Eval injection is a security flaw in which untrusted input is passed into a dynamic evaluation function and treated as code. Instead of handling data safely, the application executes attacker-controlled content, which can lead to arbitrary command execution, data theft, or full server compromise. This is especially dangerous in web applications that use eval-style functions, template runtimes, expression parsers, or scripting features to process user input.
In real attacks, eval injection often appears when a request parameter reaches a code path that was meant for trusted logic only. A small parsing mistake or weak validation can turn a harmless-looking field into a remote code execution vector, sometimes even before login. Defenders should avoid dynamic evaluation of user input, replace it with safe parsers or allowlisted logic, and review templates and expression handlers for unsafe code paths. Monitoring for unusual requests and patching vulnerable components are also essential.