XSS Detection

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 three-phase approach with subagents: recon (find sink sites), batched verify (trace taint for parallel batches of up to 3 sinks each), and merge (consolidate batch results into one report).

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 three phases using subagents. Pass the contents of sast/architecture.md to all 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 and Phase 3 entirely. Instead, write the following content to sast/xss-results.md and stop (you may delete sast/xss-recon.md after writing):

# XSS Analysis Results

No vulnerabilities found.

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

Phase 2: Verify — Trace User Input to Sinks (Batched)

After Phase 1 completes, read sast/xss-recon.md and split the sink sites into batches of up to 3 sink sites each. Launch one subagent per batch in parallel. Each subagent traces taint only for its assigned sinks and writes results to its own batch file.

Batching procedure (you, the orchestrator, do this — not a subagent):

Read sast/xss-recon.md and count the numbered sink sections (### 1., ### 2., etc.).
Divide them into batches of up to 3. For example, 8 sinks → 3 batches (1-3, 4-6, 7-8).
For each batch, extract the full text of those sink sections from the recon file.
Launch all batch subagents in parallel, passing each one only its assigned sinks.
Each subagent writes to sast/xss-batch-N.md where N is the 1-based batch number.
Identify the project's primary language/framework from sast/architecture.md and select only the matching examples from the "Vulnerable vs. Secure Examples" section above. For example, if the project uses React with an Express API, include the relevant Node.js and React examples. Include these selected examples in each subagent's instructions where indicated by [TECH-STACK EXAMPLES] below.

Give each batch subagent the following instructions (substitute the batch-specific values):

Goal: For each assigned XSS sink site, determine whether a user-supplied value reaches the output variable. Write results to sast/xss-batch-[N].md.

Your assigned sink sites (from the recon phase):

[Paste the full text of the assigned sink sections here, preserving the original numbering]

Context: You will be given the project's architecture summary. Use it 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:

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

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

Stored (second-order) input — the variable is read from persistent storage (database, file, cache), but the stored value or

README

Cross-Site Scripting (XSS) Detection

What is XSS

XSS Types

What XSS IS

What XSS is NOT

Patterns That Prevent XSS

Vulnerable vs. Secure Examples

Python — Flask / Jinja2

Python — Django

PHP

Node.js — Express (string concatenation)

Node.js / EJS

Node.js / Handlebars

JavaScript — DOM Sinks

React

Angular

Ruby on Rails

Java — JSP

Go — html/template vs. text/template

Execution

Phase 1: Find XSS Sink Sites

After Phase 1: Check for Candidates Before Proceeding

Phase 2: Verify — Trace User Input to Sinks (Batched)

More from utkusen

SAST Analysis

Business Logic Vulnerability Detection

Insecure File Upload Detection

GraphQL Injection Detection

Hardcoded Secrets Detection

IDOR Detection

More Quality skills

A/B Testing

Accessibility Audit

AccessLint Audit