diff --git a/ChangeLog b/ChangeLog
index f41af12..4e39a70 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -10,6 +10,14 @@
      data is all zeros.
    * Fix unsafe bounds check in ssl_parse_client_psk_identity() when adding
      64kB to the address of the SSL buffer wraps around.
+   * Fix a potential heap buffer overflow in mbedtls_ssl_write. When the (by
+     default enabled) maximum fragment length extension is disabled in the
+     config and the application data buffer passed to mbedtls_ssl_write
+     is larger than the internal message buffer (16384 bytes by default), the
+     latter overflows. The exploitability of this issue depends on whether the
+     application layer can be forced into sending such large packets. The issue
+     was independently reported by Tim Nordell via e-mail and by Florin Petriuc
+     and sjorsdewit on GitHub. Fix proposed by Florin Petriuc in #1022. Fixes #707.
 
 Features
    * Allow comments in test data files.
diff --git a/library/ssl_tls.c b/library/ssl_tls.c
index 8467b13..e19dfc9 100644
--- a/library/ssl_tls.c
+++ b/library/ssl_tls.c
@@ -7078,7 +7078,9 @@
     int ret;
 #if defined(MBEDTLS_SSL_MAX_FRAGMENT_LENGTH)
     size_t max_len = mbedtls_ssl_get_max_frag_len( ssl );
-
+#else
+    size_t max_len = MBEDTLS_SSL_MAX_CONTENT_LEN;
+#endif /* MBEDTLS_SSL_MAX_FRAGMENT_LENGTH */
     if( len > max_len )
     {
 #if defined(MBEDTLS_SSL_PROTO_DTLS)
@@ -7093,7 +7095,6 @@
 #endif
             len = max_len;
     }
-#endif /* MBEDTLS_SSL_MAX_FRAGMENT_LENGTH */
 
     if( ssl->out_left != 0 )
     {
@@ -7124,7 +7125,7 @@
  *
  * With non-blocking I/O, ssl_write_real() may return WANT_WRITE,
  * then the caller will call us again with the same arguments, so
- * remember wether we already did the split or not.
+ * remember whether we already did the split or not.
  */
 #if defined(MBEDTLS_SSL_CBC_RECORD_SPLITTING)
 static int ssl_write_split( mbedtls_ssl_context *ssl,
diff --git a/programs/ssl/ssl_client2.c b/programs/ssl/ssl_client2.c
index 5032a9f..8e2feb1 100644
--- a/programs/ssl/ssl_client2.c
+++ b/programs/ssl/ssl_client2.c
@@ -63,6 +63,9 @@
 #include <stdlib.h>
 #include <string.h>
 
+#define MAX_REQUEST_SIZE      20000
+#define MAX_REQUEST_SIZE_STR "20000"
+
 #define DFL_SERVER_NAME         "localhost"
 #define DFL_SERVER_ADDR         NULL
 #define DFL_SERVER_PORT         "4433"
@@ -242,8 +245,8 @@
     "    server_addr=%%s      default: given by name\n"     \
     "    server_port=%%d      default: 4433\n"              \
     "    request_page=%%s     default: \".\"\n"             \
-    "    request_size=%%d     default: about 34 (basic request)\n" \
-    "                        (minimum: 0, max: 16384)\n" \
+    "    request_size=%%d     default: about 34 (basic request)\n"           \
+    "                        (minimum: 0, max: " MAX_REQUEST_SIZE_STR " )\n" \
     "    debug_level=%%d      default: 0 (disabled)\n"      \
     "    nbio=%%d             default: 0 (blocking I/O)\n"  \
     "                        options: 1 (non-blocking), 2 (added delays)\n" \
@@ -437,7 +440,9 @@
 {
     int ret = 0, len, tail_len, i, written, frags, retry_left;
     mbedtls_net_context server_fd;
-    unsigned char buf[MBEDTLS_SSL_MAX_CONTENT_LEN + 1];
+
+    unsigned char buf[MAX_REQUEST_SIZE + 1];
+
 #if defined(MBEDTLS_KEY_EXCHANGE__SOME__PSK_ENABLED)
     unsigned char psk[MBEDTLS_PSK_MAX_LEN];
     size_t psk_len = 0;
@@ -602,7 +607,8 @@
         else if( strcmp( p, "request_size" ) == 0 )
         {
             opt.request_size = atoi( q );
-            if( opt.request_size < 0 || opt.request_size > MBEDTLS_SSL_MAX_CONTENT_LEN )
+            if( opt.request_size < 0 ||
+                opt.request_size > MAX_REQUEST_SIZE )
                 goto usage;
         }
         else if( strcmp( p, "ca_file" ) == 0 )
@@ -1494,8 +1500,8 @@
     mbedtls_printf( "  > Write to server:" );
     fflush( stdout );
 
