diff --git a/WHATSNEW b/WHATSNEW
index 34a0c275b..215f8cb21 100644
--- a/WHATSNEW
+++ b/WHATSNEW
@@ -92,6 +92,12 @@ Other changes:
* it is now possible to use references to Ant types and classloaders
built around Ant
<xsl:param>
declaration.
This task supports the use of a nested xmlcatalog element which is used to perform Entity and URI resolution.
+ +Note on XSLT extension functions: when using the default
+ TraX implementation of the Java class library and
+ a SecurityManager
is active - e.g. when running from
+ within an IDE - XSLT extension functions cannot be used as "secure
+ processing" is active in Java 7 and above. Ant contains a special
+ "hack" that allows XSLT extensions to be used in Java 7 and 8, but
+ this hack fails for Java 9. If you want to use extensions like the
+ redirect extension that are provided by the Java class library
+ itself, you can allow them by enabling the
+ feature http://www.oracle.com/xml/jaxp/properties/enableExtensionFunctions
+ in Java 9 and above. If you need to use an extension function not
+ provided by Java itself you can set the
+ attribute jdk.xml.transform.extensionClassLoader
to a
+ classloader (reference to an Ant PATH-like structure).