diff --git a/library/pem.c b/library/pem.c
index 9eff10ea1db5..8c07048708ba 100644
--- a/library/pem.c
+++ b/library/pem.c
@@ -510,6 +510,9 @@ int mbedtls_pem_write_buffer( const char *header, const char *footer,
     *p++ = '\0';
     *olen = p - buf;
 
+    /* Clean any remaining data previously written to the buffer */
+    memset( buf + *olen, 0, buf_len - *olen );
+
     mbedtls_free( encode_buf );
     return( 0 );
 }
diff --git a/tests/suites/test_suite_pkwrite.function b/tests/suites/test_suite_pkwrite.function
index 71aa595202ad..c6aa05a8aa6b 100644
--- a/tests/suites/test_suite_pkwrite.function
+++ b/tests/suites/test_suite_pkwrite.function
@@ -17,7 +17,7 @@ void pk_write_pubkey_check( char *key_file )
     unsigned char check_buf[5000];
     int ret;
     FILE *f;
-    size_t ilen;
+    size_t ilen, pem_len, buf_index;
 
     memset( buf, 0, sizeof( buf ) );
     memset( check_buf, 0, sizeof( check_buf ) );
@@ -28,12 +28,20 @@ void pk_write_pubkey_check( char *key_file )
     ret = mbedtls_pk_write_pubkey_pem( &key, buf, sizeof( buf ));
     TEST_ASSERT( ret == 0 );
 
+    pem_len = strlen( (char *) buf );
+
+    // check that the rest of the buffer remains clear
+    for( buf_index = pem_len; buf_index < sizeof( buf ); ++buf_index )
+    {
+        TEST_ASSERT( buf[buf_index] == 0 );
+    }
+
     f = fopen( key_file, "r" );
     TEST_ASSERT( f != NULL );
     ilen = fread( check_buf, 1, sizeof( check_buf ), f );
     fclose( f );
 
-    TEST_ASSERT( ilen == strlen( (char *) buf ) );
+    TEST_ASSERT( ilen == pem_len );
     TEST_ASSERT( memcmp( (char *) buf, (char *) check_buf, ilen ) == 0 );
 
 exit:
@@ -49,7 +57,7 @@ void pk_write_key_check( char *key_file )
     unsigned char check_buf[5000];
     int ret;
     FILE *f;
-    size_t ilen;
+    size_t ilen, pem_len, buf_index;
 
     memset( buf, 0, sizeof( buf ) );
     memset( check_buf, 0, sizeof( check_buf ) );
@@ -60,12 +68,20 @@ void pk_write_key_check( char *key_file )
     ret = mbedtls_pk_write_key_pem( &key, buf, sizeof( buf ));
     TEST_ASSERT( ret == 0 );
 
+    pem_len = strlen( (char *) buf );
+
+    // check that the rest of the buffer remains clear
+    for( buf_index = pem_len; buf_index < sizeof( buf ); ++buf_index )
+    {
+        TEST_ASSERT( buf[buf_index] == 0 );
+    }
+
     f = fopen( key_file, "r" );
     TEST_ASSERT( f != NULL );
     ilen = fread( check_buf, 1, sizeof( check_buf ), f );
     fclose( f );
 
-    TEST_ASSERT( ilen == strlen( (char *) buf ) );
+    TEST_ASSERT( ilen == pem_len );
     TEST_ASSERT( memcmp( (char *) buf, (char *) check_buf, ilen ) == 0 );
 
 exit:
diff --git a/tests/suites/test_suite_x509write.function b/tests/suites/test_suite_x509write.function
index aba23d4c91ad..f5223110543d 100644
--- a/tests/suites/test_suite_x509write.function
+++ b/tests/suites/test_suite_x509write.function
@@ -44,7 +44,7 @@ void x509_csr_check( char * key_file, char * cert_req_check_file, int md_type,
     unsigned char buf[4096];
     unsigned char check_buf[4000];
     int ret;
-    size_t olen = 0, pem_len = 0;
+    size_t olen = 0, pem_len = 0, buf_index;
     int der_len = -1;
     FILE *f;
     const char *subject_name = "C=NL,O=PolarSSL,CN=PolarSSL Server 1";
@@ -70,6 +70,11 @@ void x509_csr_check( char * key_file, char * cert_req_check_file, int md_type,
 
     pem_len = strlen( (char *) buf );
 
+    for( buf_index = pem_len; buf_index < sizeof( buf ); ++buf_index )
+    {
+        TEST_ASSERT( buf[buf_index] == 0 );
+    }
+
     f = fopen( cert_req_check_file, "r" );
     TEST_ASSERT( f != NULL );
     olen = fread( check_buf, 1, sizeof( check_buf ), f );
@@ -112,7 +117,7 @@ void x509_crt_check( char *subject_key_file, char *subject_pwd,
     unsigned char check_buf[5000];
     mbedtls_mpi serial;
     int ret;
-    size_t olen = 0, pem_len = 0;
+    size_t olen = 0, pem_len = 0, buf_index;
     int der_len = -1;
     FILE *f;
     rnd_pseudo_info rnd_info;
@@ -181,6 +186,11 @@ void x509_crt_check( char *subject_key_file, char *subject_pwd,
 
     pem_len = strlen( (char *) buf );
 
+    for( buf_index = pem_len; buf_index < sizeof( buf ); ++buf_index )
+    {
+        TEST_ASSERT( buf[buf_index] == 0 );
+    }
+
     f = fopen( cert_check_file, "r" );
     TEST_ASSERT( f != NULL );
     olen = fread( check_buf, 1, sizeof( check_buf ), f );