diff --git a/ChangeLog b/ChangeLog
index 16982a0..d84a364 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -2,6 +2,13 @@
 
 = mbed TLS 2.7.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.
+
 Changes
    * Add unit tests for AES-GCM when called through mbedtls_cipher_auth_xxx()
      from the cipher abstraction layer. Fixes #2198.
diff --git a/library/ecdsa.c b/library/ecdsa.c
index c635a50..24bf734 100644
--- a/library/ecdsa.c
+++ b/library/ecdsa.c
@@ -153,6 +153,7 @@
         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( &k, &k, &t ) );
+        MBEDTLS_MPI_CHK( mbedtls_mpi_mod_mpi( &k, &k, &grp->N ) );
         MBEDTLS_MPI_CHK( mbedtls_mpi_inv_mod( s, &k, &grp->N ) );
         MBEDTLS_MPI_CHK( mbedtls_mpi_mul_mpi( s, s, &e ) );
         MBEDTLS_MPI_CHK( mbedtls_mpi_mod_mpi( s, s, &grp->N ) );