CVE-2021-21659: XXE vulnerability in Jenkins URLTrigger Plugin

The vulnerability stems from insecure XML parser configuration in the URLTrigger Plugin. The advisory explicitly states the parser wasn't configured to prevent XXE, and the fix involved disabling external entity resolution. The most likely vulnerable functions are those handling XML content parsing from URLs. The 'checkURL' method would initiate content retrieval/processing, while 'XMLContentType#parseContent' would directly handle XML parsing. Both would require secure parser configuration (disabling DTDs/external entities) to mitigate XXE, which was missing in vulnerable versions.