YOURLS has stored XSS in referrer statistics chart via crafted Referer header

Summary

YOURLS stores the HTTP Referer header for short URL redirects and later renders aggregated referrer domains in the per-link statistics page. An unauthenticated attacker can send a crafted Referer header to any existing short URL. When an authenticated administrator or stats-page viewer opens that short URL's statistics page, the crafted referrer is embedded into Google Charts JavaScript without JavaScript-string escaping, causing stored cross-site scripting.

This is reachable in default private installations when authenticated users view stats, and in documented configurations where YOURLS_PRIVATE_INFOS is set to false to make statistics pages public.

Details

The vulnerable source-to-sink path is:

HTTP Referer header
  -> yourls_get_referrer()
  -> yourls_sanitize_url_safe()
  -> yourls_log_redirect()
  -> log table referrer column
  -> yourls-infos.php referrer aggregation
  -> yourls_get_domain()
  -> yourls_stats_pie()
  -> yourls_google_array_to_data_table()
  -> inline JavaScript

Relevant code:

• includes/functions.php:240-243: yourls_get_referrer() reads $_SERVER['HTTP_REFERER'], calls yourls_sanitize_url_safe(), and truncates the result to 200 bytes.

• includes/functions.php:294-307: yourls_redirect_shorturl() calls yourls_log_redirect() before redirecting the visitor.

• includes/functions.php:516-545: yourls_log_redirect() stores the sanitized referrer in the log table.

• yourls-infos.php:60-82: the statistics page reads logged referrers and groups them by yourls_get_domain($row->referrer).

• yourls-infos.php:493-498: the statistics page passes referrer domains to yourls_stats_pie().

• includes/functions-infos.php:338-355: yourls_google_array_to_data_table() manually concatenates labels into JavaScript as ['$label', ...] without escaping single quotes, backslashes, or other JavaScript string metacharacters.

• includes/functions-formatting.php:141-143 and includes/functions-formatting.php:555-609: yourls_sanitize_url_safe() removes some unsafe characters and CRLF sequences, but still permits the characters needed for JavaScript-string breakout, including ', [, ], ,, and parentheses.

A crafted referrer host such as:

x',1],['marker',alert(1)],['z.tld

survives sanitization when used in a URL like:

http://x',1],['marker',alert(1)],['z.tld/path

It is then rendered into chart JavaScript like:

var data = google.visualization.arrayToDataTable([
    ['x',1],['marker',alert(1)],['z.tld',1]
]);