diff --git a/ChangeLog b/ChangeLog
index 85909ca..1447fdc 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -9,6 +9,10 @@
    * Fix potential heap corruption on Windows when
      mbedtls_x509_crt_parse_path() is passed a path longer than 2GB. Cannot be
      triggered remotely. Found by Guido Vranken, Interlworks.
+   * Fix potential buffer overflow in some asn1_write_xxx() functions.
+     Cannot be triggered remotely unless you create X.509 certificates based
+     on untrusted input or write keys of untrusted origin. Found by Guido
+     Vranken, Interlworks.
    * The X509 max_pathlen constraint was not enforced on intermediate
      certificates. Found by Nicholas Wilson, fix and tests provided by
      Janos Follath. #280 and #319
diff --git a/library/asn1write.c b/library/asn1write.c
index 849e8c1..456660d 100644
--- a/library/asn1write.c
+++ b/library/asn1write.c
@@ -87,7 +87,7 @@
 {
     size_t len = 0;
 
-    if( *p - start < (int) size )
+    if( *p < start || (size_t)( *p - start ) < size )
         return( MBEDTLS_ERR_ASN1_BUF_TOO_SMALL );
 
     len = size;
@@ -107,7 +107,7 @@
     //
     len = mbedtls_mpi_size( X );
 
-    if( *p - start < (int) len )
+    if( *p < start || (size_t)( *p - start ) < len )
         return( MBEDTLS_ERR_ASN1_BUF_TOO_SMALL );
 
     (*p) -= len;
@@ -270,7 +270,7 @@
 
     // Calculate byte length
     //
-    if( *p - start < (int) size + 1 )
+    if( *p < start || (size_t)( *p - start ) < size + 1 )
         return( MBEDTLS_ERR_ASN1_BUF_TOO_SMALL );
 
     len = size + 1;