diff --git a/ChangeLog b/ChangeLog
index 0cf927e..243bd6b 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -28,6 +28,8 @@
      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
    * Send fatal alerts in many more cases instead of dropping the connection.
diff --git a/include/mbedtls/bignum.h b/include/mbedtls/bignum.h
index a713313..1a5b4b6 100644
--- a/include/mbedtls/bignum.h
+++ b/include/mbedtls/bignum.h
@@ -340,7 +340,7 @@
 
 #if defined(MBEDTLS_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
@@ -349,6 +349,15 @@
  * \return         0 if successful, MBEDTLS_ERR_MPI_BUFFER_TOO_SMALL if
  *                 the file read buffer is too small or a
  *                 MBEDTLS_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 mbedtls_mpi_read_file( mbedtls_mpi *X, int radix, FILE *fin );
 
diff --git a/library/bignum.c b/library/bignum.c
index 8df2510..d3a150c 100644
--- a/library/bignum.c
+++ b/library/bignum.c
@@ -616,11 +616,11 @@
     if( slen == sizeof( s ) - 2 )
         return( MBEDTLS_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;