sast-skills/sast-files/.agents/skills/sast-xss/SKILL.md

name, description

name

description

sast-xss

Detect Cross-Site Scripting (XSS) vulnerabilities in a codebase using a two-phase approach: first find all HTML, JavaScript, and DOM output sinks where data is rendered without escaping, then trace whether user-supplied input reaches those sinks. Requires sast/architecture.md (run sast-analysis first). Outputs findings to sast/xss-results.md. Use when asked to find XSS or cross-site scripting bugs.

Cross-Site Scripting (XSS) Detection

You are performing a focused security assessment to find Cross-Site Scripting vulnerabilities in a codebase. This skill uses a two-phase approach with subagents: sink discovery (find all places where data is rendered into HTML, JavaScript, or the DOM without proper escaping) then taint (confirm whether user-supplied input reaches those sinks).

Prerequisites: sast/architecture.md must exist. Run the analysis skill first if it doesn't.

---

What is XSS

XSS occurs when user-supplied input is incorporated into a web page's HTML, JavaScript, or DOM without proper escaping or sanitization. This allows attackers to inject and execute arbitrary scripts in victims' browsers, leading to session hijacking, credential theft, defacement, and malware distribution.

The core pattern: unescaped, unsanitized user input reaches an HTML/JS output sink.

XSS Types

• Reflected XSS: User input is immediately echoed back in the HTTP response (e.g., a search term rendered directly into the page HTML).
• Stored XSS: User input is saved to persistent storage (database, file) and later rendered in HTML for other users.
• DOM-based XSS: Client-side JavaScript reads from an attacker-controlled source (location.search, location.hash, document.cookie) and writes to a dangerous DOM sink (innerHTML, eval, document.write) without server involvement.

What XSS IS

Server-side HTML sinks — rendering user data into HTML responses without escaping:

• Python/Jinja2: {{ var | safe }}, {% autoescape off %}...{{ var }}...{% endautoescape %}
• Python/Django: mark_safe(var), format_html(...) with %s and unescaped input, {{ var | safe }} in templates
• Python/Flask: Markup(var), render_template_string(f"...{var}...")
• PHP: echo $var, print $var, <?= $var ?> without htmlspecialchars()
• Ruby/Rails: raw(var), var.html_safe, <%= raw var %>, content_tag with .html_safe
• Java/JSP: <%= var %>, ${var} without <c:out> or fn:escapeXml()
• Java/Thymeleaf: th:utext="${var}" (unescaped), [(${var})]
• Go/html-template misuse: using template.HTML(var), template.JS(var), template.URL(var) to bypass auto-escaping
• C#/Razor: @Html.Raw(var), MvcHtmlString.Create(var)
• Node.js/EJS: <%- var %> (unescaped), vs <%= var %> (safe)
• Node.js/Handlebars: {{{ var }}} (triple-brace, unescaped)
• Node.js/Pug: !{var} (unescaped)
• Express: res.send("<html>..." + var + "..."), res.write("<p>" + var + "</p>")

Client-side DOM sinks — JavaScript writing user-controlled data to the DOM unsafely:

• element.innerHTML = var
• element.outerHTML = var
• document.write(var), document.writeln(var)
• element.insertAdjacentHTML('beforeend', var)
• jQuery: $(element).html(var), $(element).append(var) (when var contains HTML), $('<div>' + var + '</div>')
• React: dangerouslySetInnerHTML={{ __html: var }}
• Angular: [innerHTML]="var", bypassSecurityTrustHtml(var), bypassSecurityTrustScript(var), bypassSecurityTrustUrl(var)
• Vue: v-html="var"

JavaScript execution sinks — user-controlled data evaluated as code:

• eval(var)
• setTimeout(var, delay) / setInterval(var, delay) when var is a string
• new Function(var)()
• element.setAttribute('onclick', var), element.setAttribute('href', 'javascript:' + var)
• location.href = var, location.replace(var), location.assign(var) (when var is user-controlled and can be javascript:...)
• element.src = var, element.action = var (script injection via javascript: URIs)
• scriptElement.text = var, scriptElement.textContent = var

