diff --git a/ChangeLog b/ChangeLog
index dd6a69ea2..8449329b0 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -25,6 +25,8 @@ Bugfix
      Found by blaufish. Fixes #641.
    * Fix incorrect sign computation in modular exponentiation
      when dealing with negative MPI. Found by Guido Vranken.
+   * Fix potential stack underflow in mpi_read_file.
+     Found by Guido Vranken.
 
 Changes
    * Clarify ECDSA documentation and improve the sample code to avoid
diff --git a/include/polarssl/bignum.h b/include/polarssl/bignum.h
index dc54af8cd..c11db96dc 100644
--- a/include/polarssl/bignum.h
+++ b/include/polarssl/bignum.h
@@ -377,7 +377,7 @@ int mpi_write_string( const mpi *X, int radix, char *s, size_t *slen );
 
 #if defined(POLARSSL_FS_IO)
 /**
- * \brief          Read X from an opened file
+ * \brief          Read MPI from a line in an opened file
  *
  * \param X        Destination MPI
  * \param radix    Input numeric base
@@ -386,6 +386,14 @@ int mpi_write_string( const mpi *X, int radix, char *s, size_t *slen );
  * \return         0 if successful, POLARSSL_ERR_MPI_BUFFER_TOO_SMALL if
  *                 the file read buffer is too small or a
  *                 POLARSSL_ERR_MPI_XXX error code
+ *
+ * \note           On success, this function advances the file stream
+ *                 to the end of the current line or to EOF.
+ *
+ *                 The function returns 0 on an empty line.
+ *
+ *                 Leading whitespaces are ignored, as is a
+ *                 '0x' prefix for radix 16.
  */
 int mpi_read_file( mpi *X, int radix, FILE *fin );
 
diff --git a/library/bignum.c b/library/bignum.c
index 9d12a86cc..4829d916b 100644
--- a/library/bignum.c
+++ b/library/bignum.c
@@ -620,11 +620,11 @@ int mpi_read_file( mpi *X, int radix, FILE *fin )
     if( slen == sizeof( s ) - 2 )
         return( POLARSSL_ERR_MPI_BUFFER_TOO_SMALL );
 
-    if( s[slen - 1] == '\n' ) { slen--; s[slen] = '\0'; }
-    if( s[slen - 1] == '\r' ) { slen--; s[slen] = '\0'; }
+    if( slen > 0 && s[slen - 1] == '\n' ) { slen--; s[slen] = '\0'; }
+    if( slen > 0 && s[slen - 1] == '\r' ) { slen--; s[slen] = '\0'; }
 
     p = s + slen;
-    while( --p >= s )
+    while( p-- > s )
         if( mpi_get_digit( &d, radix, *p ) != 0 )
             break;