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GHSA-W67G-2H6V-VJGQ: Phlexing on the XSS Filters: A Comedy of Errors in Ruby Views

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Phlexing on the XSS Filters: A Comedy of Errors in Ruby Views

Vulnerability ID: GHSA-W67G-2H6V-VJGQ
CVSS Score: 8.1
Published: 2026-02-06

Phlex, a Ruby library designed to write object-oriented HTML, suffered from a catastrophic failure in its Cross-Site Scripting (XSS) mitigation logic. Despite intended protections, the library failed to account for the chaotic nature of HTML parsing, allowing attackers to inject malicious JavaScript via four distinct vectors: attribute splatting, dynamic tag generation, HTML entity encoding, and SVG context manipulation. The vulnerability stems from an over-reliance on regular expressions for sanitization without understanding the full breadth of browser parsing behaviors.

TL;DR

Phlex attempted to sanitize HTML generation using blocklists but failed to account for browser parsing quirks. Attackers can bypass XSS protections using HTML entities (e.g., 'javascript:'), attribute injection via whitespace/slashes, or malicious dynamic tag names. Update immediately.

⚠️ Exploit Status: POC

Technical Details

  • Attack Vector: Network (Input to View Layer)
  • CVSS: 8.1 (High)
  • CWEs: CWE-79 (XSS), CWE-116 (Improper Encoding)
  • Exploit Status: PoC Available / Functional Exploit
  • Components: Attribute Splatting, Dynamic Tag Helper, Protocol Filter
  • Fix Complexity: Low (Library Update)

Affected Systems

  • Ruby on Rails applications using Phlex
  • Sinatra applications using Phlex
  • Hanami applications using Phlex
  • Any Ruby application using Phlex for HTML generation
  • phlex: < Feb 6 2026 Release (Fixed in: Feb 6 2026 Release)

Code Analysis

Commit: 9f56ad1

Main fix implementation for attribute splatting and entity decoding

UNSAFE_ATTRIBUTE_NAME_CHARS = %r([<>&"'/=\s\x00])
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Commit: 74e3d86

Refinement of fix across branches

...
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Exploit Details

  • GitHub Advisory: Proof of Concept provided in the advisory and test suite commits.

Mitigation Strategies

  • Update Phlex gem to patched version
  • Sanitize keys in attribute hashes before passing to Phlex
  • Strictly validate URLs using a proper URI parser, not regex
  • Avoid passing user input directly to dynamic tag generation

Remediation Steps:

  1. Check current version: bundle list phlex
  2. Update gem: bundle update phlex
  3. Verify update: Ensure version corresponds to post-Feb 6, 2026 release
  4. Review codebase for usage of tag with user input
  5. Review codebase for usage of double-splat ** attributes with user input

References

Read the full report for GHSA-W67G-2H6V-VJGQ on our website for more details including interactive diagrams and full exploit analysis.

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