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Workaround
Update outdated clients.
Error Handling Templates and Template Use in General
The SP’s support of a primitive HTML templating language will be removed, and error handling in general will be very simplified and stripped back to rely on a combination of native web server error code use (typically allowing native support for custom error pages) and some form of the existing feature to redirect to a specified URL with parameters for error handling. This will be examined for safety in the face of injection attacks, so it’s likely some aspects of this will be scaled back.
Workaround
Our impression is that the vast majority deployments have not used these templates even a little, with logos absent, error text unchanged, and even contact links left invalid, which looks embarassing and reflects badly on our software. Using native error handling will typically mask most of this from users, and logs continue to be the only real source of resolution information anyway. Existing usage of the redirect-based error handling feature seems relatively uncommon, but may require some minor changes.