diff --git a/ChangeLog b/ChangeLog
index ed7818e..65c5d65 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -62,6 +62,8 @@
    * Fix issue in RSA key generation program programs/x509/rsa_genkey
      where the failure of CTR DRBG initialization lead to freeing an
      RSA context without proper initialization beforehand.
+   * Fix memory allocation corner cases in memory_buffer_alloc.c module. Found
+     by Guido Vranken. #639
 
 Changes
    * Extend cert_write example program by options to set the CRT version
diff --git a/library/memory_buffer_alloc.c b/library/memory_buffer_alloc.c
index 545d5a2..0d3342d 100644
--- a/library/memory_buffer_alloc.c
+++ b/library/memory_buffer_alloc.c
@@ -182,9 +182,9 @@
 
 static int verify_chain()
 {
-    memory_header *prv = heap.first, *cur = heap.first->next;
+    memory_header *prv = heap.first, *cur;
 
-    if( verify_header( heap.first ) != 0 )
+    if( heap.first == NULL || verify_header( heap.first ) != 0 )
     {
 #if defined(MBEDTLS_MEMORY_DEBUG)
         mbedtls_fprintf( stderr, "FATAL: verification of first header "
@@ -202,6 +202,8 @@
         return( 1 );
     }
 
+    cur = heap.first->next;
+
     while( cur != NULL )
     {
         if( verify_header( cur ) != 0 )
@@ -245,7 +247,9 @@
 
     original_len = len = n * size;
 
-    if( n != 0 && len / n != size )
+    if( n == 0 || size == 0 || len / n != size )
+        return( NULL );
+    else if( len > (size_t)-MBEDTLS_MEMORY_ALIGN_MULTIPLE )
         return( NULL );
 
     if( len % MBEDTLS_MEMORY_ALIGN_MULTIPLE )
@@ -386,7 +390,7 @@
     if( ptr == NULL || heap.buf == NULL || heap.first == NULL )
         return;
 
-    if( p < heap.buf || p > heap.buf + heap.len )
+    if( p < heap.buf || p >= heap.buf + heap.len )
     {
 #if defined(MBEDTLS_MEMORY_DEBUG)
         mbedtls_fprintf( stderr, "FATAL: mbedtls_free() outside of managed "
@@ -570,8 +574,7 @@
 
 void mbedtls_memory_buffer_alloc_init( unsigned char *buf, size_t len )
 {
-    memset( &heap, 0, sizeof(buffer_alloc_ctx) );
-    memset( buf, 0, len );
+    memset( &heap, 0, sizeof( buffer_alloc_ctx ) );
 
 #if defined(MBEDTLS_THREADING_C)
     mbedtls_mutex_init( &heap.mutex );
@@ -581,20 +584,24 @@
     mbedtls_platform_set_calloc_free( buffer_alloc_calloc, buffer_alloc_free );
 #endif
 
-    if( (size_t) buf % MBEDTLS_MEMORY_ALIGN_MULTIPLE )
+    if( len < sizeof( memory_header ) + MBEDTLS_MEMORY_ALIGN_MULTIPLE )
+        return;
+    else if( (size_t)buf % MBEDTLS_MEMORY_ALIGN_MULTIPLE )
     {
         /* Adjust len first since buf is used in the computation */
         len -= MBEDTLS_MEMORY_ALIGN_MULTIPLE
-             - (size_t) buf % MBEDTLS_MEMORY_ALIGN_MULTIPLE;
+             - (size_t)buf % MBEDTLS_MEMORY_ALIGN_MULTIPLE;
         buf += MBEDTLS_MEMORY_ALIGN_MULTIPLE
-             - (size_t) buf % MBEDTLS_MEMORY_ALIGN_MULTIPLE;
+             - (size_t)buf % MBEDTLS_MEMORY_ALIGN_MULTIPLE;
     }
 
+    memset( buf, 0, len );
+
     heap.buf = buf;
     heap.len = len;
 
-    heap.first = (memory_header *) buf;
-    heap.first->size = len - sizeof(memory_header);
+    heap.first = (memory_header *)buf;
+    heap.first->size = len - sizeof( memory_header );
     heap.first->magic1 = MAGIC1;
     heap.first->magic2 = MAGIC2;
     heap.first_free = heap.first;
diff --git a/library/pem.c b/library/pem.c
index 87401ba..d726bd6 100644
--- a/library/pem.c
+++ b/library/pem.c
@@ -423,7 +423,7 @@
                       unsigned char *buf, size_t buf_len, size_t *olen )
 {
     int ret;
-    unsigned char *encode_buf, *c, *p = buf;
+    unsigned char *encode_buf = NULL, *c, *p = buf;
     size_t len = 0, use_len, add_len = 0;
 
     mbedtls_base64_encode( NULL, 0, &use_len, der_data, der_len );
@@ -435,7 +435,7 @@
         return( MBEDTLS_ERR_BASE64_BUFFER_TOO_SMALL );
     }
 
-    if( ( encode_buf = mbedtls_calloc( 1, use_len ) ) == NULL )
+    if( use_len != 0 && ( encode_buf = mbedtls_calloc( 1, use_len ) ) == NULL )
         return( MBEDTLS_ERR_PEM_ALLOC_FAILED );
 
     if( ( ret = mbedtls_base64_encode( encode_buf, use_len, &use_len, der_data,
diff --git a/library/x509_crl.c b/library/x509_crl.c
index 91bbf05..33ad19a 100644
--- a/library/x509_crl.c
+++ b/library/x509_crl.c
@@ -257,7 +257,7 @@
 {
     int ret;
     size_t len;
-    unsigned char *p, *end;
+    unsigned char *p = NULL, *end;
     mbedtls_x509_buf sig_params1, sig_params2, sig_oid2;
     mbedtls_x509_crl *crl = chain;
 
@@ -294,7 +294,7 @@
     /*
      * Copy raw DER-encoded CRL
      */
-    if( ( p = mbedtls_calloc( 1, buflen ) ) == NULL )
+    if( buflen != 0 && ( p = mbedtls_calloc( 1, buflen ) ) == NULL )
         return( MBEDTLS_ERR_X509_ALLOC_FAILED );
 
     memcpy( p, buf, buflen );