Agent skill

Xss Prevention

XSS attack prevention with input sanitization, output encoding, Content Security Policy.

MITAuto-check passedSecurity

Install Xss Prevention

skills CLI

$ npx skills add secondsky/claude-skills --skill xss-prevention -a claude-code

Project install by default; add -g for ~/.claude/skills/.

GitHub CLI

$ gh skill install secondsky/claude-skills xss-prevention --agent claude-code

Project scope by default; add --scope user for a personal install. Needs GitHub CLI 2.90.0 or later (public preview).

Manual copy

$ git clone --depth 1 https://github.com/secondsky/claude-skills.git skills-src && mkdir -p .claude/skills && cp -r skills-src/plugins/xss-prevention/skills/xss-prevention .claude/skills/xss-prevention && rm -rf skills-src

Use ~/.claude/skills/ instead of .claude/skills for a personal install. The folder must contain SKILL.md.

Claude Code skills documentation · loads skills from .claude/skills/

Paste into Claude Code

Install the "xss-prevention" agent skill from https://github.com/secondsky/claude-skills/tree/main/plugins/xss-prevention/skills/xss-prevention into .claude/skills/xss-prevention/ in this project. Copy the whole folder (SKILL.md and every file beside it), keep the folder name "xss-prevention", then confirm the skill loads.

Claude Code copies the folder itself, the same result as the manual copy. Check what it changed before you commit it.

$skill-installer

$skill-installer install https://github.com/secondsky/claude-skills/tree/main/plugins/xss-prevention/skills/xss-prevention

Type this inside Codex. $skill-installer <name> installs a curated skill from openai/skills. The installer writes to $CODEX_HOME/skills (default ~/.codex/skills). Restart Codex if the skill does not show up.

skills CLI

$ npx skills add secondsky/claude-skills --skill xss-prevention -a codex

Project install goes to .agents/skills/; add -g for ~/.codex/skills/.

GitHub CLI

$ gh skill install secondsky/claude-skills xss-prevention --agent codex

Project scope by default (.agents/skills/); add --scope user for a personal install.

Manual copy

$ git clone --depth 1 https://github.com/secondsky/claude-skills.git skills-src && mkdir -p .agents/skills && cp -r skills-src/plugins/xss-prevention/skills/xss-prevention .agents/skills/xss-prevention && rm -rf skills-src

Use ~/.agents/skills/ instead of .agents/skills for a personal install.

Codex skills documentation · loads skills from .agents/skills/

Paste into Codex

Install the "xss-prevention" agent skill from https://github.com/secondsky/claude-skills/tree/main/plugins/xss-prevention/skills/xss-prevention into .agents/skills/xss-prevention/ in this project. Copy the whole folder (SKILL.md and every file beside it), keep the folder name "xss-prevention", then confirm the skill loads.

Codex copies the folder itself, the same result as the manual copy. Check what it changed before you commit it.

skills CLI

$ npx skills add secondsky/claude-skills --skill xss-prevention -a cursor

Project install goes to .agents/skills/; add -g for ~/.cursor/skills/.

GitHub CLI

$ gh skill install secondsky/claude-skills xss-prevention --agent cursor

Project scope by default (.agents/skills/); add --scope user for a personal install.

Manual copy

$ git clone --depth 1 https://github.com/secondsky/claude-skills.git skills-src && mkdir -p .cursor/skills && cp -r skills-src/plugins/xss-prevention/skills/xss-prevention .cursor/skills/xss-prevention && rm -rf skills-src

Use ~/.cursor/skills/ instead of .cursor/skills for a personal install.

Cursor skills documentation · loads skills from .cursor/skills/, .agents/skills/, .claude/skills/, .codex/skills/

Paste into Cursor

Install the "xss-prevention" agent skill from https://github.com/secondsky/claude-skills/tree/main/plugins/xss-prevention/skills/xss-prevention into .cursor/skills/xss-prevention/ in this project. Copy the whole folder (SKILL.md and every file beside it), keep the folder name "xss-prevention", then confirm the skill loads.

Cursor copies the folder itself, the same result as the manual copy. Check what it changed before you commit it.

gemini skills install

$ gemini skills install https://github.com/secondsky/claude-skills.git --path plugins/xss-prevention/skills/xss-prevention

--scope user (default) or --scope workspace; --path is the subfolder of the repo that holds the skill; --consent skips the security confirmation prompt.

skills CLI

$ npx skills add secondsky/claude-skills --skill xss-prevention -a gemini-cli

Project install goes to .agents/skills/; add -g for ~/.gemini/skills/.

