The vulnerability stems from struct field serialization in the API response. The affected structs (Basic, Digest, ClientTLS) had their sensitive fields (Users/Key) configured with 'json' tags that allowed inclusion in API responses. The patch fixes this by setting 'json:"-"' to exclude these fields from serialization. While these are struct fields rather than traditional functions, they represent the vulnerable data exposure points in the configuration handling logic. The high confidence comes from the direct correlation between the CWE-522 pattern and the patch changes to sensitive field serialization behavior.