DOM-based sources — attacker-controlled inputs read by client-side JavaScript:

• location.search (URL query string)
• location.hash (URL fragment)
• location.href
• document.referrer
• document.URL, document.documentURI
• document.cookie
• postMessage event data (event.data)
• window.name
• localStorage.getItem(...), sessionStorage.getItem(...) (if populated from URL or postMessage)

What XSS is NOT

Do not flag these as XSS:

• CSRF: Forging requests on behalf of a user — a separate vulnerability class
• SQLi via XSS: Injecting SQL through an XSS vector — the SQL injection itself is the primary finding
• Clickjacking: Embedding pages in iframes — different vulnerability class
• Header injection: Injecting newlines into HTTP response headers — separate class (HTTP Response Splitting)
• Safe template output: Auto-escaped {{ var }} in Jinja2/Django/Twig/Blade/Handlebars double-brace syntax with auto-escaping on — these are safe
• textContent / innerText: These write plain text only; no HTML parsing occurs — safe

Patterns That Prevent XSS

When you see these patterns, the code is likely not vulnerable:

1. Context-aware auto-escaping (most template engines default)

# Jinja2 / Django (auto-escape on by default)
{{ var }}          # HTML-escaped → safe

# EJS
<%= var %>         # HTML-escaped → safe

# Handlebars
{{ var }}          # HTML-escaped → safe

# Pug
= var              # HTML-escaped → safe

# Thymeleaf
th:text="${var}"   # HTML-escaped → safe

# Razor (C#)
@var               # HTML-encoded → safe

2. Explicit escaping before output

// PHP
echo htmlspecialchars($var, ENT_QUOTES, 'UTF-8');

# Rails
<%= h(var) %>
<%= ERB::Util.html_escape(var) %>

// JSP with JSTL
<c:out value="${var}"/>
// or fn:escapeXml()
${fn:escapeXml(var)}

// html/template — auto-escapes by context (HTML, JS, URL, CSS)
{{.Var}}   // safe inside html/template

3. DOM manipulation using safe properties

element.textContent = userInput;   // plain text, no HTML parsing — safe
element.innerText = userInput;     // plain text — safe

4. Sanitization with an allowlisted HTML library

// DOMPurify
element.innerHTML = DOMPurify.sanitize(userInput);

// sanitize-html with strict config
const clean = sanitizeHtml(userInput, { allowedTags: [], allowedAttributes: {} });

5. React / Angular / Vue auto-escaping

// React JSX — auto-escaped
return <div>{userInput}</div>;

<!-- Angular — auto-escaped -->
<div>{{ userInput }}</div>
<!-- Vue — auto-escaped -->
<div>{{ userInput }}</div>

---

Vulnerable vs. Secure Examples

Python — Flask / Jinja2

# VULNERABLE: Markup() bypasses Jinja2 auto-escaping
@app.route('/greet')
def greet():
    name = request.args.get('name', '')
    return render_template_string(f"<h1>Hello, {name}!</h1>")   # raw f-string, no template escaping

# VULNERABLE: mark_safe equivalent
@app.route('/profile')
def profile():
    bio = request.args.get('bio', '')
    return render_template('profile.html', bio=Markup(bio))      # Markup() marks it as safe, bypassing escaping

# SECURE: use template with auto-escaping (never pass Markup around user input)
@app.route('/greet')
def greet():
    name = request.args.get('name', '')
    return render_template('greet.html', name=name)              # template: {{ name }} — auto-escaped

Python — Django

# VULNERABLE: mark_safe() with user input
def user_bio(request):
    bio = request.GET.get('bio', '')
    safe_bio = mark_safe(bio)   # user input bypasses Django's auto-escaping
    return render(request, 'bio.html', {'bio': safe_bio})

# SECURE: pass raw string; template handles escaping
def user_bio(request):
    bio = request.GET.get('bio', '')
    return render(request, 'bio.html', {'bio': bio})   # template: {{ bio }} — auto-escaped

PHP

// VULNERABLE: echo without escaping
function showUsername($username) {
    echo "<p>Welcome, " . $username . "</p>";
}

