Critical arbitrary code execution flaw reported in JavaScript expression parser expr-eval
Take action: If you use the expr-eval JavaScript library (or have dependencies that use it), be aware that uncontrolled user input can exploit a flaw and run arbitrary code. Sanitize user input as much as possible and plan a very quick switch to the actively maintained expr-eval-fork version 3.0.0 since the original package is not updated and still vulnerable to remote code execution. This is urgent if your application processes user-supplied mathematical expressions in calculators, educational tools, financial platforms, or AI systems like LangChain implementations.
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A critical security vulnerability is reported in the popular expr-eval JavaScript library. The expr-eval library, originally developed by Matthew Crumley, serves as a JavaScript expression parser and evaluator designed to provide safer alternatives to JavaScript's native eval() function when processing user-supplied mathematical expressions at runtime.
The library has become integral to numerous applications including online calculators, educational software suites, simulation tools, financial analysis platforms, and increasingly, generative AI systems that interpret mathematical expressions within user prompts.
The vulnerability is tracked as CVE-2025-12735 (CVSS score 9.8) and is caused by insufficient input validation within the expr-eval library's core Parser.evaluate() function. It fails to properly validate the variables or context object passed during expression evaluation and allows attackers to influence input fields processed by expr-eval to define arbitrary functions within the context object used by the parser. Once injected, these malicious function objects are invoked by the parser during the evaluation process, enabling attackers to execute system-level commands with the privileges of the running application.
The vulnerability affects two related npm packages that have achieved substantial adoption across the JavaScript development community:
- expr-eval: The original library with over 800,000 weekly downloads on the NPM package registry. The last stable release was published six years ago, and the project appears to be unmaintained by its original author, with no indication of when security patches will be merged or released.
- expr-eval-fork: An actively maintained fork created to address a prior Prototype Pollution vulnerability (Issue #266) that remained unresolved in the original repository. This fork processes over 80,000 weekly downloads and has successfully implemented the security fix in version 3.0.0.
More than 250 packages are dependent on expr-eval across the npm ecosystem including oplangchain, a JavaScript implementation of the popular LangChain framework used extensively in artificial intelligence application development. AI and natural language processing systems is particularly concerning frequently process untrusted user input, the exact attack vector exploited by CVE-2025-12735.
Applications utilizing expr-eval for mathematical expression evaluation in contexts where user input influences the variables or context object passed to Parser.evaluate() face immediate exploitation risk. This includes web-based calculators accepting mathematical expressions from users, educational platforms processing student-submitted formulas, financial modeling tools evaluating user-defined calculations, and AI systems interpreting mathematical content from text prompts or chat interactions.
A security fix has been released in Pull Request #288 in the expr-eval repository. The expr-eval-fork project has integrated the fix in version 3.0.0. Development teams using expr-eval-fork should upgrade to this patched version as their highest priority security action.
For users of the original expr-eval package, the situation is not so clear. The project's original maintainers have remained unresponsive, creating uncertainty about when, or if these patches will be merged into an official release. Developers and organizations currently depending on the original expr-eval package should plan migration to the actively maintained expr-eval-fork version 3.0.0 instead of waiting for an uncertain official patch release.
