Before starting, I would like to say that I am not convinced that this is a problem with the “Plugin as Parameter” code (with Paul’s comment above it) in libraries/Templates.php. That is simply where I stopped digging because I found a work-around.

My code looks roughly like this, where I am editing an entry with SafeCracker:

{exp:foo:base_url}
{exp:safecracker channel="test" return="{exp:foo:base_url}/thank_you”}
<input type=“hidden” name=“entry_id” value=”{entry_id}” />
<input type=“hidden” name=“title” value=”{title}” />
<input type=“submit” value=“Submit” />
{/exp:safecracker}

Expected Result: To see value from {exp:foo:base_url} appear within SafeCracker’s return and return_url form field.

Actual Result: The tag is replaced with a random strings of letters and numbers. Here is an example (abbreviated):

<input type=“hidden” value=“M00o93H7pQ09L8X1t49cHY01Z5j4TT91fGfr/thank_you” name=“return_url”>
<input type=“hidden” value=“M00o93H7pQ09L8X1t49cHY01Z5j4TT91fGfr/thank_you” name=“return”>

The replacement happens regardless of whether {exp:foo:base_url} returns an empty or non-empty string.

So far I have found two ways to correct this. I can either remove the additional usages of {exp:foo:base_url} or add a bogus parameter to the tag.

Either way, this seems like a caching issue related to the use of a plugin in a tag parameter. Reproduced on 2.2 and 2.4.