Executive Summary

CVE-2026-12209 is a vulnerability in the RubyLouvre/avalon JavaScript library versions up to 2.2.10. It arises from insecure handling of template filters stored in a plain object without proper validation, allowing attackers to manipulate the object's prototype chain.

Specifically, attackers can use filter names like '__proto__' or 'constructor' to access and execute Object.prototype properties as filter functions. Since the template parser uses 'new Function()' to compile expressions, this flaw enables execution of arbitrary JavaScript code remotely.

This prototype escape and remote code execution (RCE) vulnerability is due to insecure filter storage and lookup mechanisms in files such as src/filters/index.js and src/parser/index.js.

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Impact Analysis

This vulnerability allows remote attackers to execute arbitrary JavaScript code on systems using the affected RubyLouvre/avalon library versions. This can lead to unauthorized actions such as running malicious commands or processes.

For example, an attacker could spawn system processes like a calculator application, demonstrating the ability to execute arbitrary commands, which could be leveraged for more harmful activities such as data theft, system compromise, or further network attacks.

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Detection Guidance

This vulnerability affects RubyLouvre/avalon JavaScript library versions v0.9.9 through v2.2.10, specifically involving insecure filter storage and lookup mechanisms in the files src/filters/index.js and src/parser/index.js.

To detect if your system is vulnerable, you can check the version of the avalon2 npm package used in your project.

• Run the command `npm list avalon2` in your project directory to see the installed version.
• Search your codebase for usage of the vulnerable files or functions, for example: `grep -r 'src/filters/index.js' ./` or `grep -r 'new Function' ./` to identify potential vulnerable code.

Network detection of exploitation attempts may be difficult since the attack involves remote code execution via template parsing, but monitoring for unusual process executions like spawning 'calc.exe' or other suspicious commands on affected hosts could help.

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Mitigation Strategies

Immediate mitigation steps include:

• Identify and upgrade the RubyLouvre/avalon library to a version later than 2.2.10 if available, or apply patches if provided.
• If upgrading is not possible, restrict or sanitize template inputs to prevent attacker-controlled content from reaching the vulnerable filter functions.
• Avoid using `new Function()` with untrusted input in template parsing to prevent arbitrary code execution.
• Monitor and restrict execution of suspicious processes on affected systems.

Since the vendor has not responded and the exploit is publicly disclosed, applying these mitigations promptly is critical.

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Compliance Impact

The vulnerability allows remote code execution through prototype pollution and insecure template filter handling, which could lead to unauthorized system access or data manipulation.

Such unauthorized access or manipulation may result in breaches of data confidentiality and integrity, potentially violating compliance requirements under standards like GDPR or HIPAA that mandate protection of personal and sensitive data.

However, the provided information does not explicitly detail the impact on compliance with these regulations.

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