diff --git a/Doc/library/binascii.rst b/Doc/library/binascii.rst
@@ -50,11 +50,23 @@ The :mod:`binascii` module defines the following functions:
       Added the *backtick* parameter.
 
 
-.. function:: a2b_base64(string)
+.. function:: a2b_base64(string, strict_mode=False)
 
    Convert a block of base64 data back to binary and return the binary data. More
    than one line may be passed at a time.
 
+   If *strict_mode* is true, only valid base64 data will be converted. Invalid base64
+   data will raise :exc:`binascii.Error`.
+
+   Valid base64:
+      * Conforms to :rfc:`3548`.
+      * Contains only characters from the base64 alphabet.
+      * Contains no excess data after padding (including excess padding, newlines, etc.).
+      * Does not start with a padding.
+
+   .. versionchanged:: 3.11
+      Added the *strict_mode* parameter.
+
 
 .. function:: b2a_base64(data, *, newline=True)
 

diff --git a/Lib/test/test_binascii.py b/Lib/test/test_binascii.py
@@ -114,6 +114,47 @@ def addnoise(line):
         # empty strings. TBD: shouldn't it raise an exception instead ?
         self.assertEqual(binascii.a2b_base64(self.type2test(fillers)), b'')
 
+    def test_base64_strict_mode(self):
+        # Test base64 with strict mode on
+        def _assertRegexTemplate(assert_regex: str, data: bytes, non_strict_mode_expected_result: bytes):
+            with self.assertRaisesRegex(binascii.Error, assert_regex):
+                binascii.a2b_base64(self.type2test(data), strict_mode=True)
+            self.assertEqual(binascii.a2b_base64(self.type2test(data), strict_mode=False),
+                             non_strict_mode_expected_result)
+            self.assertEqual(binascii.a2b_base64(self.type2test(data)),
+                             non_strict_mode_expected_result)
+
+        def assertExcessData(data, non_strict_mode_expected_result: bytes):
+            _assertRegexTemplate(r'(?i)Excess data', data, non_strict_mode_expected_result)
+
+        def assertNonBase64Data(data, non_strict_mode_expected_result: bytes):
+            _assertRegexTemplate(r'(?i)Only base64 data', data, non_strict_mode_expected_result)
+
+        def assertMalformedPadding(data, non_strict_mode_expected_result: bytes):
+            _assertRegexTemplate(r'(?i)Leading padding', data, non_strict_mode_expected_result)
+
+        # Test excess data exceptions
+        assertExcessData(b'ab==a', b'i')
+        assertExcessData(b'ab===', b'i')
+        assertExcessData(b'ab==:', b'i')
+        assertExcessData(b'abc=a', b'i\xb7')
+        assertExcessData(b'abc=:', b'i\xb7')
+        assertExcessData(b'ab==\n', b'i')
+
+        # Test non-base64 data exceptions
+        assertNonBase64Data(b'\nab==', b'i')
+        assertNonBase64Data(b'ab:(){:|:&};:==', b'i')
+        assertNonBase64Data(b'a\nb==', b'i')
+        assertNonBase64Data(b'a\x00b==', b'i')
+
+        # Test malformed padding
+        assertMalformedPadding(b'=', b'')
+        assertMalformedPadding(b'==', b'')
+        assertMalformedPadding(b'===', b'')
+        assertMalformedPadding(b'ab=c=', b'i\xb7')
+        assertMalformedPadding(b'ab=ab==', b'i\xb6\x9b')
+
+
     def test_base64errors(self):
         # Test base64 with invalid padding
         def assertIncorrectPadding(data):

diff --git a/Misc/NEWS.d/next/Library/2021-01-31-18-24-54.bpo-43086.2_P-SH.rst b/Misc/NEWS.d/next/Library/2021-01-31-18-24-54.bpo-43086.2_P-SH.rst
@@ -0,0 +1,3 @@
+Added a new optional :code:`strict_mode` parameter to *binascii.a2b_base64*.
+When :code:`scrict_mode` is set to :code:`True`, the *a2b_base64* function will accept only valid base64 content.
+More details about what "valid base64 content" is, can be found in the function's documentation.
diff --git a/Modules/binascii.c b/Modules/binascii.c
@@ -433,18 +433,26 @@ binascii.a2b_base64
 