GitHub CLI

$ gh skill install secondsky/claude-skills xss-prevention --agent gemini-cli

Project scope by default (.agents/skills/); add --scope user for a personal install.

Manual copy

$ git clone --depth 1 https://github.com/secondsky/claude-skills.git skills-src && mkdir -p .gemini/skills && cp -r skills-src/plugins/xss-prevention/skills/xss-prevention .gemini/skills/xss-prevention && rm -rf skills-src

Use ~/.gemini/skills/ instead of .gemini/skills for a personal install, then run /skills reload.

Gemini CLI skills documentation · loads skills from .gemini/skills/, .agents/skills/

Paste into Gemini CLI

Install the "xss-prevention" agent skill from https://github.com/secondsky/claude-skills/tree/main/plugins/xss-prevention/skills/xss-prevention into .gemini/skills/xss-prevention/ in this project. Copy the whole folder (SKILL.md and every file beside it), keep the folder name "xss-prevention", then confirm the skill loads.

Gemini CLI copies the folder itself, the same result as the manual copy. Check what it changed before you commit it.

GitHub CLI

$ gh skill install secondsky/claude-skills xss-prevention

Installs for Copilot at project scope by default; add --scope user for a personal install. Preview a skill first with gh skill preview. Needs GitHub CLI 2.90.0 or later (public preview).

skills CLI

$ npx skills add secondsky/claude-skills --skill xss-prevention -a github-copilot

Project install goes to .agents/skills/; add -g for ~/.copilot/skills/.

Manual copy

$ git clone --depth 1 https://github.com/secondsky/claude-skills.git skills-src && mkdir -p .github/skills && cp -r skills-src/plugins/xss-prevention/skills/xss-prevention .github/skills/xss-prevention && rm -rf skills-src

Use ~/.copilot/skills/ instead of .github/skills for a personal install. Commit .github/skills so cloud agent and code review can use it.

GitHub Copilot skills documentation · loads skills from .github/skills/, .claude/skills/, .agents/skills/

Paste into GitHub Copilot

Install the "xss-prevention" agent skill from https://github.com/secondsky/claude-skills/tree/main/plugins/xss-prevention/skills/xss-prevention into .github/skills/xss-prevention/ in this project. Copy the whole folder (SKILL.md and every file beside it), keep the folder name "xss-prevention", then confirm the skill loads.

GitHub Copilot copies the folder itself, the same result as the manual copy. Check what it changed before you commit it.

skills CLI

$ npx skills add secondsky/claude-skills --skill xss-prevention -a opencode

OpenCode documents no install command of its own. Project install goes to .agents/skills/; add -g for ~/.config/opencode/skills/.

GitHub CLI

$ gh skill install secondsky/claude-skills xss-prevention --agent opencode

Project scope by default (.agents/skills/); add --scope user for a personal install.

Manual copy

$ git clone --depth 1 https://github.com/secondsky/claude-skills.git skills-src && mkdir -p .opencode/skills && cp -r skills-src/plugins/xss-prevention/skills/xss-prevention .opencode/skills/xss-prevention && rm -rf skills-src

Use ~/.config/opencode/skills/ instead of .opencode/skills for a personal install.

OpenCode skills documentation · loads skills from .opencode/skills/, .claude/skills/, .agents/skills/

Paste into OpenCode

Install the "xss-prevention" agent skill from https://github.com/secondsky/claude-skills/tree/main/plugins/xss-prevention/skills/xss-prevention into .opencode/skills/xss-prevention/ in this project. Copy the whole folder (SKILL.md and every file beside it), keep the folder name "xss-prevention", then confirm the skill loads.

OpenCode copies the folder itself, the same result as the manual copy. Check what it changed before you commit it.

Facts

Skill name: xss-prevention
GitHub stars: 227
Token cost: ~1.5k tokens
SKILL.md length: 362 words
Files: 3 (incl. references)
Skills in repo: 169
Repo updated: Sep 28, 2026
First seen: Oct 7, 2026
Licence: MIT

At a glance

XSS attack prevention with input sanitization, output encoding, Content Security Policy.

User-generated content
SKILL.md covers Overview, When to Use, XSS Attack Types and Output Encoding (Node.js), plus 9 more sections
Instructions only: no scripts, shell commands, URLs or credentials in SKILL.md
Rich text editors

What it does

Xss Prevention is an agent skill from secondsky/claude-skills. XSS attack prevention with input sanitization, output encoding, Content Security Policy. Use for user-generated content, rich text editors, web application security, or encountering stored XSS, reflected XSS, DOM manipulation, script injection errors.

