Bun is a JavaScript runtime that lets JavaScript run outside the browser, similar to Node.js. It includes a runtime, package manager, and tooling in one fast executable. Security teams care about Bun because any runtime that can execute scripts on a developer or server machine can also be abused to launch malware, download payloads, or run hidden second-stage code.
In real attacks, threat actors may use Bun after an initial foothold in another language or package ecosystem. A malicious installer or startup hook can fetch Bun, then hand off to JavaScript for credential theft, token harvesting, or system discovery. Defenders should watch for unexpected runtime downloads, obfuscated JavaScript, and suspicious child processes created by package installs or build steps. In practice, Bun is not dangerous by itself; the risk comes from trust in a fast, flexible runtime being turned into an execution path for unreviewed code.