     data: ascii_buffer
     /
+    *
+    strict_mode: bool(accept={int}) = False
 
 Decode a line of base64 data.
+
+  strict_mode
+    When set to True, bytes that are not part of the base64 standard are not allowed.
+    The same applies to excess data after padding (= / ==).
 [clinic start generated code]*/
 
 static PyObject *
-binascii_a2b_base64_impl(PyObject *module, Py_buffer *data)
-/*[clinic end generated code: output=0628223f19fd3f9b input=5872acf6e1cac243]*/
+binascii_a2b_base64_impl(PyObject *module, Py_buffer *data, int strict_mode)
+/*[clinic end generated code: output=5409557788d4f975 input=3a30c4e3528317c6]*/
 {
     assert(data->len >= 0);
 
     const unsigned char *ascii_data = data->buf;
     size_t ascii_len = data->len;
+    binascii_state *state = NULL;
+    char padding_started = 0;
 
     /* Allocate the buffer */
     Py_ssize_t bin_len = ((ascii_len+3)/4)*3; /* Upper bound, corrected later */
@@ -455,6 +463,15 @@ binascii_a2b_base64_impl(PyObject *module, Py_buffer *data)
         return NULL;
     unsigned char *bin_data_start = bin_data;
 
+    if (strict_mode && ascii_len > 0 && ascii_data[0] == '=') {
+        malformed_padding:
+        state = get_binascii_state(module);
+        if (state) {
+            PyErr_SetString(state->Error, "Leading padding not allowed");
+        }
+        goto error_end;
+    }
+
     int quad_pos = 0;
     unsigned char leftchar = 0;
     int pads = 0;
@@ -465,20 +482,42 @@ binascii_a2b_base64_impl(PyObject *module, Py_buffer *data)
         ** the invalid ones.
         */
         if (this_ch == BASE64_PAD) {
+            padding_started = 1;
+
             if (quad_pos >= 2 && quad_pos + ++pads >= 4) {
-                /* A pad sequence means no more input.
-                ** We've already interpreted the data
-                ** from the quad at this point.
+                /* A pad sequence means we should not parse more input.
+                ** We've already interpreted the data from the quad at this point.
+                ** in strict mode, an error should raise if there's excess data after the padding.
                 */
+                if (strict_mode && i + 1 < ascii_len) {
+                    state = get_binascii_state(module);
+                    if (state) {
+                        PyErr_SetString(state->Error, "Excess data after padding");
+                    }
+                    goto error_end;
+                }
+
                 goto done;
             }
             continue;
         }
 
         this_ch = table_a2b_base64[this_ch];
         if (this_ch >= 64) {
+            if (strict_mode) {
+                state = get_binascii_state(module);
+                if (state) {
+                    PyErr_SetString(state->Error, "Only base64 data is allowed");
+                }
+                goto error_end;
+            }
             continue;
         }
+
+        // Characters that are not '=', in the middle of the padding, are not allowed
+        if (strict_mode && padding_started) {
+            goto malformed_padding;
+        }
         pads = 0;
 
         switch (quad_pos) {
@@ -505,7 +544,7 @@ binascii_a2b_base64_impl(PyObject *module, Py_buffer *data)
     }
 
     if (quad_pos != 0) {
-        binascii_state *state = get_binascii_state(module);
+        state = get_binascii_state(module);
         if (state == NULL) {
             /* error already set, from get_binascii_state */
         } else if (quad_pos == 1) {
@@ -522,6 +561,7 @@ binascii_a2b_base64_impl(PyObject *module, Py_buffer *data)
         } else {
             PyErr_SetString(state->Error, "Incorrect padding");
         }
+        error_end:
         _PyBytesWriter_Dealloc(&writer);
         return NULL;
     }

diff --git a/Modules/clinic/binascii.c.h b/Modules/clinic/binascii.c.h