CVE-2020-35730

Roundcube Webmail contains a cross-site scripting (XSS) flaw (CWE-79) in the link-reference handling of rcube_string_replacer.php, where the linkref_addindex function mishandles JavaScript embedded in a link element of a plain-text email. An attacker triggers the flaw simply by sending a crafted plain-text message to a victim; when the message is processed/displayed in the Roundcube interface, the embedded script executes in the context of the victim's webmail session. Successful exploitation can lead to session hijacking, theft of webmail cookies or credentials, and arbitrary actions in the victim's mailbox. Any deployment of Roundcube Webmail is affected, which includes self-hosted instances and webmail offered by hosting providers, ISPs, and universities. Although no public proof-of-concept is known, CISA added this vulnerability to the Known Exploited Vulnerabilities catalog on 2023-06-22, confirming exploitation in the wild; ransomware association is unknown, and no CVSS score is yet available, though EPSS puts 30-day exploitation probability at 32.7% (98th percentile).

What to do: Apply the vendor's updated Roundcube release per CISA's required action (updates per vendor instructions); since no specific fixed versions appear in this data, install the latest patched release of your deployed 1.x branch and verify with the vendor advisory. Check webmail servers for processing of plain-text messages with link-reference elements and review logs for anomalous webmail sessions; treat KEV-listed status as evidence of active exploitation and prioritize internet-exposed Roundcube instances.