-    len = mbedtls_snprintf( (char *) buf, sizeof(buf) - 1, GET_REQUEST,
-                    opt.request_page );
+    len = mbedtls_snprintf( (char *) buf, sizeof( buf ) - 1, GET_REQUEST,
+                            opt.request_page );
     tail_len = (int) strlen( GET_REQUEST_END );
 
     /* Add padding to GET request to reach opt.request_size in length */
@@ -1506,7 +1512,7 @@
         len += opt.request_size - len - tail_len;
     }
 
-    strncpy( (char *) buf + len, GET_REQUEST_END, sizeof(buf) - len - 1 );
+    strncpy( (char *) buf + len, GET_REQUEST_END, sizeof( buf ) - len - 1 );
     len += tail_len;
 
     /* Truncate if request size is smaller than the "natural" size */
@@ -1550,6 +1556,12 @@
 
         frags = 1;
         written = ret;
+
+        if( written < len )
+        {
+            mbedtls_printf( " warning\n  ! request didn't fit into single datagram and "
+                            "was truncated to size %u", (unsigned) written );
+        }
     }
 
     buf[written] = '\0';
diff --git a/tests/scripts/all.sh b/tests/scripts/all.sh
index d9c5bbf..258141d 100755
--- a/tests/scripts/all.sh
+++ b/tests/scripts/all.sh
@@ -413,6 +413,16 @@
 scripts/config.pl set MBEDTLS_NO_PLATFORM_ENTROPY # uses syscall() on GNU/Linux
 CC=gcc CFLAGS='-Werror -Wall -Wextra -O0 -std=c99 -pedantic' make lib
 
+msg "build: default config except MFL extension (ASan build)" # ~ 30s
+cleanup
+cp "$CONFIG_H" "$CONFIG_BAK"
+scripts/config.pl unset MBEDTLS_SSL_MAX_FRAGMENT_LENGTH
+CC=gcc cmake -D CMAKE_BUILD_TYPE:String=Asan .
+make
+
+msg "test: ssl-opt.sh, MFL-related tests"
+tests/ssl-opt.sh -f "Max fragment length"
+
 msg "build: default config with  MBEDTLS_TEST_NULL_ENTROPY (ASan build)"
 cleanup
 cp "$CONFIG_H" "$CONFIG_BAK"
@@ -628,4 +638,3 @@
 
 msg "Done, cleaning up"
 cleanup
-
diff --git a/tests/ssl-opt.sh b/tests/ssl-opt.sh
index 64f26a0..5dc47a3 100755
--- a/tests/ssl-opt.sh
+++ b/tests/ssl-opt.sh
@@ -1348,7 +1348,23 @@
 
