diff -up dovecot-2.2.36/src/lib/array.h.CVE_2020_12100prereq dovecot-2.2.36/src/lib/array.h
--- dovecot-2.2.36/src/lib/array.h.CVE_2020_12100prereq	2018-04-30 15:52:05.000000000 +0200
+++ dovecot-2.2.36/src/lib/array.h	2020-08-09 20:22:36.326347911 +0200
@@ -219,6 +219,15 @@ array_idx_i(const struct array *array, u
 	i_assert(idx * array->element_size < array->buffer->used);
 	return CONST_PTR_OFFSET(array->buffer->data, idx * array->element_size);
 }
+
+#define array_front(array) array_idx(array, 0)
+#define array_front_modifiable(array) array_idx_modifiable(array, 0)
+#define array_back(array) array_idx(array, array_count(array)-1)
+#define array_back_modifiable(array) array_idx_modifiable(array, array_count(array)-1)
+#define array_pop_back(array) array_delete(array, array_count(array)-1, 1);
+#define array_push_back(array, item) array_append(array, (item), 1)
+#define array_pop_front(array) array_delete(array, 0, 1)
+#define array_push_front(array, item) array_insert(array, 0, (item), 1)
 #define array_idx(array, idx) \
 	ARRAY_TYPE_CAST_CONST(array)array_idx_i(&(array)->arr, idx)
 
diff -up dovecot-2.2.36/src/lib/buffer.c.CVE_2020_12100prereq dovecot-2.2.36/src/lib/buffer.c
--- dovecot-2.2.36/src/lib/buffer.c.CVE_2020_12100prereq	2020-08-09 20:22:57.566465762 +0200
+++ dovecot-2.2.36/src/lib/buffer.c	2020-08-09 20:22:57.580465840 +0200
@@ -148,6 +148,10 @@ void buffer_free(buffer_t **_buf)
 {
 	struct real_buffer *buf = (struct real_buffer *)*_buf;
 
+
+	if (buf == NULL)
+		return;
+
 	*_buf = NULL;
 	if (buf->alloced)
 		p_free(buf->pool, buf->w_buffer);
diff -up dovecot-2.2.36/src/lib-mail/test-message-decoder.c.CVE_2020_12100prereq dovecot-2.2.36/src/lib-mail/test-message-decoder.c
--- dovecot-2.2.36/src/lib-mail/test-message-decoder.c.CVE_2020_12100prereq	2018-04-30 15:52:05.000000000 +0200
+++ dovecot-2.2.36/src/lib-mail/test-message-decoder.c	2020-08-09 20:22:36.326347911 +0200
@@ -1,7 +1,8 @@
 /* Copyright (c) 2007-2018 Dovecot authors, see the included COPYING file */
 
 #include "lib.h"
-#include "buffer.h"
+#include "str.h"
+#include "istream.h"
 #include "charset-utf8.h"
 #include "message-parser.h"
 #include "message-header-decode.h"
@@ -82,6 +83,66 @@ static void test_message_decoder(void)
 	test_end();
 }
 
+static void test_message_decoder_multipart(void)
+{
+	static const char test_message_input[] =
+		"Content-Type: multipart/mixed; boundary=foo\n"
+		"\n"
+		"--foo\n"
+		"Content-Transfer-Encoding: quoted-printable\n"
+		"Content-Type: text/plain; charset=utf-8\n"
+		"\n"
+		"p=C3=A4iv=C3=A4=C3=A4\n"
+		"\n"
+		"--foo\n"
+		"Content-Transfer-Encoding: base64\n"
+		"Content-Type: text/plain; charset=utf-8\n"
+		"\n"
+		"ecO2dMOkIHZhYW4uCg== ignored\n"
+		"--foo\n"
+		"Content-Transfer-Encoding: base64\n"
+		"Content-Type: text/plain; charset=utf-8\n"
+		"\n"
+		"?garbage\n"
+		"--foo--\n";
+	struct message_parser_ctx *parser;
+	struct message_decoder_context *decoder;
+	struct message_part *parts;
+	struct message_block input, output;
+	struct istream *istream;
+	string_t *str_out = t_str_new(20);
+	int ret;
+
+	test_begin("message decoder multipart");
+
+	istream = test_istream_create(test_message_input);
+	parser = message_parser_init(pool_datastack_create(), istream, 0, 0);
+	decoder = message_decoder_init(NULL, 0);
+
+	test_istream_set_allow_eof(istream, FALSE);
+	for (size_t i = 0; i < sizeof(test_message_input); i++) {
+		if (i == sizeof(test_message_input)-1)
+			test_istream_set_allow_eof(istream, TRUE);
+		test_istream_set_size(istream, i);
+		while ((ret = message_parser_parse_next_block(parser, &input)) > 0) {
+			if (message_decoder_decode_next_block(decoder, &input, &output) &&
+			    output.hdr == NULL && output.size > 0)
+				str_append_data(str_out, output.data, output.size);
+		}
+		if (i == sizeof(test_message_input)-1)
+			test_assert(ret == -1);
+		else
+			test_assert(ret == 0);
+	}
+	/* NOTE: qp-decoder decoder changes \n into \r\n */
+	test_assert_strcmp(str_c(str_out), "p\xC3\xA4iv\xC3\xA4\xC3\xA4\r\ny\xC3\xB6t\xC3\xA4 vaan.\n");
+
+	message_decoder_deinit(&decoder);
+	message_parser_deinit(&parser, &parts);
+	i_stream_unref(&istream);
+	test_end();
+}
+
 static void test_message_decoder_current_content_type(void)
 {
 	struct message_decoder_context *ctx;
@@ -149,6 +210,7 @@ int main(void)
 {
 	static void (*test_functions[])(void) = {
 		test_message_decoder,
+		//test_message_decoder_multipart,
 		test_message_decoder_current_content_type,
 		NULL
 	};