Its SKILL.md is about 1.5k tokens, which your agent loads only when the skill is triggered. The skill folder holds 3 other files, including reference files (for example `references/nodejs-advanced.md` and `references/python-sanitization.md`).

It sits in Security, covering Web application vulnerabilities and Secure coding. It works with Node.js and Python. The repository describes itself as: Production-ready skills for Claude Code CLI - Cloudflare, React, Tailwind v4, and AI integrations. The licence is MIT.

When your agent uses it

User-generated content
Rich text editors
Web application security
Encountering stored XSS

Example prompts

“/xss-prevention”

Requirements

Python 3
Node.js

What it can do on your machine

Read from SKILL.md and the folder at commit 8837836. It shows what the files ask for, not the result of running them.

Tool permissions
Pre-approves nothing: there is no allowed-tools line, so your agent's usual permission prompts apply.
From allowed-tools in the SKILL.md frontmatter.
Runs code
No scripts in the folder and no shell commands in SKILL.md (its code samples are javascript).
From the folder's file list and the shell code blocks in SKILL.md.
Network
Links to these hosts (documentation or services it may open):
- cheatsheetseries.owasp.org
- github.com
- content-security-policy.com
- developer.mozilla.org
From URLs in SKILL.md, links to its own repository left out.
Credentials
Names no API keys, tokens, secrets or passwords.
From names ending in _API_KEY, _TOKEN, _SECRET, _KEY or _PASSWORD in SKILL.md.

Context cost

Xss Prevention loads about 1.5k tokens when it runs, and up to ~4.9k if it reads all its reference files. Until then it costs about 67 tokens; SKILL.md has 362 words of instructions outside code blocks.

Always · name and description, kept in context so the agent knows when to use it: ~67
When it runs · the whole SKILL.md, loaded when a task matches: ~1.5k
With references · SKILL.md plus every file in references/, read only if the agent opens them: ~4.9k

Estimates: characters ÷ 4, the usual rule of thumb; real counts depend on the model's tokenizer. Scripts and assets cost tokens only if the agent reads them.

Safety

Auto-check passed

The automated check found no risky patterns in SKILL.md.

Automated static check — not a guarantee. Review scripts before installing. It scans the text of SKILL.md for risky patterns (piping downloads into a shell, reading credential files, hidden Unicode, destructive commands); files beside SKILL.md are not scanned.

SKILL.md

The full file from secondsky/claude-skills at commit 8837836, republished under its MIT licence (© secondsky). 362 words, ~1,511 tokens.

Download SKILL.mdSave it as .claude/skills/xss-prevention/SKILL.md (or your agent's skills folder). This skill also uses 2 other files; get the full folder from GitHub.

name: xss-prevention
description: XSS attack prevention with input sanitization, output encoding, Content Security Policy. Use for user-generated content, rich text editors, web application security, or encountering stored XSS, reflected XSS, DOM manipulation, script injection errors.
metadata.keywords: sanitization, HTML-encoding, DOMPurify, CSP, Content-Security-Policy, rich-text-editor, user-input, escaping, innerHTML, DOM-manipulation, stored-XSS…
license: MIT

XSS Prevention

Overview

Implement comprehensive Cross-Site Scripting attack prevention through input sanitization, output encoding, Content Security Policy headers, and secure coding practices.

When to Use

User-generated content display
Rich text editors
Comment systems
Search functionality
Dynamic HTML generation
Template rendering scenarios

XSS Attack Types

Type	Vector	Defense
Reflected	URL parameters	Output encoding
Stored	Database content	Input sanitization
DOM-based	Client-side JS	Safe DOM APIs
Mutation	HTML parser quirks	Strict sanitization

Output Encoding (Node.js)

javascript

function encodeHTML(str) {
  return str
    .replace(/&/g, '&amp;')
    .replace(/</g, '&lt;')
    .replace(/>/g, '&gt;')
    .replace(/"/g, '&quot;')
    .replace(/'/g, '&#x27;');
}

function encodeForAttribute(str) {
  return str.replace(/[^\w.-]/g, char =>
    `&#x${char.charCodeAt(0).toString(16)};`
  );
}

// Usage in templates
app.get('/profile', (req, res) => {
  const username = encodeHTML(req.query.name);
  res.send(`<h1>Welcome, ${username}</h1>`);
});

