diff --git a/ChangeLog b/ChangeLog
index da29d70..0e9fa29 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -13,6 +13,16 @@
      dates on leap years with 100 and 400 intervals are handled correctly. Found
      by Nicholas Wilson. #694
 
+Security
+   * Fix a potential heap buffer overflow in mbedtls_ssl_write. When the (by
+     default enabled) maximum fragment length extension is disabled in the
+     config and the application data buffer passed to mbedtls_ssl_write
+     is larger than the internal message buffer (16384 bytes by default), the
+     latter overflows. The exploitability of this issue depends on whether the
+     application layer can be forced into sending such large packets. The issue
+     was independently reported by Tim Nordell via e-mail and by Florin Petriuc
+     and sjorsdewit on GitHub. Fix proposed by Florin Petriuc in #1022. Fixes #707.
+
 = mbed TLS 2.1.9 branch released 2017-08-10
 
 Security