Expression evaluation is the runtime processing of text or data as logic, such as formulas, predicates, or method calls. In enterprise software, frameworks may use it to route requests, read object fields, bind parameters, or compute access rules. This is useful because it makes applications flexible and configurable without changing code.
From a security perspective, expression evaluation becomes dangerous when untrusted input can influence the expression itself or the data it resolves against. An attacker may inject a payload that reads sensitive values, triggers unwanted methods, or changes control flow. Defenders treat these features as part of the attack surface and reduce risk by avoiding dynamic evaluation on user input, using allowlists, disabling unnecessary expression engines, and separating data from executable logic. In Java ecosystems, expression handling is common in frameworks and therefore needs careful review during patching and code audits.