Burp Suite is extensible via the Burp Extender interfaces. This allows third-party developers to extend the functionality of Burp in numerous ways. Extensions can perform a wide range of functions, including processing and modifying HTTP requests made via all Burp tools, issuing arbitrary HTTP requests via Burp, extending Burp's UI with custom menu items, querying and modifying Burp's configuration data, and accessing key runtime information, including the Proxy history, Target site map and Scanner results.
See the Burp Extender source code and Javadoc for more details.
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This release fixes a bug which was introduced in the v1.5rc2 release, and which caused the active scan checks for XSS to fail to execute in some situations