The vulnerability resides in css_parser’s read_remote_file method, which processes URLs in CSS without any scheme allowlist or host filtering. An attacker can supply a CSS file that references any HTTP, HTTPS, or file:// URI, causing the parser to perform an HTTP request or local file read. A malicious HTTP redirect can turn a simple external retrieval into an arbitrary local file disclosure, exposing sensitive files on the server. The flaw is a classic SSRF (CWE‑918) that enables remote hosts to read local data.

The affected library is Premailer’s css_parser, versions 2.2.0 through 2.9.x. Any application that uses css_parser to parse attacker‑controlled CSS and supplies a base_uri option is vulnerable. The problem is fixed in 3.0.0 and later releases.

The CVSS score of 8.9 indicates high severity, EPSS score of less than 1% shows that exploitation is considered rare at present, and the vulnerability is not listed in CISA’s KEV catalog. Nonetheless, the flaw is exploitable from any web application that processes CSS from external or untrusted sources; the attack vector is a remote HTTP request or redirect, inferred from the description of recursive redirects and file:// handling.