Jenkins 2.218 and earlier, LTS 2.204.1 and earlier does not serve the X-Frame-Options: deny HTTP header on REST API responses to protect against clickjacking attacks. An attacker could exploit this by routing the victim through a specially crafted web page that embeds a REST API endpoint in an iframe and tricking the user into performing an action which would allow for the attacker to learn the content of that REST API endpoint.

Jenkins 2.219, LTS 2.204.2 now adds the X-Frame-Options: deny HTTP header to REST API responses, which prevents these types of clickjacking attacks.

Jenkins 2.218 and earlier, LTS 2.204.1 and earlier allowed users with Overall/Read access to view a JVM memory usage chart.

Jenkins 2.218 and earlier, LTS 2.204.1 and earlier exposed session identifiers on a user's detail object in the whoAmI diagnostic page.

Jenkins 2.218 and earlier, LTS 2.204.1 and earlier does not use a constant-time comparison when checking whether two HMACs are equal. This could potentially allow attackers to use statistical methods to obtain a valid HMAC for an attacker-controlled input value.

Jenkins 2.219, LTS 2.204.2 now uses a constant-time comparison when validating HMACs.

Jenkins 2.218 and earlier, LTS 2.204.1 and earlier supports two network discovery services (UDP multicast/broadcast and DNS multicast) by default.

The UDP multicast/broadcast service can be used in an amplification reflection attack, as very few bytes sent to the respective endpoint result in much larger responses: A single byte request to this service would respond with more than 100 bytes of Jenkins metadata which could be used in a DDoS attack on a Jenkins controller. Within the same network, spoofed UDP packets could also be sent to make two Jenkins controllers go into an infinite loop of replies to one another, thus causing a denial of service.

Jenkins 2.219, LTS 2.204.2 now disables both UDP multicast/broadcast and DNS multicast by default.

Administrators that need these features can re-enable them again by setting the system property hudson.DNSMultiCast.disabled to false (for DNS multicast) or the system property hudson.udp to 33848, or another port (for UDP broadcast/multicast). These are the same system properties that controlled whether these features were enabled in the past, so any instances explicitly enabling these features by setting these system properties will continue to have them enabled.

Jenkins 2.218 and earlier, LTS 2.204.1 and earlier did not use a constant-time comparison function for validating connection secrets, which could potentially allow an attacker to use a timing attack to obtain this secret.