Make posix.putenv() raise ValueError if name contains an '=' character.

diff --git a/Doc/library/os.rst b/Doc/library/os.rst
--- a/Doc/library/os.rst
+++ b/Doc/library/os.rst
@@ -253,7 +253,8 @@ process and user.
 
    Set the environment variable named *varname* to the string *value*.  Such
    changes to the environment affect subprocesses started with :func:`os.system`,
-   :func:`popen` or :func:`fork` and :func:`execv`.
+   :func:`popen` or :func:`fork` and :func:`execv`.  Raises :exc:`ValueError`
+   if *varname* contains an ``'='`` character.
 
    Availability: most flavors of Unix, Windows.
 
diff --git a/Lib/test/test_os.py b/Lib/test/test_os.py
--- a/Lib/test/test_os.py
+++ b/Lib/test/test_os.py
@@ -345,6 +345,14 @@ class EnvironTests(mapping_tests.BasicTe
             value = os.popen("/bin/sh -c 'echo $HELLO'").read().strip()
             self.assertEquals(value, "World")
 
+    def test_bad_keys(self):
+        # Issue #4926
+        os.environ.clear()
+        self.assertRaises(ValueError, os.environ.__setitem__,
+                          "PYTHONTESTVAROS=SOMETHING", "value")
+        self.assertTrue("PYTHONTESTVAROS=SOMETHING" not in os.environ)
+        self.assertTrue("PYTHONTESTVAROS" not in os.environ)
+
 class WalkTests(unittest.TestCase):
     """Tests for os.walk()."""
 
diff --git a/Lib/test/test_posix.py b/Lib/test/test_posix.py
--- a/Lib/test/test_posix.py
+++ b/Lib/test/test_posix.py
@@ -320,6 +320,27 @@ class PosixTester(unittest.TestCase):
             if hasattr(st, 'st_flags'):
                 posix.lchflags(test_support.TESTFN, st.st_flags)
 
+    def test_putenv(self):
+        if not hasattr(posix, 'putenv'):
+            return
+        # XXX: we can't read the values back, as there's no wrapper
+        # for getenv() - os.getenv() reads the value out of
+        # os.environ, which, like posix.environ, doesn't reflect
+        # changes made with putenv() or unsetenv().
+        try:
+            posix.putenv("PYTHONTESTVARA", "value1")
+            posix.putenv("PYTHONTESTVARA", "value2")
+            if hasattr(posix, 'unsetenv'):
+                posix.unsetenv("PYTHONTESTVARA")
+                posix.putenv("PYTHONTESTVARA", "value3")
+                posix.putenv("PYTHONTESTVARA", "value4")
+        finally:
+            if hasattr(posix, 'unsetenv'):
+                posix.unsetenv("PYTHONTESTVARA")
+        # See issue #4926
+        self.assertRaises(ValueError, posix.putenv,
+                          "PYTHONTESTVARB=SOMETHING", "value")
+
     def test_getcwd_long_pathnames(self):
         if hasattr(posix, 'getcwd'):
             dirname = 'getcwd-test-directory-0123456789abcdef-01234567890abcdef'
diff --git a/Modules/posixmodule.c b/Modules/posixmodule.c
--- a/Modules/posixmodule.c
+++ b/Modules/posixmodule.c
@@ -6868,6 +6868,13 @@ posix_putenv(PyObject *self, PyObject *a
     if (!PyArg_ParseTuple(args, "ss:putenv", &s1, &s2))
         return NULL;
 
+    /* An '=' character would be interpreted as the end of the name. */
+    if (strchr(s1, '=') != NULL) {
+        PyErr_SetString(PyExc_ValueError, "environment variable name must "
+                        "not contain the '=' character");
+        return NULL;
+    }
+
 #if defined(PYOS_OS2)
     if (stricmp(s1, "BEGINLIBPATH") == 0) {
         APIRET rc;