diff --git a/ChangeLog b/ChangeLog
index 71f927a..994e18d 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -3,6 +3,11 @@
 = mbed TLS 2.16.x branch released xxxx-xx-xx
 
 Security
+   * Fix side channel vulnerability in ECDSA. Our bignum implementation is not
+     constant time/constant trace, so side channel attacks can retrieve the
+     blinded value, factor it (as it is smaller than RSA keys and not guaranteed
+     to have only large prime factors), and then, by brute force, recover the
+     key. Reported by Alejandro Cabrera Aldaya and Billy Brumley.
    * Zeroize local variables in mbedtls_internal_aes_encrypt() and
      mbedtls_internal_aes_decrypt() before exiting the function. The value of
      these variables can be used to recover the last round key. To follow best
diff --git a/library/ecdsa.c b/library/ecdsa.c
index 2b48006..3cf3d7c 100644
--- a/library/ecdsa.c
+++ b/library/ecdsa.c
@@ -363,6 +363,7 @@
         MBEDTLS_MPI_CHK( mbedtls_mpi_add_mpi( &e, &e, s ) );
         MBEDTLS_MPI_CHK( mbedtls_mpi_mul_mpi( &e, &e, &t ) );
         MBEDTLS_MPI_CHK( mbedtls_mpi_mul_mpi( pk, pk, &t ) );
+        MBEDTLS_MPI_CHK( mbedtls_mpi_mod_mpi( pk, pk, &grp->N ) );
         MBEDTLS_MPI_CHK( mbedtls_mpi_inv_mod( s, pk, &grp->N ) );
         MBEDTLS_MPI_CHK( mbedtls_mpi_mul_mpi( s, s, &e ) );
         MBEDTLS_MPI_CHK( mbedtls_mpi_mod_mpi( s, s, &grp->N ) );