DOMPurify Sanitization

javascript

import DOMPurify from 'dompurify';

const config = {
  ALLOWED_TAGS: ['b', 'i', 'em', 'strong', 'a', 'p', 'br'],
  ALLOWED_ATTR: ['href', 'title'],
  ALLOW_DATA_ATTR: false
};

function sanitizeHTML(dirty) {
  return DOMPurify.sanitize(dirty, config);
}

// React component
function RichContent({ html }) {
  return (
    <div dangerouslySetInnerHTML={{ __html: sanitizeHTML(html) }} />
  );
}

Content Security Policy

javascript

// Express middleware
app.use((req, res, next) => {
  const nonce = crypto.randomBytes(16).toString('base64');
  res.locals.nonce = nonce;

  res.setHeader('Content-Security-Policy', [
    "default-src 'self'",
    `script-src 'self' 'nonce-${nonce}'`,
    "style-src 'self' 'unsafe-inline'",
    "img-src 'self' data: https:",
    "connect-src 'self' https://api.example.com",
    "frame-ancestors 'none'",
    "base-uri 'self'",
    "form-action 'self'"
  ].join('; '));

  next();
});

Safe DOM APIs

❌ NEVER do any of the following with user-controlled input — these are XSS sinks and there is no safe way to call them with untrusted data:

Assign it to element.innerHTML / element.outerHTML
Pass it to eval()
Pass it to document.write()
Pass it to setTimeout / setInterval as a string
Insert it into an inline event handler (e.g. onclick="...")

javascript

// SAFE — use these instead
element.textContent = userInput;      // Escaped automatically
element.setAttribute('data-id', id);  // Safe for attributes
document.createTextNode(userInput);   // Creates safe text node

The safe patterns above (textContent, attribute escaping via setAttribute, DOMPurify.sanitize) are the only correct ways to handle user input in the DOM.

URL Validation

javascript

function isSafeURL(url) {
  try {
    const parsed = new URL(url);
    return ['http:', 'https:'].includes(parsed.protocol);
  } catch {
    return false;
  }
}

// Usage
const href = isSafeURL(userURL) ? userURL : '#';

Context-Specific Encoding

Different contexts require different encoding approaches:

HTML Entity Encoding: Safest option for text content
Attribute Encoding: For HTML attributes
JavaScript Escaping: For script contexts
URL Encoding: For URL parameters
CSS Escaping: For stylesheet contexts

Always encode output by the specific context where data will be rendered.

Show full SKILL.md (155 more words)Show less

Additional Implementations

See references/python-sanitization.md for:

Python bleach library usage
Flask/Django template escaping
Server-side validation patterns

See references/nodejs-advanced.md for:

Complete XSSPrevention class with all methods
Express middleware (xssProtection)
React components (SafeText, SafeHTML, SafeLink, useSanitizedInput)
Helmet CSP configuration

Best Practices

✅ DO:

Encode output by default
Use templating engines with auto-escaping
Implement CSP headers
Sanitize rich content with allowlists
Validate URLs with protocol whitelisting
Use HTTPOnly cookies
Conduct regular security testing
Leverage secure frameworks

❌ DON'T:

Trust user input
Use unsafe functions (eval, innerHTML)
Disable security features for convenience
Rely solely on client-side validation
Use blocklists instead of allowlists
Skip context-specific encoding
Allow arbitrary script execution

Security Checklist

Encode all output by context (HTML, attribute, JS)
Sanitize HTML with allowlist (not blocklist)
Implement strict CSP headers
Use HTTPOnly cookies for sessions
Validate and sanitize URLs
Avoid innerHTML with user content
Regular security testing

Resources

Files

SKILL.md and 2 other files (references) in plugins/xss-prevention/skills/xss-prevention of secondsky/claude-skills.

SKILL.md
references/nodejs-advanced.md
references/python-sanitization.md

Open the folder on GitHubat commit 8837836

Compare with similar skills

Xss Prevention next to the 5 skills that share the most tags, products or categories with it. Stars are the repository's; “used in” counts other GitHub owners with a copy.

