imaplib: fix CRAM-MD5 on FIPS-only environments

Author: picnixz

Lib/imaplib.py

@@ -725,9 +725,17 @@ def login_cram_md5(self, user, password):
     def _CRAM_MD5_AUTH(self, challenge):
         """ Authobject to use with CRAM-MD5 authentication. """
         import hmac
-        pwd = (self.password.encode('utf-8') if isinstance(self.password, str)
-                                             else self.password)
-        return self.user + " " + hmac.HMAC(pwd, challenge, 'md5').hexdigest()
+
+        if isinstance(self.password, str):
+            password = self.password.encode('utf-8')
+        else:
+            password = self.password
+
+        try:
+            authcode = hmac.HMAC(password, challenge, 'md5')
+        except ValueError:  # HMAC-MD5 is not available
+            raise self.error("CRAM-MD5 authentication is not supported")
+        return f"{self.user} {authcode.hexdigest()}"
 
 
     def logout(self):

Lib/test/test_imaplib.py

@@ -12,8 +12,7 @@
 import socket
 
 from test.support import verbose, run_with_tz, run_with_locale, cpython_only
-from test.support import hashlib_helper
-from test.support import threading_helper
+from test.support import hashlib_helper, threading_helper
 import unittest
 from unittest import mock
 from datetime import datetime, timezone, timedelta
@@ -256,7 +255,20 @@ def cmd_IDLE(self, tag, args):
             self._send_tagged(tag, 'BAD', 'Expected DONE')
 
 
-class NewIMAPTestsMixin():
+class AuthHandler_CRAM_MD5(SimpleIMAPHandler):
+    capabilities = 'LOGINDISABLED AUTH=CRAM-MD5'
+    def cmd_AUTHENTICATE(self, tag, args):
+        self._send_textline('+ PDE4OTYuNjk3MTcwOTUyQHBvc3RvZmZpY2Uucm'
+                            'VzdG9uLm1jaS5uZXQ=')
+        r = yield
+        if (r == b'dGltIGYxY2E2YmU0NjRiOWVmYT'
+                 b'FjY2E2ZmZkNmNmMmQ5ZjMy\r\n'):
+            self._send_tagged(tag, 'OK', 'CRAM-MD5 successful')
+        else:
+            self._send_tagged(tag, 'NO', 'No access')
+
+
+class NewIMAPTestsMixin:
     client = None
 
     def _setup(self, imap_handler, connect=True):
@@ -439,40 +451,26 @@ def cmd_AUTHENTICATE(self, tag, args):
 
     @hashlib_helper.requires_hashdigest('md5', openssl=True)
     def test_login_cram_md5_bytes(self):
-        class AuthHandler(SimpleIMAPHandler):
-            capabilities = 'LOGINDISABLED AUTH=CRAM-MD5'
-            def cmd_AUTHENTICATE(self, tag, args):
-                self._send_textline('+ PDE4OTYuNjk3MTcwOTUyQHBvc3RvZmZpY2Uucm'
-                                    'VzdG9uLm1jaS5uZXQ=')
-                r = yield
-                if (r == b'dGltIGYxY2E2YmU0NjRiOWVmYT'
-                         b'FjY2E2ZmZkNmNmMmQ5ZjMy\r\n'):
-                    self._send_tagged(tag, 'OK', 'CRAM-MD5 successful')
-                else:
-                    self._send_tagged(tag, 'NO', 'No access')
-        client, _ = self._setup(AuthHandler)
-        self.assertTrue('AUTH=CRAM-MD5' in client.capabilities)
+        client, _ = self._setup(AuthHandler_CRAM_MD5)
+        self.assertIn('AUTH=CRAM-MD5', client.capabilities)
         ret, _ = client.login_cram_md5("tim", b"tanstaaftanstaaf")
         self.assertEqual(ret, "OK")
 
     @hashlib_helper.requires_hashdigest('md5', openssl=True)
     def test_login_cram_md5_plain_text(self):
-        class AuthHandler(SimpleIMAPHandler):
-            capabilities = 'LOGINDISABLED AUTH=CRAM-MD5'
-            def cmd_AUTHENTICATE(self, tag, args):
-                self._send_textline('+ PDE4OTYuNjk3MTcwOTUyQHBvc3RvZmZpY2Uucm'
-                                    'VzdG9uLm1jaS5uZXQ=')
-                r = yield
-                if (r == b'dGltIGYxY2E2YmU0NjRiOWVmYT'
-                         b'FjY2E2ZmZkNmNmMmQ5ZjMy\r\n'):
-                    self._send_tagged(tag, 'OK', 'CRAM-MD5 successful')
-                else:
-                    self._send_tagged(tag, 'NO', 'No access')
-        client, _ = self._setup(AuthHandler)
-        self.assertTrue('AUTH=CRAM-MD5' in client.capabilities)
+        client, _ = self._setup(AuthHandler_CRAM_MD5)
+        self.assertIn('AUTH=CRAM-MD5', client.capabilities)
         ret, _ = client.login_cram_md5("tim", "tanstaaftanstaaf")
         self.assertEqual(ret, "OK")
 
+    @hashlib_helper.block_algorithm("md5")
+    def test_login_cram_md5_blocked(self):
+        client, _ = self._setup(AuthHandler_CRAM_MD5)
+        self.assertIn('AUTH=CRAM-MD5', client.capabilities)
+        msg = re.escape("Server does not support CRAM-MD5 authentication")
+        with self.assertRaisesRegex(imaplib.IMAP4.error, msg):
+            client.login_cram_md5("tim", b"tanstaaftanstaaf")
+
     def test_aborted_authentication(self):
         class MyServer(SimpleIMAPHandler):
             def cmd_AUTHENTICATE(self, tag, args):

Misc/NEWS.d/next/Library/2025-07-13-11-20-05.gh-issue-136134.xhh0Kq.rst

@@ -0,0 +1,2 @@
+:mod:`imaplib`: raise an error if CRAM-MD5 authentication is not supported.
+Patch by Bénédikt Tran.