Obfuscated javascript: XSS — Evasion Variants (href, style-expression, null-byte)

Detects obfuscated XSS payloads that use encoding evasion to bypass naive javascript: scheme filters. Covers three distinct evasion sub-techniques seen in the redteam corpus: (1) href with null-byte/whitespace-prefix before 'javascript:' — e.g. <a href="\x0Bjavascript:..."> where \x0B is vertical tab inserted to evade literal "javascript:" detection; (2) CSS expression() injection via style attribute — e.g. <div style="x:\xE2\x80\x89expression(javascript:alert(1))"> where Unicode whitespace is used to avoid keyword matching; (3) Double-javascript: to evade single-pass stripping — <a href="javascript:javascript:alert(1)">. These are distinct from the canonical ATR-2026-00571 (which covers onload/onerror event handlers). Maturity=experimental because the obfuscation patterns may need tightening to avoid FP on security research text that discusses them.

Response Actions

References

1. 01

   Hex-escaped whitespace/null prefix before javascript: scheme in href

   field: contentop: regex
2. 02

   Double javascript: scheme — evades single-pass javascript: stripping

   field: contentop: regex
3. 03

   CSS expression() with javascript payload or known exfil functions

   field: contentop: regex
4. 04

   IMG onerror with hex-escaped whitespace prefix (evasion variant)

   field: contentop: regex

Attack Examples (Rule Triggers)

1. <a href="\x0Bjavascript:javascript:alert(1)" id="fuzzelement1">test</a>

2. ABC<div style="x:\xE2\x80\x89expression(javascript:alert(1))">DEF

3. <img src=x onerror=\x11"javascript:alert(1)">

4. <a href="javascript:javascript:eval(document.cookie)">click</a>

Real-world attack payloads (sanitized). Used for regression testing.

Benign Examples (Rule Doesn't Trigger)

1. Security education text discussing evasion — no actual payload tags

   XSS filter evasion techniques include encoding the javascript: scheme with null bytes.

2. Legitimate https href

   <a href="https://example.com">Link text</a>

3. Unrelated coding request

   Can you help me write a Python function to sort a list?

4. Style with 'expression' as a plain word not as CSS expression()

   <div style="color: expression">text</div>

Known False Positive Contexts

• ▸Security research papers or blog posts that quote these evasion payloads in code blocks as examples
• ▸Penetration testing reports that describe XSS evasion techniques

title: "Obfuscated javascript: XSS — Evasion Variants (href, style-expression, null-byte)"
id: ATR-2026-01452
rule_version: 1
status: experimental
description: >
  Detects obfuscated XSS payloads that use encoding evasion to bypass naive
  javascript: scheme filters. Covers three distinct evasion sub-techniques seen
  in the redteam corpus:
  (1) href with null-byte/whitespace-prefix before 'javascript:' — e.g.
      <a href="\x0Bjavascript:..."> where \x0B is vertical tab inserted to
      evade literal "javascript:" detection;
  (2) CSS expression() injection via style attribute — e.g.
      <div style="x:\xE2\x80\x89expression(javascript:alert(1))"> where Unicode
      whitespace is used to avoid keyword matching;
  (3) Double-javascript: to evade single-pass stripping —
      <a href="javascript:javascript:alert(1)">.
  These are distinct from the canonical ATR-2026-00571 (which covers onload/onerror
  event handlers). Maturity=experimental because the obfuscation patterns may
  need tightening to avoid FP on security research text that discusses them.
author: "ATR Community"
date: "2026/06/12"
schema_version: "0.1"
detection_tier: pattern
maturity: experimental
severity: high

references:
  owasp_llm:
    - "LLM01:2025 - Prompt Injection"
  owasp_agentic:
    - "ASI01:2026 - Agent Goal Hijack"
  mitre_atlas:
    - "AML.T0051 - LLM Prompt Injection"
  cwe:
    - "CWE-79"
  external:
    - "https://cheatsheetseries.owasp.org/cheatsheets/XSS_Filter_Evasion_Cheat_Sheet.html"

