The eval function is a language feature that interprets a string as code and executes it at runtime. It is frequently abused for code injection when untrusted input reaches it, so defenders should avoid it, validate inputs, and inspect code paths that build executable strings.
What the eval Function Does
The eval function turns a string into executable code at runtime, which makes it powerful for dynamic behaviour but dangerous when that string can be influenced by users, configuration, or other external input.
Its main security concern is code injection: if an attacker can shape the string, they may be able to run arbitrary expressions or commands in the application's execution context. That risk is especially serious when the code path has access to secrets, files, network calls, or privileged runtime capabilities.
Why eval Is Dangerous
eval is risky because it collapses the boundary between data and code. Once that boundary is gone, ordinary input-validation mistakes can become execution flaws, and seemingly harmless text fields, query parameters, or template fragments can become code carriers.
The danger is not limited to obvious attacker-controlled input. Any path that assembles executable strings from partially trusted values can introduce logic abuse, unexpected side effects, and hard-to-review behaviour that evades normal static analysis.
Common Secure Alternatives
In most cases, the safer design is to replace eval with explicit parsing, mapping, or dispatch logic. A fixed lookup table, expression parser, or domain-specific interpreter preserves control over what operations are allowed and keeps input as data instead of code.
When dynamic behaviour is truly required, limit the accepted grammar, constrain the available operations, and keep the execution context as small as possible. That reduces the blast radius if the string is malformed or malicious.
How Teams Review eval Usage
Teams should treat every eval call as a high-value review point. The important question is not only whether the input is validated, but whether the feature can be rewritten so that runtime code generation is unnecessary.
For code paths that cannot be removed quickly, reviewers should trace where the string originates, what privileges the runtime has, and whether the evaluated content can reach files, environment variables, sessions, or administrative APIs.
Risk and Threat Considerations
When eval is fed by untrusted or weakly controlled input, the failure mode is direct code execution rather than a narrow logic error. That makes the issue attractive to attackers because a single injection point can convert input handling into full application compromise.
Failure mechanism: An attacker supplies text that is parsed as code, and the interpreter executes it inside the application's trust boundary, often with access to the same data and privileges as the parent process.
Impact: The result can include data theft, session abuse, privilege escalation, malicious outbound requests, or broader host compromise depending on what the runtime can reach.
Standards & Framework Alignment
This section maps relevant standards and security frameworks to the operational risks and controls described in this guidance.
MITRE ATT&CK addresses the attack and risk surface, while NIST SP 800-53 Rev 5, OWASP ASVS and CIS Controls v8 set the governance and control requirements practitioners need to meet.
| Framework | Control / Reference | Relevance |
|---|---|---|
| NIST SP 800-53 Rev 5 | SI-10 — Information Input Validation | eval becomes dangerous when input can be interpreted as code rather than data. |
| AC-6 — Least Privilege | A compromised eval path executes with the runtime's existing authority and reach. | |
| Recommendation — Validate and constrain all inputs before they can reach any code-execution path. Limit the runtime privileges available to any process that may execute dynamic code. | ||
| OWASP ASVS | V15 — Secure Coding and Architecture | ASVS covers unsafe code construction patterns that create injection-prone logic. |
| Recommendation — Remove dynamic code execution patterns and replace them with explicit, reviewable logic. | ||
| MITRE ATT&CK | T1059 — Command and Scripting Interpreter | eval maps to interpreter-based execution abused for code execution and scripting abuse. |
| Recommendation — Detect and restrict interpreter abuse paths that can turn injected text into execution. | ||
| CIS Controls v8 | CIS-16 — Application Software Security | Application security safeguards are needed to prevent dangerous runtime code evaluation. |
| Recommendation — Review application code for dynamic execution patterns and remove them where possible. | ||
Practitioner Guidance
Why practitioners should care: eval is rarely justified when a safer control-flow or expression mechanism exists, and its presence usually signals a maintenance and security liability that deserves design-level scrutiny.
What to watch for: Pay close attention to strings built from request parameters, configuration values, templates, rule engines, and plugin inputs, because those are common places where executable text is assembled indirectly.
Practitioner takeaway: If a code path uses eval, assume it is security-sensitive until proven otherwise, and prefer eliminating the pattern rather than trying to “sanitize” arbitrary code.
Related resources from NHI Mgmt Group
- What is the difference between running an eval and building a custom completion function?
- What is the difference between function calling and MCP for enterprise security?
- When does MCP make more sense than function calling?
- What is the difference between application RBAC and function-level permissions for MCP?
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Reviewed and updated by the NHIMG editorial team on October 17, 2026.
NHI Mgmt Group — the #1 independent authority on Non-Human Identity, IAM, and Agentic AI security. nhimg.org