 # Tests for Max Fragment Length extension
 
-run_test    "Max fragment length: not used, reference" \
+MAX_CONTENT_LEN_EXPECT='16384'
+MAX_CONTENT_LEN_CONFIG=$( ../scripts/config.pl get MBEDTLS_SSL_MAX_CONTENT_LEN)
+
+if [ -n "$MAX_CONTENT_LEN_CONFIG" ] && [ "$MAX_CONTENT_LEN_CONFIG" -ne "$MAX_CONTENT_LEN_EXPECT" ]; then
+    printf "The ${CONFIG_H} file contains a value for the configuration of\n"
+    printf "MBEDTLS_SSL_MAX_CONTENT_LEN that is different from the script’s\n"
+    printf "test value of ${MAX_CONTENT_LEN_EXPECT}. \n"
+    printf "\n"
+    printf "The tests assume this value and if it changes, the tests in this\n"
+    printf "script should also be adjusted.\n"
+    printf "\n"
+
+    exit 1
+fi
+
+requires_config_enabled MBEDTLS_SSL_MAX_FRAGMENT_LENGTH
+run_test    "Max fragment length: enabled, default" \
             "$P_SRV debug_level=3" \
             "$P_CLI debug_level=3" \
             0 \
@@ -1359,6 +1375,55 @@
             -S "server hello, max_fragment_length extension" \
             -C "found max_fragment_length extension"
 
+requires_config_enabled MBEDTLS_SSL_MAX_FRAGMENT_LENGTH
+run_test    "Max fragment length: enabled, default, larger message" \
+            "$P_SRV debug_level=3" \
+            "$P_CLI debug_level=3 request_size=16385" \
+            0 \
+            -c "Maximum fragment length is 16384" \
+            -s "Maximum fragment length is 16384" \
+            -C "client hello, adding max_fragment_length extension" \
+            -S "found max fragment length extension" \
+            -S "server hello, max_fragment_length extension" \
+            -C "found max_fragment_length extension" \
+            -c "16385 bytes written in 2 fragments" \
+            -s "16384 bytes read" \
+            -s "1 bytes read"
+
+requires_config_enabled MBEDTLS_SSL_MAX_FRAGMENT_LENGTH
+run_test    "Max fragment length, DTLS: enabled, default, larger message" \
+            "$P_SRV debug_level=3 dtls=1" \
+            "$P_CLI debug_level=3 dtls=1 request_size=16385" \
+            1 \
+            -c "Maximum fragment length is 16384" \
+            -s "Maximum fragment length is 16384" \
+            -C "client hello, adding max_fragment_length extension" \
+            -S "found max fragment length extension" \
+            -S "server hello, max_fragment_length extension" \
+            -C "found max_fragment_length extension" \
+            -c "fragment larger than.*maximum "
+
+requires_config_disabled MBEDTLS_SSL_MAX_FRAGMENT_LENGTH
+run_test    "Max fragment length: disabled, larger message" \
+            "$P_SRV debug_level=3" \
+            "$P_CLI debug_level=3 request_size=16385" \
+            0 \
+            -C "Maximum fragment length is 16384" \
+            -S "Maximum fragment length is 16384" \
+            -c "16385 bytes written in 2 fragments" \
+            -s "16384 bytes read" \
+            -s "1 bytes read"
+
+requires_config_disabled MBEDTLS_SSL_MAX_FRAGMENT_LENGTH
+run_test    "Max fragment length DTLS: disabled, larger message" \
+            "$P_SRV debug_level=3 dtls=1" \
+            "$P_CLI debug_level=3 dtls=1 request_size=16385" \
+            1 \
+            -C "Maximum fragment length is 16384" \
+            -S "Maximum fragment length is 16384" \
+            -c "fragment larger than.*maximum "
+
+requires_config_enabled MBEDTLS_SSL_MAX_FRAGMENT_LENGTH
 run_test    "Max fragment length: used by client" \
             "$P_SRV debug_level=3" \
             "$P_CLI debug_level=3 max_frag_len=4096" \
@@ -1370,6 +1435,7 @@
             -s "server hello, max_fragment_length extension" \
             -c "found max_fragment_length extension"
 
