The vulnerability stems from the LOOSE_HTTP_DATE_RE regular expression in cookiejar.py, which was modified in the patched commit to prevent ReDoS. The http2time function directly uses this regex to parse cookie expiration dates. The original regex structure allowed exponential backtracking on malicious input, as confirmed by the CVE description and commit message referencing bpo-38804 fixes. While iso2time also uses a similar regex (ISO_DATE_RE), the primary attack vector and CVE focus is on LOOSE_HTTP_DATE_RE in http2time handling Set-Cookie headers.