// SECURE: htmlspecialchars
function showUsername($username) {
    echo "<p>Welcome, " . htmlspecialchars($username, ENT_QUOTES, 'UTF-8') . "</p>";
}

Node.js — Express (string concatenation)

// VULNERABLE: user input concatenated into HTML response
app.get('/search', (req, res) => {
  const query = req.query.q;
  res.send(`<h1>Results for: ${query}</h1>`);
});

// SECURE: use a template engine with auto-escaping, or escape manually
const escapeHtml = require('escape-html');
app.get('/search', (req, res) => {
  const query = req.query.q;
  res.send(`<h1>Results for: ${escapeHtml(query)}</h1>`);
});

Node.js / EJS

<!-- VULNERABLE: unescaped output -->
<div><%- userInput %></div>

<!-- SECURE: escaped output -->
<div><%= userInput %></div>

Node.js / Handlebars

<!-- VULNERABLE: triple-brace, unescaped -->
<div>{{{ userInput }}}</div>

<!-- SECURE: double-brace, auto-escaped -->
<div>{{ userInput }}</div>

JavaScript — DOM Sinks

// VULNERABLE: innerHTML with URL fragment
const name = location.hash.substring(1);
document.getElementById('greeting').innerHTML = 'Hello, ' + name;

// SECURE: textContent
const name = location.hash.substring(1);
document.getElementById('greeting').textContent = 'Hello, ' + name;

// VULNERABLE: eval with postMessage data
window.addEventListener('message', (event) => {
  eval(event.data);
});

// SECURE: parse and validate; never eval postMessage data
window.addEventListener('message', (event) => {
  const data = JSON.parse(event.data);
  // handle data safely
});

React

// VULNERABLE: dangerouslySetInnerHTML with user input
function Comment({ content }) {
  return <div dangerouslySetInnerHTML={{ __html: content }} />;
}

// SECURE: render as text (auto-escaped by React)
function Comment({ content }) {
  return <div>{content}</div>;
}

Angular

// VULNERABLE: bypassing Angular's DomSanitizer
constructor(private sanitizer: DomSanitizer) {}
getUserHtml(input: string): SafeHtml {
  return this.sanitizer.bypassSecurityTrustHtml(input);  // unsafe if input is user-controlled
}

<!-- VULNERABLE: [innerHTML] with unsanitized value -->
<div [innerHTML]="userInput"></div>

<!-- SECURE: use interpolation (auto-escaped) -->
<div>{{ userInput }}</div>

Ruby on Rails

<%# VULNERABLE: raw() or html_safe with user input %>
<%= raw(@user.bio) %>
<%= @user.bio.html_safe %>

<%# SECURE: default ERB escaping %>
<%= @user.bio %>

Java — JSP

<%-- VULNERABLE: scriptlet echo --%>
<p>Hello, <%= request.getParameter("name") %></p>

<%-- VULNERABLE: EL without c:out --%>
<p>Hello, ${param.name}</p>

<%-- SECURE: c:out escaping --%>
<p>Hello, <c:out value="${param.name}"/></p>

Go — html/template vs. text/template

// VULNERABLE: using text/template (no HTML escaping)
import "text/template"
tmpl := template.Must(template.New("").Parse("<h1>Hello, {{.Name}}!</h1>"))
tmpl.Execute(w, data)

// VULNERABLE: using template.HTML() cast to bypass escaping
import "html/template"
name := template.HTML(r.URL.Query().Get("name"))   // bypasses auto-escaping

// SECURE: html/template with plain string value
import "html/template"
tmpl := template.Must(template.New("").Parse("<h1>Hello, {{.Name}}!</h1>"))
tmpl.Execute(w, data)   // .Name is a plain string — auto-escaped

---

Execution

This skill runs in two phases using subagents. Pass the contents of sast/architecture.md to both subagents as context.

Phase 1: Find XSS Sink Sites

Launch a subagent with the following instructions:

Goal: Find every location in the codebase where data is rendered into HTML, JavaScript, or the DOM in a way that could allow script injection — any unescaped or explicitly-marked-safe output, any dangerous DOM property assignment, any JavaScript execution sink. Write results to sast/xss-recon.md.