+requires_config_enabled MBEDTLS_SSL_MAX_FRAGMENT_LENGTH
 run_test    "Max fragment length: used by server" \
             "$P_SRV debug_level=3 max_frag_len=4096" \
             "$P_CLI debug_level=3" \
@@ -1381,6 +1447,7 @@
             -S "server hello, max_fragment_length extension" \
             -C "found max_fragment_length extension"
 
+requires_config_enabled MBEDTLS_SSL_MAX_FRAGMENT_LENGTH
 requires_gnutls
 run_test    "Max fragment length: gnutls server" \
             "$G_SRV" \
@@ -1390,6 +1457,7 @@
             -c "client hello, adding max_fragment_length extension" \
             -c "found max_fragment_length extension"
 
+requires_config_enabled MBEDTLS_SSL_MAX_FRAGMENT_LENGTH
 run_test    "Max fragment length: client, message just fits" \
             "$P_SRV debug_level=3" \
             "$P_CLI debug_level=3 max_frag_len=2048 request_size=2048" \
@@ -1403,6 +1471,7 @@
             -c "2048 bytes written in 1 fragments" \
             -s "2048 bytes read"
 
+requires_config_enabled MBEDTLS_SSL_MAX_FRAGMENT_LENGTH
 run_test    "Max fragment length: client, larger message" \
             "$P_SRV debug_level=3" \
             "$P_CLI debug_level=3 max_frag_len=2048 request_size=2345" \
@@ -1417,6 +1486,7 @@
             -s "2048 bytes read" \
             -s "297 bytes read"
 
+requires_config_enabled MBEDTLS_SSL_MAX_FRAGMENT_LENGTH
 run_test    "Max fragment length: DTLS client, larger message" \
             "$P_SRV debug_level=3 dtls=1" \
             "$P_CLI debug_level=3 dtls=1 max_frag_len=2048 request_size=2345" \
@@ -3417,6 +3487,7 @@
             "$P_CLI request_size=16384 force_version=ssl3 recsplit=0 \
              force_ciphersuite=TLS-RSA-WITH-AES-256-CBC-SHA" \
             0 \
+            -c "16384 bytes written in 1 fragments" \
             -s "Read from client: 16384 bytes read"
 
 requires_config_enabled MBEDTLS_SSL_PROTO_SSL3
@@ -3425,6 +3496,7 @@
             "$P_CLI request_size=16384 force_version=ssl3 \
              force_ciphersuite=TLS-RSA-WITH-RC4-128-SHA" \
             0 \
+            -c "16384 bytes written in 1 fragments" \
             -s "Read from client: 16384 bytes read"
 
 run_test    "Large packet TLS 1.0 BlockCipher" \
@@ -3432,6 +3504,7 @@
             "$P_CLI request_size=16384 force_version=tls1 recsplit=0 \
              force_ciphersuite=TLS-RSA-WITH-AES-256-CBC-SHA" \
             0 \
+            -c "16384 bytes written in 1 fragments" \
             -s "Read from client: 16384 bytes read"
 
 run_test    "Large packet TLS 1.0 BlockCipher truncated MAC" \
@@ -3440,6 +3513,7 @@
              force_ciphersuite=TLS-RSA-WITH-AES-256-CBC-SHA \
              trunc_hmac=1" \
             0 \
+            -c "16384 bytes written in 1 fragments" \
             -s "Read from client: 16384 bytes read"
 
 run_test    "Large packet TLS 1.0 StreamCipher truncated MAC" \
@@ -3448,6 +3522,7 @@
              force_ciphersuite=TLS-RSA-WITH-RC4-128-SHA \
              trunc_hmac=1" \
             0 \
+            -c "16384 bytes written in 1 fragments" \
             -s "Read from client: 16384 bytes read"
 
 run_test    "Large packet TLS 1.1 BlockCipher" \
@@ -3455,6 +3530,7 @@
             "$P_CLI request_size=16384 force_version=tls1_1 \
              force_ciphersuite=TLS-RSA-WITH-AES-256-CBC-SHA" \
             0 \
+            -c "16384 bytes written in 1 fragments" \
             -s "Read from client: 16384 bytes read"
 
