anti-trojan-source

Detect trojan source attacks that employ unicode bidi attacks to inject malicious code

About

Detects cases of trojan source attacks that employ unicode bidi attacks to inject malicious code, as well as other attacks that use confusable characters (such as glassworm attacks). The tool uses both an explicit list of dangerous Unicode characters and category-based detection to catch invisible characters by their Unicode category (Format and Control categories).

anti-trojan-source-example.mov

If you're using ESLint:

• See: eslint-plugin-anti-trojan-source for a purpose-bulit plugin to detect anti-trojan characters.
• This plugin inspired work to create an anti-trojan rule detect-bidi-characters in eslint-plugin-security and if you're already using that security plugin then it is advised to turn on that rule.

Detection Capabilities

anti-trojan-source provides comprehensive protection by detecting:

• 281 explicit confusable scalars — bidirectional controls, zero-width characters, BMP variation selectors, a small set of non-Cf/Cc invisibles (Hangul fillers, U+034F), plus 240 supplementary variation selectors (U+E0100–U+E01EF)
• All Unicode Format characters (Cf category) — invisible formatting characters by category (including Unicode tag letters used for ASCII smuggling / hidden payloads)
• All Unicode Control characters (Cc category) — except commonly-used whitespace (TAB, LF, CR)

Category-based Cf/Cc detection keeps the tool future-proof as Unicode adds new format or control code points. The explicit list covers characters that matter for security but are not Cf/Cc (e.g. BMP variation selectors are Mn, not Cf).

Scope

This project scans decoded Unicode text — the string you get after reading a UTF‑8 (or other Unicode encoding) file the usual way. It does not inspect raw bytes, URLs, or tokenizer-specific behavior.

In scope

Topic	Detection approach
Trojan Source (bidi embeddings, overrides, isolates, PDF, etc.)	Cf ranges + explicit list
Zero-width / word joiner / BOM / soft hyphen (where Cf or listed)	Cf + explicit list
Unicode Tags block (U+E0001, U+E0020–U+E007F) — invisible ASCII-shaped payloads (background)	Cf
Variation selectors (BMP U+FE00–U+FE0F + supplement U+E0100–U+E01EF)	Explicit list (Mn in Unicode, not Cf)
Strict explicit blocklist of a few non-Cf/Cc scalars that often render invisibly (U+034F, U+115F, U+1160, U+3164)	Explicit list only
Any other Format (Cf) or Control (Cc) code point	Category tables (Cc minus TAB/LF/CR)
Dangerous confusables on the maintained explicit list (e.g. NO-BREAK SPACE)	Explicit list

Out of scope

Topic	Reason
Full homoglyph / mixed-script confusable-IDN databases (“every Cyrillic lookalike of Latin”)	Requires a large, policy-heavy confusables data set; we only flag listed confusables plus all Cf/Cc
UTF‑8 “sneaky” byte patterns, overlong encodings, non-Unicode steganography	Needs byte-level analysis, not scalar-by-scalar Unicode
URL / percent-encoded layers, HTML entities	Decode/normalize elsewhere first
Full rendering, grapheme clusters, locale-specific display rules	Tooling is scalar-based and intentionally simple
Whether a finding is malicious	High-signal alert for human review

Invisible Characters Support Matrix

The following table summarizes attack styles versus what this tool flags:

Attack Type	Supported	Notes
Trojan Source	✅	Bidi / format controls per trojansource.codes.
Glassworm / confusable identifiers	✅ (partial)	Flags explicit confusables and all Cf/Cc — not a complete homoglyph alphabet.
Unicode tag / “ASCII smuggling”	✅	Tag letters are Cf; see Embrace The Red.
Extended variation selectors	✅	U+E0100–U+E01EF on explicit list.
Category-based Cf / Cc	✅	Future-proof for new format/control code points.
Invisible letters (strict list)	✅	U+034F, Hangul fillers — explicit blocklist only.

Why is Confusable Unicode Character detection important?

The following publication on the topic of unicode characters attacks, dubbed Trojan Source: Invisible Vulnerabilities, has caused a lot of concern from potential supply chain attacks where adversaries are able to inject malicious code into the source code of a project, slipping by unseen in the code review process. This project expands on that to detect other forms of confusable characters that can be used in similar attacks.

For more information on the topic, you're welcome to read on the official website trojansource.codes and the following source code repository which contains the source code of the publication.

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Table of Contents

• About
  • Detection Capabilities
  • Scope
  • Invisible Characters Support Matrix
  • Why is Confusable Unicode Character detection important?
• Use as a CLI
  • Detect confusable characters using file globbing
  • Detect confusable characters using file paths
  • Detect confusable characters by piping input
  • Verbose output mode
  • JSON output mode
• Use as an eslint plugin
• Use as a library
  • Simple boolean check
  • Detailed findings
• Use as a pre-commit hook
• References
• Contributing
• Author

---

Use as a CLI

anti-trojan-source is an npm package that supports detecting files that contain confusable unicode characters in them, per the research.