Context: You will be given the project's architecture summary. Use it to understand the frontend stack, template engines, server-side rendering frameworks, and any client-side JavaScript patterns.

What to search for — vulnerable sink patterns:

Flag ANY dynamic variable passed to a dangerous output sink. You are not yet checking whether the variable is user-controlled — that is Phase 2's job.

1. Server-side template unescaped output:

• Jinja2/Django: {{ var | safe }}, {% autoescape off %}, Markup(var), mark_safe(var), format_html(...) with direct user-controlled format args
• EJS: <%- var %>
• Handlebars/Mustache: {{{ var }}}
• Pug: !{var}
• Thymeleaf: th:utext="${var}", [(${var})]
• Twig: {{ var | raw }}
• Blade (Laravel): {!! $var !!}
• Rails ERB: raw(var), var.html_safe, <%= raw var %>
• PHP: echo $var, print $var, <?= $var ?> without htmlspecialchars()
• Go: template.HTML(var), template.JS(var), template.URL(var), usage of text/template for HTML output
• C#/Razor: @Html.Raw(var), MvcHtmlString.Create(var)

2. Direct HTML string construction in server-side code:

• String concatenation or interpolation building an HTML response: res.send("<p>" + var + "</p>"), f"<h1>{var}</h1>", "<div>" + var + "</div>"
• render_template_string(f"...{var}...") in Flask

3. Client-side DOM sinks:

• element.innerHTML = var
• element.outerHTML = var
• document.write(var), document.writeln(var)
• element.insertAdjacentHTML(position, var)
• jQuery: $(el).html(var), $(el).append(var), $('<tag>' + var + '</tag>'), $.parseHTML(var) passed to DOM
• React: dangerouslySetInnerHTML={{ __html: var }}
• Angular: [innerHTML]="var", bypassSecurityTrustHtml(var), bypassSecurityTrustScript(var), bypassSecurityTrustUrl(var), bypassSecurityTrustStyle(var), bypassSecurityTrustResourceUrl(var)
• Vue: v-html="var"

4. JavaScript execution sinks:

• eval(var)
• setTimeout(var, ...) / setInterval(var, ...) where var is a string variable (not a function reference)
• new Function(var)()
• scriptElement.text = var, scriptElement.textContent = var
• element.setAttribute('onclick', var), element.setAttribute('href', 'javascript:' + var), and similar event-handler attribute assignments
• URL-based sinks where javascript: URIs could execute: location.href = var, location.replace(var), element.src = var, element.action = var

5. DOM-based XSS patterns — client-side code reading from attacker-controlled sources and passing to any sink above:

• Reading from: location.search, location.hash, location.href, document.referrer, document.URL, document.cookie, window.name, postMessage handler (event.data), URLSearchParams
• Then passing to an HTML or JS sink without escaping

What to skip (these are safe output patterns — do not flag):

• Auto-escaped template output: {{ var }} in Jinja2 (auto-escape on), <%= var %> in EJS, {{ var }} in Handlebars double-brace, @var in Razor, th:text in Thymeleaf
• element.textContent = var and element.innerText = var — no HTML parsing, safe
• React JSX {var} — auto-escaped
• Angular {{ var }} interpolation — auto-escaped
• Vue {{ var }} interpolation — auto-escaped
• DOMPurify.sanitize(var) wrapping an innerHTML assignment — typically safe (verify config)
• sanitize-html, xss, or similar allowlist sanitizer library wrapping output

Output format — write to sast/xss-recon.md:

# XSS Recon: [Project Name]

## Summary
Found [N] locations where data is rendered into HTML/JS/DOM without guaranteed escaping.

## Sink Sites

### 1. [Descriptive name — e.g., "innerHTML assignment in search results handler"]
- **File**: `path/to/file.ext` (lines X-Y)
- **Function / endpoint / component**: [function name, route, or component]
- **Sink type**: [server-side template / HTML string concat / DOM innerHTML / eval / JS execution sink / DOM-based source-to-sink]
- **Sink call**: [the exact API or property used — e.g., `innerHTML`, `mark_safe()`, `<%- %>`]
- **Interpolated variable(s)**: `var_name` — [brief note, e.g., "unknown origin" or "looks like user profile field"]
- **XSS type**: [Reflected / Stored / DOM-based — best guess at this stage]
- **Code snippet**:

[the vulnerable sink code]

[Repeat for each site]

After Phase 1: Check for Candidates Before Proceeding

After Phase 1 completes, read sast/xss-recon.md. If the recon found zero sink sites (the summary reports "Found 0" or the "Sink Sites" section is empty or absent), skip Phase 2 entirely. Instead, write the following content to sast/xss-results.md and stop:

# XSS Analysis Results

No vulnerabilities found.

