YOURLS is a self-hosted, customizable URL shortener written in PHP. From 1.5.1 until 1.10.4, YOURLS stores the HTTP Referer header through yourls_get_referrer(), yourls_sanitize_url_safe(), and yourls_log_redirect(), then aggregates the value in yourls-infos.php and passes the derived domain through yourls_get_domain(), yourls_stats_pie(), and yourls_google_array_to_data_table(). The chart builder concatenates labels into inline JavaScript without JavaScript-string escaping, so an unauthenticated attacker can poison the statistics of an existing short URL with a crafted referrer. When an administrator or public stats-page viewer opens the affected statistics page, attacker-controlled JavaScript executes in the YOURLS origin and can access admin-visible data, the API signature token, and privileged same-origin actions. This issue is fixed in version 1.10.4.

YOURLS, a self-hosted PHP URL shortener, contains a stored XSS vulnerability affecting versions 1.5.1 through 1.10.3. The application stores the HTTP Referer header without proper sanitization and later concatenates derived domain values into inline JavaScript within statistics pages without JavaScript-string escaping. An unauthenticated attacker can craft a malicious Referer header to poison the statistics of any existing short URL. When an administrator or public stats-page viewer opens the affected statistics page, attacker-controlled JavaScript executes within the YOURLS origin. This can expose admin-visible data, the API signature token, and allow privileged same-origin actions. The attack chain involves multiple functions: yourls_get_referrer(), yourls_sanitize_url_safe(), yourls_log_redirect(), yourls_get_domain(), yourls_stats_pie(), and yourls_google_array_to_data_table(). The vulnerability is fixed in YOURLS version 1.10.4.