Affected: Go: github.com/expr-lang/expr

Several builtin functions in Expr, including `flatten`, `min`, `max`, `mean`, and `median`, perform recursive traversal over user-provided data structures without enforcing a maximum recursion depth.

If the evaluation environment contains **deeply nested** or **cyclic** data structures, these functions may recurse indefinitely until exceed the Go runtime stack limit. This results in a **stack overflow panic**, causing the host application to crash.

While exploitability depends on whether an attacker can influence or inject cyclic or pathologically deep data into the evaluation environment, this behavior represents a denial-of-service (DoS) risk and affects overall library robustness. Instead of returning a recoverable evaluation error, the process may terminate unexpectedly.

### Impact

In affected versions, evaluation of expressions that invoke certain builtin functions on untrusted or insufficiently validated data structures can lead to a **process-level crash** due to stack exhaustion.

This issue is most relevant in scenarios where:

* Expr is used to evaluate expressions against externally supplied or dynamically constructed environments. * Cyclic references (directly or indirectly) can be introduced into arrays, maps, or structs. * There are no application-level safeguards preventing deeply nested input data.

In typical use cases with controlled, acyclic data, the issue may not manifest. However, when present, the resulting panic can be used to reliably crash the a…