diff --git a/HISTORY.md b/HISTORY.md
index 4fda387fbe..aec56d70ae 100644
--- a/HISTORY.md
+++ b/HISTORY.md
@@ -6,6 +6,27 @@ dev
 
 - \[Short description of non-trivial change.\]
 
+2.27.1.post4 (2026-11-05)
+-------------------------
+
+**Security hardening**
+
+- `no_proxy` entries now match whole domain labels only:
+  `no_proxy=example.com` still bypasses the proxy for `example.com` and
+  `sub.example.com`, but no longer for `badexample.com`. A leading dot on an
+  entry is ignored. Backport of requests #7427 (2.34.0), the port of CPython
+  bpo-39057. No CVE or GHSA. Python 2 compatible.
+
+**Bugfix**
+
+- `check_compatibility()` accepts urllib3 post-release versions such as the
+  ActiveState `1.26.20.post4` build. It used to raise ValueError on them,
+  which `import requests` turned into a `RequestsDependencyWarning`.
+
+This release uses the `X.Y.Z.postN` version scheme, replacing the
+`+security.N` local versions of earlier ActiveState releases (inventory
+2.27.1.4).
+
 2.27.1+security.3 (2026-06-12)
 -------------------------------
 
diff --git a/requests/__init__.py b/requests/__init__.py
index ab56c206d5..e1ebdf83e9 100644
--- a/requests/__init__.py
+++ b/requests/__init__.py
@@ -62,6 +62,11 @@ def check_compatibility(urllib3_version, chardet_version, charset_normalizer_ver
     urllib3_version = urllib3_version.split('+', 1)[0].split('.')
     assert urllib3_version != ['dev']  # Verify urllib3 isn't installed from git.
 
+    # Only major.minor.patch matter. Drop a PEP 440 post-release segment such
+    # as the ActiveState '1.26.20.post4' builds, which would otherwise break
+    # the three-way unpack below.
+    urllib3_version = urllib3_version[:3]
+
     # Sometimes, urllib3 only reports its version as 16.1.
     if len(urllib3_version) == 2:
         urllib3_version.append('0')
diff --git a/requests/__version__.py b/requests/__version__.py
index 0c956c86f7..eff6ce07b1 100644
--- a/requests/__version__.py
+++ b/requests/__version__.py
@@ -5,7 +5,7 @@
 __title__ = 'requests'
 __description__ = 'Python HTTP for Humans.'
 __url__ = 'https://requests.readthedocs.io'
-__version__ = '2.27.1+security.3'
+__version__ = '2.27.1.post4'
 __build__ = 0x022701
 __author__ = 'Kenneth Reitz'
 __author_email__ = 'me@kennethreitz.org'
diff --git a/requests/utils.py b/requests/utils.py
index 0ac98e6ace..ed5f062e1c 100644
--- a/requests/utils.py
+++ b/requests/utils.py
@@ -778,6 +778,13 @@ def should_bypass_proxies(url, no_proxy):
                 host_with_port += ':{}'.format(parsed.port)
 
             for host in no_proxy:
+                # Match whole domain labels only, so that "example.com" does
+                # not also bypass the proxy for "badexample.com". A leading
+                # dot on the entry is ignored. (CPython bpo-39057)
+                host = host.lstrip('.')
+                if parsed.hostname == host or host_with_port == host:
+                    return True
+                host = '.' + host
                 if parsed.hostname.endswith(host) or host_with_port.endswith(host):
                     # The URL does match something in no_proxy, so we don't want
                     # to apply the proxies on this URL.
diff --git a/tests/test_check_compatibility.py b/tests/test_check_compatibility.py
new file mode 100644
index 0000000000..c93df6a9bc
--- /dev/null
+++ b/tests/test_check_compatibility.py
@@ -0,0 +1,26 @@
+# -*- coding: utf-8 -*-
+"""check_compatibility must accept the version strings that ActiveState's
+urllib3 builds report: plain, PEP 440 local (+security.N) and post-release
+(.postN)."""
+
+import pytest
+
+from requests import check_compatibility
+
+
+@pytest.mark.parametrize('urllib3_version', (
+    '1.26.20',
+    '1.26',
+    '1.26.20+security.2',
+    '1.26.20.post3',
+    '1.26.20.post4',
+    '1.21.1',
+))
+def test_supported_urllib3_versions(urllib3_version):
+    check_compatibility(urllib3_version, '3.0.4', None)
+
+
+@pytest.mark.parametrize('urllib3_version', ('2.0.7', '1.20.0', '1.27.0.post1'))
+def test_unsupported_urllib3_versions(urllib3_version):
+    with pytest.raises(AssertionError):
+        check_compatibility(urllib3_version, '3.0.4', None)
diff --git a/tests/test_utils.py b/tests/test_utils.py
index 8e98397204..26b959a5a0 100644
--- a/tests/test_utils.py
+++ b/tests/test_utils.py
@@ -728,6 +728,28 @@ def test_should_bypass_proxies_no_proxy(
     assert should_bypass_proxies(url, no_proxy=no_proxy) == expected
 
 
+@pytest.mark.parametrize(
+    'url, expected', (
+            ('http://localhost/', True),
+            ('http://anotherdomain.com:8888/', True),
+            ('http://newdomain.com:1234/', True),
+            ('http://www.newdomain.com:1234/', True),
+            ('http://foo.d.o.t/', True),
+            ('http://d.o.t/', True),
+            ('http://prelocalhost/', False),
+            ('http://badanotherdomain.com/', False),
+            ('http://newdomain.com/', False),
+            ('http://newdomain.com:1235/', False),
+    ))
+def test_should_bypass_proxies_no_proxy_domain_boundary(url, expected):
+    """Ensure no_proxy matching respects domain boundaries and does not
+    greedily match domains that merely end with the no_proxy entry.
+    See CPython bpo-39057 and requests #7427.
+    """
+    no_proxy = 'localhost, anotherdomain.com, newdomain.com:1234, .d.o.t'
+    assert should_bypass_proxies(url, no_proxy=no_proxy) == expected
+
+
 @pytest.mark.skipif(os.name != 'nt', reason='Test only on Windows')
 @pytest.mark.parametrize(
     'url, expected, override', (