a/libs/langchain/langchain_classic/agents/mrkl/output_parser.py
+++ b/libs/langchain/langchain_classic/agents/mrkl/output_parser.py
@@ -41,9 +41,7 @@ class MRKLOutputParser(AgentOutputParser):
             OutputParserException: If the output could not be parsed.
         """
         includes_answer = FINAL_ANSWER_ACTION in text
-        regex = (
-            r"Action\s*\d*\s*:[\s]*(.*?)[\s]*Action\s*\d*\s*Input\s*\d*\s*:[\s]*(.*)"
-        )
+        regex = r"Action\s*\d*\s*:[\s]*(.*?)Action\s*\d*\s*Input\s*\d*\s*:[\s]*(.*)"
         action_match = re.search(regex, text, re.DOTALL)
         if action_match and includes_answer:
             if text.find(FINAL_ANSWER_ACTION) < text.find(action_match.group(0)):
diff --git a/libs/langchain/langchain_classic/agents/output_parsers/react_single_input.py b/libs/langchain/langchain_classic/agents/output_parsers/react_single_input.py
index ae7634f9636..a74cc65a164 100644
--- a/libs/langchain/langchain_classic/agents/output_parsers/react_single_input.py
+++ b/libs/langchain/langchain_classic/agents/output_parsers/react_single_input.py
@@ -52,9 +52,7 @@ class ReActSingleInputOutputParser(AgentOutputParser):
     @override
     def parse(self, text: str) -> AgentAction | AgentFinish:
         includes_answer = FINAL_ANSWER_ACTION in text
-        regex = (
-            r"Action\s*\d*\s*:[\s]*(.*?)[\s]*Action\s*\d*\s*Input\s*\d*\s*:[\s]*(.*)"
-        )
+        regex = r"Action\s*\d*\s*:[\s]*(.*?)Action\s*\d*\s*Input\s*\d*\s*:[\s]*(.*)"
         action_match = re.search(regex, text, re.DOTALL)
         if action_match:
             if includes_answer:
diff --git a/libs/langchain/tests/unit_tests/agents/output_parsers/test_react_single_input.py b/libs/langchain/tests/unit_tests/agents/output_parsers/test_react_single_input.py
index b8f3b612274..eed59a6cfa4 100644
--- a/libs/langchain/tests/unit_tests/agents/output_parsers/test_react_single_input.py
+++ b/libs/langchain/tests/unit_tests/agents/output_parsers/test_react_single_input.py
@@ -1,3 +1,6 @@
+import signal
+import sys
+
 import pytest
 from langchain_core.agents import AgentAction, AgentFinish
 from langchain_core.exceptions import OutputParserException
@@ -43,3 +46,32 @@ Action: search Final Answer:
 Action Input: what is the temperature in SF?"""
     with pytest.raises(OutputParserException):
         parser.invoke(_input)
+
+
+def _timeout_handler(_signum: int, _frame: object) -> None:
+    msg = "ReDoS: regex took too long"
+    raise TimeoutError(msg)
+
+
+@pytest.mark.skipif(
+    sys.platform == "win32", reason="SIGALRM is not available on Windows"
+)
+def test_react_single_input_no_redos() -> None:
+    """Regression test for ReDoS caused by catastrophic backtracking."""
+    parser = ReActSingleInputOutputParser()
+    malicious = "Action: " + " \t" * 1000 + "Action "
+    old = signal.signal(signal.SIGALRM, _timeout_handler)
+    signal.alarm(2)
+    try:
+        try:
+            parser.parse(malicious)
+        except OutputParserException:
+            pass
+        except TimeoutError:
+            pytest.fail(
+                "ReDoS detected: ReActSingleInputOutputParser.parse() "
+                "hung on crafted input"
+            )
+    finally:
+        signal.alarm(0)
+        signal.signal(signal.SIGALRM, old)
diff --git a/libs/langchain/tests/unit_tests/agents/test_mrkl_output_parser.py b/libs/langchain/tests/unit_tests/agents/test_mrkl_output_parser.py
index 9112a997d84..58731439dc8 100644
--- a/libs/langchain/tests/unit_tests/agents/test_mrkl_output_parser.py
+++ b/libs/langchain/tests/unit_tests/agents/test_mrkl_output_parser.py
@@ -1,3 +1,6 @@
+import signal
+import sys
+
 import pytest
 from langchain_core.agents import AgentAction, AgentFinish
 from langchain_core.exceptions import OutputParserException
@@ -79,3 +82,30 @@ def test_final_answer_after_parsable_action() -> None:
         "Parsing LLM output produced both a final answer and a parse-able action"
         in exception_info.value.args[0]
     )
+
+
+def _timeout_handler(_signum: int, _frame: object) -> None:
+    msg = "ReDoS: regex took too long"
+    raise TimeoutError(msg)
+
+
+@pytest.mark.skipif(
+    sys.platform == "win32", reason="SIGALRM is not available on Windows"
+)
+def test_mrkl_output_parser_no_redos() -> None:
+    """Regression test for ReDoS caused by catastrophic backtracking."""
+    malicious = "Action: " + " \t" * 1000 + "Action "
+    old = signal.signal(signal.SIGALRM, _timeout_handler)
+    signal.alarm(2)
+    try:
+        try:
+            mrkl_output_parser.parse(malicious)
+        except OutputParserException:
+            pass
+        except TimeoutError:
+            pytest.fail(
+                "ReDoS detected: MRKLOutputParser.parse() hung on crafted input"
+            )
+    finally:
+        signal.alarm(0)
+        signal.signal(signal.SIGALRM, old)