diff --git a/git/objects/commit.py b/git/objects/commit.py
@@ -10,7 +10,6 @@
 from io import BytesIO
 import logging
 import os
-import re
 from subprocess import Popen, PIPE
 from time import altzone, daylight, localtime, time, timezone
 import warnings
@@ -964,12 +963,21 @@ def co_authors(self) -> List[Actor]:
         co_authors = []
 
         if self.message:
-            results = re.findall(
-                r"^Co-authored-by: (.*) <(.*?)>$",
-                str(self.message),
-                re.MULTILINE,
-            )
-            for author in results:
-                co_authors.append(Actor(*author))
+            # Scan line by line instead of matching `(.*) <(.*?)>` across the whole
+            # message. On a single trailer line that repeats " <" without ever closing
+            # a ">", greedy backtracking over each " <" made the regex run in O(n^2)
+            # time, so a large (fully attacker-controlled) commit message could stall
+            # any caller of this property. A trailer is "Co-authored-by: <name> <email>"
+            # with the email in the final angle brackets, so the name ends at the last
+            # " <" and the line ends at ">".
+            prefix = "Co-authored-by: "
+            for line in str(self.message).split("\n"):
+                if not line.startswith(prefix) or not line.endswith(">"):
+                    continue
+                identity = line[len(prefix) :]
+                separator = identity.rfind(" <")
+                if separator == -1:
+                    continue
+                co_authors.append(Actor(identity[:separator], identity[separator + 2 : -1]))
 
         return co_authors
diff --git a/test/test_commit.py b/test/test_commit.py
@@ -590,6 +590,23 @@ def test_commit_co_authors(self):
             Actor("test_user_3", "test_user_3@github.com"),
         ]
 
+    def test_commit_co_authors_bounds_malformed_trailer(self):
+        """A malformed trailer line must not make co_authors run in quadratic time."""
+        commit = copy.copy(self.rorepo.commit("4251bd5"))
+        # An unterminated trailer repeating " <" without a closing ">". The old
+        # `(.*) <(.*?)>` regex backtracked over every " <" (O(n^2)); the crafted line
+        # is fully attacker-controlled through the commit message.
+        commit.message = (
+            "Subject\n\nCo-authored-by: " + ("a <" * 20_000) + "\nCo-authored-by: Real Name <real@example.com>"
+        )
+        start = time.process_time()
+        result = commit.co_authors
+        elapsed = time.process_time() - start
+        # Leave ample CPU time for slow runners, but catch quadratic backtracking.
+        self.assertLess(elapsed, 1.0)
+        # The malformed line yields nothing; the well-formed trailer still parses.
+        assert result == [Actor("Real Name", "real@example.com")]
+
     @with_rw_directory
     def test_create_from_tree_with_trailers_dict(self, rw_dir):
         """Test that create_from_tree supports adding trailers via a dict."""