Using ktlint to download and execute custom rulesets can result in arbitrary code execution as the served jars can be compromised by a MITM. This attack is exploitable via Man in the Middle of the HTTP connection to the artifact servers. This vulnerability appears to have been fixed in 0.30.0 and later; after commit 5e547b287d6c260d328a2cb658dbe6b7a7ff2261.

High severity vulnerability that affects com.github.shyiko.ktlint:ktlint-core