The vulnerability stems from the regex construction in index.js's exported function. The commit diff shows replacement of native RegExp with RE2 (a backtracking-resistant engine), confirming the original regex was vulnerable. The function builds patterns with nested quantifiers and alternations (visible in variables like protocol/auth/host) that could cause exponential backtracking. Attack vectors in test fixtures demonstrate exploitation via overly long auth components and domains. The patch's core mitigation was switching regex engines rather than pattern adjustments, indicating the regex-building function itself was the vulnerable component.