diff --git a/library/ctr_drbg.c b/library/ctr_drbg.c
index ec7d9a7..54feb71 100644
--- a/library/ctr_drbg.c
+++ b/library/ctr_drbg.c
@@ -61,6 +61,10 @@
 void mbedtls_ctr_drbg_init( mbedtls_ctr_drbg_context *ctx )
 {
     memset( ctx, 0, sizeof( mbedtls_ctr_drbg_context ) );
+
+#if defined(MBEDTLS_THREADING_C)
+    mbedtls_mutex_init( &ctx->mutex );
+#endif
 }
 
 /*
@@ -115,6 +119,9 @@
     if( ctx == NULL )
         return;
 
+#if defined(MBEDTLS_THREADING_C)
+    mbedtls_mutex_free( &ctx->mutex );
+#endif
     mbedtls_aes_free( &ctx->aes_ctx );
     mbedtls_zeroize( ctx, sizeof( mbedtls_ctr_drbg_context ) );
 }
@@ -392,7 +399,22 @@
 
 int mbedtls_ctr_drbg_random( void *p_rng, unsigned char *output, size_t output_len )
 {
-    return mbedtls_ctr_drbg_random_with_add( p_rng, output, output_len, NULL, 0 );
+    int ret;
+    mbedtls_ctr_drbg_context *ctx = (mbedtls_ctr_drbg_context *) p_rng;
+
+#if defined(MBEDTLS_THREADING_C)
+    if( ( ret = mbedtls_mutex_lock( &ctx->mutex ) ) != 0 )
+        return( ret );
+#endif
+
+    ret = mbedtls_ctr_drbg_random_with_add( ctx, output, output_len, NULL, 0 );
+
+#if defined(MBEDTLS_THREADING_C)
+    if( mbedtls_mutex_unlock( &ctx->mutex ) != 0 )
+        return( MBEDTLS_ERR_THREADING_MUTEX_ERROR );
+#endif
+
+    return( ret );
 }
 
 #if defined(MBEDTLS_FS_IO)
@@ -537,6 +559,8 @@
     CHK( mbedtls_ctr_drbg_random( &ctx, buf, MBEDTLS_CTR_DRBG_BLOCKSIZE ) );
     CHK( memcmp( buf, result_pr, MBEDTLS_CTR_DRBG_BLOCKSIZE ) );
 
+    mbedtls_ctr_drbg_free( &ctx );
+
     if( verbose != 0 )
         mbedtls_printf( "passed\n" );
 
@@ -546,6 +570,8 @@
     if( verbose != 0 )
         mbedtls_printf( "  CTR_DRBG (PR = FALSE): " );
 
+    mbedtls_ctr_drbg_init( &ctx );
+
     test_offset = 0;
     CHK( mbedtls_ctr_drbg_seed_entropy_len( &ctx, ctr_drbg_self_test_entropy,
                             (void *) entropy_source_nopr, nonce_pers_nopr, 16, 32 ) );
@@ -554,6 +580,8 @@
     CHK( mbedtls_ctr_drbg_random( &ctx, buf, 16 ) );
     CHK( memcmp( buf, result_nopr, 16 ) );
 
+    mbedtls_ctr_drbg_free( &ctx );
+
     if( verbose != 0 )
         mbedtls_printf( "passed\n" );