diff --git a/ChangeLog b/ChangeLog
index 83fd5ac..6f5d24b 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -16,6 +16,12 @@
    * Fixed multiple buffer overreads in mbedtls_pem_read_buffer() when parsing
      the input string in PEM format to extract the different components. Found
      by Eyal Itkin.
+   * Fixed potential arithmetic overflow in mbedtls_ctr_drbg_reseed() that could
+     cause buffer bound checks to be bypassed. Found by Eyal Itkin.
+   * Fixed potential arithmetic overflows in mbedtls_cipher_update() that could
+     cause buffer bound checks to be bypassed. Found by Eyal Itkin.
+   * Fixed potential arithmetic overflow in mbedtls_md2_update() that could
+     cause buffer bound checks to be bypassed. Found by Eyal Itkin.
 
 = mbed TLS 1.3.18 branch 2016-10-17
 
diff --git a/library/cipher.c b/library/cipher.c
index b69d331..7ea25cf 100644
--- a/library/cipher.c
+++ b/library/cipher.c
@@ -315,9 +315,9 @@
          * If there is not enough data for a full block, cache it.
          */
         if( ( ctx->operation == POLARSSL_DECRYPT &&
-                ilen + ctx->unprocessed_len <= cipher_get_block_size( ctx ) ) ||
+                ilen <= cipher_get_block_size( ctx ) - ctx->unprocessed_len ) ||
              ( ctx->operation == POLARSSL_ENCRYPT &&
-                ilen + ctx->unprocessed_len < cipher_get_block_size( ctx ) ) )
+                ilen < cipher_get_block_size( ctx ) - ctx->unprocessed_len ) )
         {
             memcpy( &( ctx->unprocessed_data[ctx->unprocessed_len] ), input,
                     ilen );
diff --git a/library/ctr_drbg.c b/library/ctr_drbg.c
index 24adff0..7b315e8 100644
--- a/library/ctr_drbg.c
+++ b/library/ctr_drbg.c
@@ -277,7 +277,8 @@
     unsigned char seed[CTR_DRBG_MAX_SEED_INPUT];
     size_t seedlen = 0;
 
-    if( ctx->entropy_len + len > CTR_DRBG_MAX_SEED_INPUT )
+    if( ctx->entropy_len > CTR_DRBG_MAX_SEED_INPUT ||
+        len > CTR_DRBG_MAX_SEED_INPUT - ctx->entropy_len )
         return( POLARSSL_ERR_CTR_DRBG_INPUT_TOO_BIG );
 
     memset( seed, 0, CTR_DRBG_MAX_SEED_INPUT );
diff --git a/library/md2.c b/library/md2.c
index 110cd95..2ac7eba 100644
--- a/library/md2.c
+++ b/library/md2.c
@@ -155,7 +155,7 @@
 
     while( ilen > 0 )
     {
-        if( ctx->left + ilen > 16 )
+        if( ilen > 16 - ctx->left )
             fill = 16 - ctx->left;
         else
             fill = ilen;