Only proceed to Phase 2 if Phase 1 found at least one sink site.

Phase 2: Trace User Input to Sink Sites

Launch a second subagent after Phase 1 completes with the following instructions:

Goal: For each XSS sink site in sast/xss-recon.md, determine whether a user-supplied value reaches the output variable. Write final results to sast/xss-results.md.

Context: You will be given the project's architecture summary and the Phase 1 recon output. Use the architecture to understand request entry points, data flows, middleware, and client-side data sources.

For each sink site, trace the interpolated variable(s) backwards to their origin:

User-controlled sources to look for:

1. HTTP request sources (server-side):

   • Query parameters: request.GET.get(...), req.query.x, params[:x], $_GET['x'], c.Query("x"), r.URL.Query().Get("x")
   • Path parameters: request.path_params['id'], req.params.id, params[:id], $_GET['id']
   • Request body / form fields: request.POST.get(...), req.body.x, request.form.get(...), $_POST['x']
   • HTTP headers: request.headers.get(...), req.headers['x'], $_SERVER['HTTP_X_CUSTOM']
   • Cookies: request.COOKIES.get(...), req.cookies.x, $_COOKIE['x']
   • File upload filenames or content: request.files['x'].filename

2. Attacker-controlled DOM sources (client-side / DOM-based XSS):

   • location.search, location.hash, location.href, document.referrer, document.URL
   • window.name, document.cookie
   • postMessage event: window.addEventListener('message', (e) => { ... e.data ... })
   • URLSearchParams values derived from location.search
   • localStorage / sessionStorage values written from URL or postMessage

3. Stored (second-order) input — the variable is read from persistent storage (database, file, cache), but the stored value originally came from user input:

   • Find the write path: where was this field stored? Was it user-supplied at write time?
   • Was any escaping or sanitization applied at write time? (Note: HTML-escaping at write time is fragile — it may be double-encoded or stripped elsewhere)
   • Stored XSS is still a vulnerability even if it was validated or stored safely; track whether the read-back path escapes before rendering

4. Server-side / hardcoded value — the variable comes from config, environment, a hardcoded constant, or server-side logic with no user influence — this site is NOT exploitable.

For each sink site, also check for mitigations that would prevent exploitation:

• Is the output explicitly escaped with a safe function just before the sink? (htmlspecialchars(), escapeHtml(), h(), fn:escapeXml())
• Is a sanitization library applied with a strict allowlist config? (DOMPurify.sanitize(input) — check if the config strips scripts)
• Is the HTTP response Content-Type set to application/json or text/plain (no HTML rendering)?
• Is a Content Security Policy header present that blocks inline scripts? (CSP reduces impact but is not a full fix)
• Is there a WAF or input validation that strictly allowlists the expected format (e.g., a numeric ID)?

Classification:

• Vulnerable: User input demonstrably reaches the sink with no effective escaping or sanitization.
• Likely Vulnerable: User input probably reaches the sink (indirect/stored flow) or only weak mitigation is present (CSP-only, WAF-only, partial sanitization, incomplete allowlist).
• Not Vulnerable: The variable is server-side only with no user influence, OR proper context-aware escaping is applied immediately before the sink.
• Needs Manual Review: Cannot determine the variable's origin with confidence (opaque helpers, complex conditional flows, external libraries, or cross-service data flows).

