CVE-2023-43770

Roundcube Webmail versions before 1.4.14, 1.5.x before 1.5.4, and 1.6.x before 1.6.3 contain a persistent cross-site scripting (XSS) flaw (CWE-79) caused by how program/lib/Roundcube/rcube_string_replacer.php converts plain text into clickable links. An attacker sends a text/plain email containing crafted links; when the recipient views the message, the crafted link text is turned into HTML that runs attacker-controlled script, which persists and executes in the victim's webmail session. Successful exploitation lets the attacker execute JavaScript with the victim's session, enabling mailbox access, theft of session credentials, and actions performed as the user (CVSS 6.1, scope-changed with limited confidentiality and integrity impact). Anyone running an affected Roundcube instance is exposed, including the roundcube package shipped with Debian Linux. The flaw is being actively exploited: CISA added it to the Known Exploited Vulnerabilities catalog on 2024-02-12, and EPSS assigns a 58.5% probability of exploitation within 30 days (99th percentile).

What to do: Upgrade Roundcube to 1.4.14, 1.5.4, or 1.6.3 (or later, per branch), or install the updated roundcube package on Debian. Because the bug is in CISA's KEV catalog, U.S. federal agencies must apply the vendor fix by the catalog due date, and other defenders should prioritize patching internet-facing webmail servers. Review webmail access logs for suspicious message views or account activity, and consider forcing session re-authentication for accounts that opened crafted plain-text messages.