CVE-2026-12209 Overview
CVE-2026-12209 is a prototype pollution vulnerability affecting RubyLouvre avalon versions up to 2.2.10. The flaw resides in the Template Filter Handler component, specifically in the src/filters/index.js file. Attackers can manipulate object prototype attributes remotely without authentication or user interaction, which falls under [CWE-94] Improper Control of Generation of Code.
The exploit has been publicly disclosed and may be leveraged by threat actors. The vendor was contacted prior to public disclosure but did not respond. Organizations using avalon in client-side rendering pipelines should treat this as actionable risk given the publicly available proof-of-concept.
Critical Impact
Remote attackers can pollute object prototypes through manipulated template filter input, potentially altering application logic and integrity without authentication.
Affected Products
- RubyLouvre avalon versions up to and including 2.2.10
- Applications embedding the avalon Template Filter Handler (src/filters/index.js)
- Web frontends depending on avalon for MVVM data binding
Discovery Timeline
- 2026-06-15 - CVE-2026-12209 published to the National Vulnerability Database (NVD)
- 2026-06-17 - Last updated in NVD database
Technical Details for CVE-2026-12209
Vulnerability Analysis
The vulnerability resides in the avalon Template Filter Handler implemented in src/filters/index.js. The handler processes template filter expressions without validating the keys used to write properties on filter or context objects. Attackers can supply specially crafted filter strings that reach Object.prototype through unsafe property assignment patterns.
Property writes that traverse __proto__, constructor, or prototype chains modify the base Object prototype shared by all JavaScript objects in the runtime. This is classified as [CWE-94], improperly controlled modification of object prototype attributes. Polluted prototype properties can influence downstream logic, including authentication branches, configuration lookups, and template rendering.
The attack proceeds over the network without authentication or user interaction. The proof-of-concept published in the OriginSecurityX avalon-filter-rce repository demonstrates injection through the template filter parsing path.
Root Cause
The root cause is missing key sanitization during recursive property assignment in the filter handler. The parser walks user-controlled segments and writes them onto an internal object without rejecting dangerous keys such as __proto__ or nested constructor.prototype chains.
Attack Vector
An unauthenticated attacker submits a malicious filter expression to an avalon-powered template. The expression triggers prototype property writes that persist across the JavaScript runtime. Subsequent code paths consuming the polluted properties exhibit attacker-controlled behavior. Refer to the VulDB entry for CVE-2026-12209 for additional technical context.
Detection Methods for CVE-2026-12209
Indicators of Compromise
- Unexpected properties appearing on Object.prototype at runtime, such as injected flags read by downstream conditionals
- Inbound HTTP requests containing __proto__, constructor, or prototype tokens in template parameters or filter expressions
- Client-side errors or behavioral anomalies in avalon-rendered views following untrusted input
- Outbound connections from browser sessions to the IP referenced in the published proof-of-concept repository
Detection Strategies
- Inspect web application logs for query strings and POST bodies containing prototype-related keywords targeting avalon endpoints
- Add runtime guards that freeze Object.prototype and alert on assignment attempts in non-production builds
- Perform software composition analysis (SCA) to identify avalon versions at or below 2.2.10 across frontend dependencies
- Review code paths that pass user input into avalon filters registration or template compilation
Monitoring Recommendations
- Enable web application firewall (WAF) rules that block request payloads containing __proto__ and constructor.prototype patterns
- Forward frontend error telemetry to a centralized analytics pipeline for anomaly review
- Track dependency versions in CI/CD and alert when avalon resolves to a vulnerable release
How to Mitigate CVE-2026-12209
Immediate Actions Required
- Inventory all applications bundling RubyLouvre avalon and identify versions at or below 2.2.10
- Restrict or sanitize user input that flows into avalon template filters and bindings
- Deploy WAF signatures to block requests containing prototype pollution payloads
- Audit application code for downstream logic that trusts unvalidated object properties
Patch Information
No vendor patch is available. The vendor did not respond to disclosure attempts according to the published advisory. Maintainers and consumers should evaluate migration to an actively maintained MVVM framework or apply local forks that sanitize filter keys.
Workarounds
- Freeze Object.prototype at application startup using Object.freeze(Object.prototype) to block runtime pollution
- Reject request parameters containing __proto__, constructor, or prototype keys at the API gateway
- Wrap avalon filter registration in a validator that whitelists permitted property names
- Isolate avalon-rendered components from sensitive logic until a fix or replacement is in place
# Configuration example: block prototype pollution payloads at the edge (nginx)
if ($args ~* "(__proto__|constructor\[prototype\]|prototype\[)") {
return 403;
}
if ($request_body ~* "(__proto__|constructor\.prototype)") {
return 403;
}
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