Output format — write to sast/xss-results.md:

# XSS Analysis Results: [Project Name]

## Executive Summary
- Sink sites analyzed: [N]
- Vulnerable: [N]
- Likely Vulnerable: [N]
- Not Vulnerable: [N]
- Needs Manual Review: [N]

## Findings

### [VULNERABLE] Descriptive name
- **File**: `path/to/file.ext` (lines X-Y)
- **Endpoint / function / component**: [route, function, or component name]
- **XSS type**: [Reflected / Stored / DOM-based]
- **Issue**: [e.g., "HTTP query param `q` flows directly into innerHTML without escaping"]
- **Taint trace**: [Step-by-step from source to sink — e.g., "req.query.q → query → `<h1>${query}</h1>` → res.send()"]
- **Impact**: [What an attacker can do — session hijacking, credential theft, keylogging, defacement, redirects to malicious sites, etc.]
- **Remediation**: [Specific fix — escape with the correct function, switch to textContent, use auto-escaping template syntax, apply DOMPurify]
- **Dynamic Test**:

[curl command or browser payload to confirm the finding. Show the exact parameter, payload, and what to observe. Example: curl "https://app.example.com/search?q=<script>alert(1)</script>" Or: Visit https://app.example.com/# and observe alert box]

### [LIKELY VULNERABLE] Descriptive name
- **File**: `path/to/file.ext` (lines X-Y)
- **Endpoint / function / component**: [route, function, or component name]
- **XSS type**: [Reflected / Stored / DOM-based]
- **Issue**: [e.g., "Stored user bio likely rendered via innerHTML; write path confirmed from user input"]
- **Taint trace**: [Best-effort trace, with uncertain steps identified]
- **Concern**: [Why it's still a risk — e.g., "Sanitization library present but configured to allow script-capable tags"]
- **Remediation**: [Specific fix]
- **Dynamic Test**:

[payload to attempt]

### [NOT VULNERABLE] Descriptive name
- **File**: `path/to/file.ext` (lines X-Y)
- **Endpoint / function / component**: [route, function, or component name]
- **Reason**: [e.g., "Output wrapped in htmlspecialchars() before echo" or "Variable is a hardcoded server constant"]

### [NEEDS MANUAL REVIEW] Descriptive name
- **File**: `path/to/file.ext` (lines X-Y)
- **Endpoint / function / component**: [route, function, or component name]
- **Uncertainty**: [Why the variable's origin or escaping status could not be determined]
- **Suggestion**: [What to trace manually — e.g., "Follow `buildProfileHtml()` in utils.js to check where its return value originates"]

---

Important Reminders

• Read sast/architecture.md and pass its content to both subagents as context.
• Phase 2 must run AFTER Phase 1 completes — it depends on the recon output.
• Phase 1 is purely structural: flag any dynamic variable passed to an HTML/JS/DOM sink, regardless of origin. Do not attempt to trace user input in Phase 1 — that is Phase 2's job.
• Phase 2 is purely taint analysis: for each sink found in Phase 1, trace the variable back to its origin. If it comes from a user-controlled source with no effective escaping, the site is a real vulnerability.
• Context matters: the same variable may be safe in one output context (HTML body with escaping) and dangerous in another (JavaScript string literal, URL attribute, or event handler attribute). Check the exact rendering context.
• Custom sanitization (homegrown regex stripping, blacklisting <script>, etc.) is not sufficient — flag as Likely Vulnerable. Only DOMPurify with a strict config or equivalent allowlist library is acceptable.
• Stored XSS is easy to miss: trace the write path to confirm the field is user-supplied, then separately verify the read/render path lacks escaping. Both legs must be true for the vulnerability to be exploitable.
• DOM-based XSS lives entirely in client-side JavaScript: look for location.*, document.referrer, event.data, and other attacker-controlled properties flowing into DOM sinks without passing through the server.
• CSP headers reduce XSS exploitability but are not a fix — still flag the underlying injection point.
• When in doubt, classify as "Needs Manual Review" rather than "Not Vulnerable". False negatives are worse than false positives in security assessment.
• Angular's DomSanitizer.bypassSecurityTrust* methods are always suspicious — flag them whenever the argument is not a hardcoded constant.
• For JavaScript execution sinks (eval, setTimeout with string arg), even seemingly innocuous data (error messages, IDs) can be dangerous if an attacker can influence them.