compliance:
  owasp_llm:
    - id: LLM01:2025
      context: "Obfuscated javascript: XSS evades literal-keyword filters; this rule closes the evasion gap left by simpler XSS detection."
      strength: primary
  eu_ai_act:
    - article: "15"
      context: "Article 15 requires robustness against adversarial attempts including obfuscated XSS variants that evade standard filtering."
      strength: primary
  nist_ai_rmf:
    - subcategory: "MS.2.7"
      context: "NIST AI RMF MEASURE 2.7 (security and resilience) is supported by detection of obfuscated XSS evasion techniques."
      strength: primary
  iso_42001:
    - clause: "8.1"
      context: "ISO/IEC 42001 Clause 8.1 (operational planning and control) is operationalised by detecting obfuscated XSS in agent inputs."
      strength: primary
    - clause: "6.2"
      context: "ISO/IEC 42001 Clause 6.2 calls for risk treatment of known XSS evasion patterns."
      strength: secondary

tags:
  category: context-exfiltration
  subcategory: xss-injection
  scan_target: mcp
  confidence: medium

agent_source:
  type: llm_io
  framework:
    - any
  provider:
    - any

detection:
  condition: any
  false_positives:
    - "Security research papers or blog posts that quote these evasion payloads in code blocks as examples"
    - "Penetration testing reports that describe XSS evasion techniques"
  conditions:
    # Layer 1: Null-byte / whitespace prefix before javascript: in href
    # Covers: href="\x0Bjavascript:..." where \x0B is vertical tab
    - field: content
      operator: regex
      value: "(?i)href\\s*=\\s*[\"']?\\\\x[0-9a-fA-F]{2}javascript:"
      description: "Hex-escaped whitespace/null prefix before javascript: scheme in href"

    # Layer 2: Double javascript: (single-pass stripping evasion)
    # <a href="javascript:javascript:alert(1)">
    - field: content
      operator: regex
      value: "(?i)href\\s*=\\s*[\"']?javascript:\\s*javascript:"
      description: "Double javascript: scheme — evades single-pass javascript: stripping"

    # Layer 3: CSS expression() with javascript: payload
    # <div style="x:\xE2\x80\x89expression(javascript:alert(1))">
    - field: content
      operator: regex
      value: "(?i)style\\s*=\\s*[\"'][^\"']{0,100}expression\\s*\\([^)]{0,100}(javascript:|alert|document\\.cookie|fetch)"
      description: "CSS expression() with javascript payload or known exfil functions"

    # Layer 4: img src with hex-escaped whitespace before onerror
    # <img src=x onerror=\x11"javascript:alert(1)">
    - field: content
      operator: regex
      value: "(?i)<img\\b[^>]{0,100}onerror\\s*=\\s*\\\\x[0-9a-fA-F]{2}"
      description: "IMG onerror with hex-escaped whitespace prefix (evasion variant)"

response:
  actions:
    - alert
    - snapshot
  auto_response_threshold: high
  message_template: >
    [ATR-2026-01452] HIGH: Obfuscated javascript: XSS evasion detected. Input
    contains encoding tricks (null-byte prefix, double-javascript:, CSS expression)
    to bypass javascript: scheme filters.

test_cases:
  true_positives:
    - input: "<a href=\"\\x0Bjavascript:javascript:alert(1)\" id=\"fuzzelement1\">test</a>"
      expected: triggered
      description: "Null-byte prefix + double javascript: evasion (evasion-xss corpus)"
    - input: "ABC<div style=\"x:\\xE2\\x80\\x89expression(javascript:alert(1))\">DEF"
      expected: triggered
      description: "CSS expression with unicode-whitespace prefix (evasion-xss corpus)"
    - input: "<img src=x onerror=\\x11\"javascript:alert(1)\">"
      expected: triggered
      description: "IMG onerror with hex-escape evasion (evasion-xss corpus)"
    - input: "<a href=\"javascript:javascript:eval(document.cookie)\">click</a>"
      expected: triggered
      description: "Double javascript: with eval"
  true_negatives:
    - input: "XSS filter evasion techniques include encoding the javascript: scheme with null bytes."
      expected: not_triggered
      description: "Security education text discussing evasion — no actual payload tags"
    - input: "<a href=\"https://example.com\">Link text</a>"
      expected: not_triggered
      description: "Legitimate https href"
    - input: "Can you help me write a Python function to sort a list?"
      expected: not_triggered
      description: "Unrelated coding request"
    - input: "<div style=\"color: expression\">text</div>"
      expected: not_triggered
      description: "Style with 'expression' as a plain word not as CSS expression()"