 run_test    "Large packet TLS 1.1 StreamCipher" \
@@ -3462,6 +3538,7 @@
             "$P_CLI request_size=16384 force_version=tls1_1 \
              force_ciphersuite=TLS-RSA-WITH-RC4-128-SHA" \
             0 \
+            -c "16384 bytes written in 1 fragments" \
             -s "Read from client: 16384 bytes read"
 
 run_test    "Large packet TLS 1.1 BlockCipher truncated MAC" \
@@ -3470,6 +3547,7 @@
              force_ciphersuite=TLS-RSA-WITH-AES-256-CBC-SHA \
              trunc_hmac=1" \
             0 \
+            -c "16384 bytes written in 1 fragments" \
             -s "Read from client: 16384 bytes read"
 
 run_test    "Large packet TLS 1.1 StreamCipher truncated MAC" \
@@ -3478,6 +3556,7 @@
              force_ciphersuite=TLS-RSA-WITH-RC4-128-SHA \
              trunc_hmac=1" \
             0 \
+            -c "16384 bytes written in 1 fragments" \
             -s "Read from client: 16384 bytes read"
 
 run_test    "Large packet TLS 1.2 BlockCipher" \
@@ -3485,6 +3564,7 @@
             "$P_CLI request_size=16384 force_version=tls1_2 \
              force_ciphersuite=TLS-RSA-WITH-AES-256-CBC-SHA" \
             0 \
+            -c "16384 bytes written in 1 fragments" \
             -s "Read from client: 16384 bytes read"
 
 run_test    "Large packet TLS 1.2 BlockCipher larger MAC" \
@@ -3492,6 +3572,7 @@
             "$P_CLI request_size=16384 force_version=tls1_2 \
              force_ciphersuite=TLS-ECDHE-RSA-WITH-AES-256-CBC-SHA384" \
             0 \
+            -c "16384 bytes written in 1 fragments" \
             -s "Read from client: 16384 bytes read"
 
 run_test    "Large packet TLS 1.2 BlockCipher truncated MAC" \
@@ -3500,6 +3581,7 @@
              force_ciphersuite=TLS-RSA-WITH-AES-256-CBC-SHA \
              trunc_hmac=1" \
             0 \
+            -c "16384 bytes written in 1 fragments" \
             -s "Read from client: 16384 bytes read"
 
 run_test    "Large packet TLS 1.2 StreamCipher" \
@@ -3507,6 +3589,7 @@
             "$P_CLI request_size=16384 force_version=tls1_2 \
              force_ciphersuite=TLS-RSA-WITH-RC4-128-SHA" \
             0 \
+            -c "16384 bytes written in 1 fragments" \
             -s "Read from client: 16384 bytes read"
 
 run_test    "Large packet TLS 1.2 StreamCipher truncated MAC" \
@@ -3515,6 +3598,7 @@
              force_ciphersuite=TLS-RSA-WITH-RC4-128-SHA \
              trunc_hmac=1" \
             0 \
+            -c "16384 bytes written in 1 fragments" \
             -s "Read from client: 16384 bytes read"
 
 run_test    "Large packet TLS 1.2 AEAD" \
@@ -3522,6 +3606,7 @@
             "$P_CLI request_size=16384 force_version=tls1_2 \
              force_ciphersuite=TLS-RSA-WITH-AES-256-CCM" \
             0 \
+            -c "16384 bytes written in 1 fragments" \
             -s "Read from client: 16384 bytes read"
 
 run_test    "Large packet TLS 1.2 AEAD shorter tag" \
@@ -3529,6 +3614,7 @@
             "$P_CLI request_size=16384 force_version=tls1_2 \
              force_ciphersuite=TLS-RSA-WITH-AES-256-CCM-8" \
             0 \
+            -c "16384 bytes written in 1 fragments" \
             -s "Read from client: 16384 bytes read"
 
 # Tests for DTLS HelloVerifyRequest