The core vulnerability stems from the upload_log_data XMLRPC endpoint and its helper __upload_file. The commit diff shows critical security additions: 1) A new __validate_log_data_params to check types and filename validity 2) Path normalization with os.path.normpath 3) Directory containment checks in __upload_file. Pre-patch, attackers could supply filenames like '../../etc/passwd' which bypassed the weak str.translate('/','+') sanitization. The CWE-434 mapping and GHSA description directly implicate these file write functions. The test_arbitrary_file_write_1 in security_test.py confirms exploitation via path traversal.