gh-143927: Normalize all line endings (CR, CRLF, and LF) in configparser (GH-143929)

sethmlarson · miss-islington · commit 6c29d2ac15ca · 2026-06-23T13:34:21.000Z
(cherry picked from commit 5858e42) Co-authored-by: Seth Larson <seth@python.org>
diff --git a/Lib/configparser.py b/Lib/configparser.py
@@ -945,7 +945,9 @@ def _write_section(self, fp, section_name, section_items, delimiter):
             value = self._interpolation.before_write(self, section_name, key,
                                                      value)
             if value is not None or not self._allow_no_value:
-                value = delimiter + str(value).replace('\n', '\n\t')
+                # Convert all possible line-endings into '\n\t'
+                value = (delimiter + str(value).replace('\r\n', '\n')
+                         .replace('\r', '\n').replace('\n', '\n\t'))
             else:
                 value = ""
             fp.write("{}{}\n".format(key, value))
diff --git a/Lib/test/test_configparser.py b/Lib/test/test_configparser.py
@@ -528,6 +528,17 @@ def test_default_case_sensitivity(self):
             cf.get(self.default_section, "Foo"), "Bar",
             "could not locate option, expecting case-insensitive defaults")
 
+    def test_crlf_normalization(self):
+        cf = self.newconfig({"key1": "a\nb","key2": "a\rb", "key3": "a\r\nb", "key4": "a\r\nb"})
+        buf = io.StringIO()
+        cf.write(buf)
+        cf_str = buf.getvalue()
+        self.assertNotIn("\r", cf_str)
+        self.assertNotIn("\r\n", cf_str)
+        self.assertEqual(cf_str.count("\n"), 10)
+        self.assertEqual(cf_str.count("\n\t"), 4)
+        self.assertTrue(cf_str.endswith("\n\n"))
+
     def test_parse_errors(self):
         cf = self.newconfig()
         self.parse_error(cf, configparser.ParsingError,
diff --git a/Misc/NEWS.d/next/Security/2026-01-16-11-58-19.gh-issue-143927.aviFeG.rst b/Misc/NEWS.d/next/Security/2026-01-16-11-58-19.gh-issue-143927.aviFeG.rst
@@ -0,0 +1,2 @@
+Normalize all line endings (CR, CRLF, and LF) to LF+TAB when writing
+multi-line configparser values.

Original file line number	Diff line number	Diff line change
`@@ -0,0 +1,2 @@`
	`1`	`+Normalize all line endings (CR, CRLF, and LF) to LF+TAB when writing`
	`2`	`+multi-line configparser values.`