Detect confusable characters using file globbing

The following command will detect all files that contain confusable unicode characters in them based on the file matching pattern that was provided to it:

npx anti-trojan-source --files='src/**/*.js'

If it doesn't find anything it will return with a 0 exit code and print to stdout:

[✓] No confusable characters detected

Detect confusable characters using file paths

npx anti-trojan-source '/src/index.js' '/src/helper.js'

If it found any matching confusable unicode characters, it will return with an exit code of 1 and print to stderr:

[x] Detected cases of confusable characters in the following files:
|
 - /src/index.js
 - /src/helper.js
Note: For backward compatibility, `hasTrojanSource({...})` is still exported as an alias to `hasConfusables({...})`. It is deprecated and will be removed in a future major version. Prefer `hasConfusables` going forward.

Detect confusable characters by piping input

If you just run npx anti-trojan-source and pipe in a file contents, it will detect the confusable unicode characters in that file:

cat /src/index.js | npx anti-trojan-source

Verbose output mode

Use the --verbose (or -v) flag to get detailed information about each detected character, including line and column numbers, character names, and Unicode code points:

npx anti-trojan-source --files='src/**/*.js' --verbose

Example output:

[x] Detected cases of trojan source in the following files:
| 
 - src/utils.js

   Line 12:34 - U+200B ZERO WIDTH SPACE [Cf (Format)]
   Snippet: const value = getUserInput()
   Line 45:10 - U+202E RIGHT-TO-LEFT OVERRIDE [Cf (Format)]
   Snippet: if (isAdmin) { // Check permissions

This mode is particularly useful for:

• Code reviews: Quickly identify where invisible characters are located
• Debugging: Understand which specific characters are causing issues
• Security audits: Get detailed reports of all suspicious characters

JSON output mode

Use the --json (or -j) flag to get machine-readable JSON output, perfect for CI/CD integration and automated processing:

npx anti-trojan-source --files='src/**/*.js' --json

Example output:

[
  {
    "file": "src/utils.js",
    "findings": [
      {
        "line": 12,
        "column": 34,
        "codePoint": "U+200B",
        "name": "ZERO WIDTH SPACE",
        "category": "Cf (Format)",
        "snippet": "const value = getUserInput()"
      }
    ]
  }
]

This mode enables:

• CI/CD integration: Parse results programmatically in your pipeline
• Custom reporting: Build your own reporting tools on top of the detection
• Automated workflows: Trigger specific actions based on findings

Use as an eslint plugin

Refer to the ESLint Plugin for this CLI and the README on that repository which clearly explains how to set it up: eslint-plugin-anti-trojan-source.

Use as a library

Simple boolean check

To use it as a library and pass it file contents to detect (backward compatible):

import { hasConfusables } from 'anti-trojan-source'

const isDangerous = hasConfusables({
  sourceText: 'if (accessLevel != "user‮ ⁦// Check if admin⁩ ⁦") {'
})

console.log(isDangerous) // true or false

hasConfusables returns a boolean when called without the detailed option.

Detailed findings

Get comprehensive information about detected characters including their location, names, and categories:

import { hasConfusables } from 'anti-trojan-source'

const findings = hasConfusables({
  sourceText: 'const value\u200b = 123', // Contains ZERO WIDTH SPACE
  detailed: true
})

console.log(findings)
// [
//   {
//     line: 1,
//     column: 12,
//     codePoint: "U+200B",
//     name: "ZERO WIDTH SPACE",
//     category: "Cf (Format)",
//     snippet: "const value = 123"
//   }
// ]

Each finding includes:

• line: Line number where the character was found
• column: Column number where the character was found
• codePoint: Unicode code point (e.g., "U+200B")
• name: Descriptive name of the character
• category: Unicode category or classification
• snippet: Context from the line (up to 80 characters)

You can also check multiple files at once:

import { hasConfusablesInFiles } from 'anti-trojan-source'

const results = hasConfusablesInFiles({
  filePaths: ['src/index.js', 'src/utils.js'],
  detailed: true // Optional: get detailed findings
})

console.log(results)
// [
//   {
//     file: "src/index.js",
//     findings: [ /* array of findings */ ]
//   }
// ]

Use as a pre-commit hook

To add this tool to your project as a pre-commit hook, try this sample configuration in .pre-commit-config.yaml:

repos:
  - repo: https://github.com/lirantal/anti-trojan-source
    rev: v1.8.1  # choose the release you want
    hooks:
      - id: anti-trojan-source

References

• Hiding and finding text with Unicode Tags — Unicode tag letters, LLM / review bypass, and links to specs (Embrace The Red).
• ASCII Smuggler — encode/decode tool for tags, variant selectors, and related invisible patterns (Embrace The Red).
• Trojan Source — original bidi / trojan source research and paper.

Contributing

Please consult CONTRIBUTING for guidelines on contributing to this project.

Author

anti-trojan-source © Liran Tal, Released under the Apache-2.0 License.