Jenkins Convert To Pipeline Plugin 1.0 and earlier uses basic string concatenation to convert Freestyle projects' Build Environment, Build Steps, and Post-build Actions to the equivalent Pipeline step invocations, allowing attackers able to configure Freestyle projects to prepare a crafted configuration that injects Pipeline script code into the (unsandboxed) Pipeline resulting from a convertion by Jenkins Convert To Pipeline Plugin.
https://www.jenkins.io/security/advisory/2023-03-21/#SECURITY-2966