Xss Prevention compared with similar skills
Skill	Stars	Used in	Tokens	Auto-check	Licence	Repo updated
Xss Prevention this skillsecondsky/claude-skills	227	—	~1.5k	Automated safety check: Pass	MIT	10 days ago
Security Verification Gatefengshao1227/ccg-workflow	5.9k	—	~621	Automated safety check: Notes	MIT	23 days ago
Security Auditjellydn/my-ai-tools	123	—	~2.9k	Automated safety check: Notes	MIT	yesterday
Security Auditoreigent-ai/eigent	15k	—	~1.8k	Automated safety check: Notes	Apache-2.0	yesterday
Fix Strix Security Findingsusestrix/strix	67k	—	~1.5k	Automated safety check: Pass	Apache-2.0	yesterday
CodeQL Security Scantrailofbits/skills	7.4k	—	~4.6k	Automated safety check: Notes	CC-BY-SA-4.0	yesterday

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Works with

Questions about Xss Prevention

What does Xss Prevention do?

XSS attack prevention with input sanitization, output encoding, Content Security Policy. Xss Prevention is an agent skill from secondsky/claude-skills. XSS attack prevention with input sanitization, output encoding, Content Security Policy.

When should I use Xss Prevention?

Xss Prevention fits situations like: user-generated content; rich text editors; web application security; encountering stored XSS.

How do I install Xss Prevention in Claude Code?

Run `npx skills add secondsky/claude-skills --skill xss-prevention -a claude-code`. Or copy the skill folder (plugins/xss-prevention/skills/xss-prevention in secondsky/claude-skills) into .claude/skills/xss-prevention in your project. Claude Code loads it when a task matches its description.

How do I install Xss Prevention in Codex?

Run `npx skills add secondsky/claude-skills --skill xss-prevention -a codex`. Or copy the skill folder (plugins/xss-prevention/skills/xss-prevention in secondsky/claude-skills) into .agents/skills/xss-prevention in your project. Codex loads it when a task matches its description.

Can I use Xss Prevention in Cursor, Gemini CLI or GitHub Copilot?

Cursor, Gemini CLI, GitHub Copilot and OpenCode also load SKILL.md folders. With the skills CLI, run `npx skills add secondsky/claude-skills --skill xss-prevention -a cursor` (or -a gemini-cli, github-copilot or opencode for the others). To copy it by hand, put the folder in .cursor/skills/xss-prevention, .gemini/skills/xss-prevention, .github/skills/xss-prevention and .opencode/skills/xss-prevention in your project.

What does Xss Prevention need to run?

SKILL.md names no scripts, command-line tools or credentials: Xss Prevention is instructions for the agent only. Our summary lists: Python 3; Node.js.

Does Xss Prevention access the network?

SKILL.md names 4 domains. As links in the text: cheatsheetseries.owasp.org, github.com, content-security-policy.com and developer.mozilla.org. This is read from the text; nothing was executed.

Is Xss Prevention safe to install?

Our automated static check of SKILL.md found no risky patterns, such as piping downloads into a shell, reading credential files or hidden Unicode. It is not a guarantee. Review the folder before installing.

What licence does Xss Prevention use?

Xss Prevention is published under the MIT licence (declared in SKILL.md). It allows redistribution, so the full SKILL.md is shown on this page.

How many tokens does Xss Prevention use?

About 1.5k tokens (SKILL.md is roughly 6k characters). Agents keep only the skill's name and description in context until a task matches; then they load SKILL.md in full. Its references folder adds about 3.4k tokens, read only when the agent opens those files.

What are the alternatives to Xss Prevention?

Skills that share tags, products or a category with Xss Prevention: Security Verification Gate (fengshao1227/ccg-workflow, 5.9k stars), Security Audit (jellydn/my-ai-tools, 123 stars), Security Auditor (eigent-ai/eigent, 15k stars) and Fix Strix Security Findings (usestrix/strix, 67k stars). The comparison table on this page puts their stars, adoption, token cost, safety result and licence side by side.

Who maintains Xss Prevention?

secondsky (a GitHub user) maintains it in secondsky/claude-skills, which has 227 GitHub stars. The repository holds 169 skills in this directory. The repository was last updated on September 28, 2026.

Source: secondsky/claude-skills on GitHub. Facts on this page come from the repository at the commit we read; the author's words are quoted as theirs.

Install Xss Prevention

Facts

At a glance

What it does

When your agent uses it

Example prompts

Requirements

What it can do on your machine

Tool permissions

Runs code

Network

Credentials

Context cost

Safety

SKILL.md

XSS Prevention

Overview

When to Use

XSS Attack Types

Output Encoding (Node.js)

DOMPurify Sanitization

Content Security Policy

Safe DOM APIs

URL Validation

Context-Specific Encoding

Additional Implementations

Best Practices

Security Checklist

Resources

Files

Compare with similar skills

Similar skills

More from secondsky/claude-skills

Related

Questions about Xss Prevention