CVE-2025-68156

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.

Workarounds

For users who cannot immediately upgrade, the following mitigations are recommended:

• Ensure that evaluation environments cannot contain cyclic references.
• Validate or sanitize externally supplied data structures before passing them to Expr.
• Wrap expression evaluation with panic recovery to prevent a full process crash (as a last-resort defensive measure).

These workarounds reduce risk but do not fully eliminate the issue without the patch.

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 application, constituting a denial of service.

The application allocates resources such as memory, threads, or file descriptors based on untrusted input without enforcing a cap. Typical impact: resource exhaustion leading to denial of service.

CVE-2025-68156 has a CVSS score of 7.5 (High). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (1.17.7); upgrading removes the vulnerable code path.

Kodem intelligence

The issue has been fixed in the v1.17.7 versions of Expr.

The patch introduces a maximum recursion depth limit for affected builtin functions. When this limit is exceeded,
evaluation aborts gracefully and returns a descriptive error instead of panicking.

Additionally, the maximum depth can be customized by users via builtin.MaxDepth, allowing applications with legitimate
deep structures to raise the limit in a controlled manner.

Users are strongly encouraged to upgrade to the patched release, which includes both the recursion guard and
comprehensive test coverage to prevent regressions.

1. What is CVE-2025-68156? CVE-2025-68156 is a high-severity allocation of resources without limits or throttling vulnerability in github.com/expr-lang/expr (go), affecting versions < 1.17.7. It is fixed in 1.17.7. The application allocates resources such as memory, threads, or file descriptors based on untrusted input without enforcing a cap.
2. How severe is CVE-2025-68156? CVE-2025-68156 has a CVSS score of 7.5 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of github.com/expr-lang/expr are affected by CVE-2025-68156? github.com/expr-lang/expr (go) versions < 1.17.7 is affected.
4. Is there a fix for CVE-2025-68156? Yes. CVE-2025-68156 is fixed in 1.17.7. Upgrade to this version or later.
5. Is CVE-2025-68156 exploitable, and should I be worried? Whether CVE-2025-68156 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2025-68156 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2025-68156? Upgrade github.com/expr-lang/expr to 1.17.7 or later.