Xss And Injection Prevention

XSS and Injection Prevention

Use this skill when writing or reviewing Visualforce pages, Apex controllers, or LWC components that render user-supplied data, build dynamic queries, or construct redirect URLs. It covers the full set of Salesforce-specific encoding functions, the SOQL injection bind-variable pattern, CRLF injection prevention, and open redirect mitigation.

Before Starting

Gather this context before working on anything in this domain:

• Identify how user input enters the system: URL parameters, form fields, Apex method parameters from external callers, or data stored in sObject fields.
• Identify the output context for each user input: HTML body, JavaScript block, URL parameter, HTTP response header. Each context requires a different encoding function.
• Determine whether any dynamic SOQL or dynamic SOSL constructs exist in the Apex code. These are the injection surface, not static queries with bind variables.

Core Concepts

Output Context Determines Encoding Function

Salesforce provides four Visualforce encoding functions. Applying the wrong one is as bad as applying none:

The critical rule: Visualforce auto-escapes {!field} in HTML context only. Inside a <script> block, {!field} is rendered raw. Every JavaScript variable assignment from a VF expression must use JSENCODE().

SOQL Injection via String Concatenation

Dynamic SOQL built by string concatenation with user input is injectable. The attacker can escape the intended query structure and append arbitrary conditions.

Vulnerable pattern:

String query = 'SELECT Id FROM Account WHERE Name = '' + userInput + ''';
List<Account> results = Database.query(query);

Safe pattern using bind variables:

String query = 'SELECT Id FROM Account WHERE Name = :userInput';
List<Account> results = Database.query(query);

Bind variables (:varName) prevent injection because the platform treats the bound value as a literal data value, never as query syntax. This is the authoritative solution for dynamic SOQL. String.escapeSingleQuotes() is a secondary measure only — it does not fully prevent injection in all edge cases.

LWC and Locker Service / Lightning Web Security

LWC runs inside Lightning Locker Service or Lightning Web Security depending on the org configuration. These sandbox the DOM and prevent cross-component script access. However, they do not prevent stored XSS in data coming from Apex. If an Apex controller returns maliciously crafted HTML from a user-supplied field and the LWC renders it with innerHTML, the attack succeeds. Safe LWC patterns use template bindings {field} which are auto-escaped by the framework.

Common Patterns

Encode All JavaScript Variable Assignments in Visualforce

When to use: Any Visualforce page that assigns Apex controller data into a JavaScript variable.

How it works:

<!-- VULNERABLE: raw expression in script block -->
<script>
    var accountName = '{!account.Name}';
</script>

<!-- SAFE: JSENCODE applied -->
<script>
    var accountName = '{!JSENCODE(account.Name)}';
</script>

Why not rely on auto-escape: Auto-escaping only applies to HTML contexts. JavaScript rendering receives the raw string before HTML interpretation.

Always Use Bind Variables in Dynamic SOQL

When to use: Any Apex code that builds a SOQL string dynamically with user-supplied data.

How it works:

// SAFE: bind variable
public List<Contact> searchContacts(String lastName) {
    return Database.query(
        'SELECT Id, Name FROM Contact WHERE LastName = :lastName'
    );
}

Whitelist PageReference Targets to Prevent Open Redirect

When to use: Any Apex controller that constructs a PageReference using URL parameters or user input.

How it works:

private static final Set<String> ALLOWED_PATHS = new Set<String>{
    '/home/home.jsp',
    '/lightning/page/home'
};

public PageReference redirect() {
    String retUrl = ApexPages.currentPage().getParameters().get('retUrl');
    if (retUrl != null && ALLOWED_PATHS.contains(retUrl)) {
        return new PageReference(retUrl);
    }
    return Page.DefaultLanding; // safe fallback
}

Decision Guidance

Recommended Workflow

Step-by-step instructions for reviewing code for XSS and injection issues:

1. Identify all input sources. Find every place user-controlled data enters the system: URL params, form fields, Apex REST endpoints, externally-loaded data. Map them to the data flow.
2. Identify all output points. For each input source, trace where the data is eventually rendered — HTML block, JavaScript block, HTML attribute, URL, or HTTP header.
3. Apply the correct encoding function for each output context. Use the context table above. Pay special attention to <script> blocks — they are the most common XSS location.
4. Audit all dynamic SOQL and SOSL. Search for Database.query( with string concatenation. Every such query must use bind variables for user input.
5. Audit PageReference construction. Any new PageReference(url) where the URL is derived from user input must be validated against an allowlist.
6. Check for CRLF injection. If the code sets HTTP response headers using user input, verify \r and \n characters are stripped before use.
7. Validate with Salesforce Code Analyzer. Run sf scanner run with PMD rules — it includes ApexXSSFromURLParam, ApexSOQLInjection, and ApexOpenRedirect rules.

Review Checklist

Run through these before marking work in this area complete:

• All <script> block variable assignments from Apex use {!JSENCODE(field)}
• No dynamic SOQL/SOSL uses string concatenation with user input — bind variables used
• All PageReference targets from user input validated against allowlist
• LWC components do not use innerHTML with user-sourced data
• HTTP response headers do not include user-supplied values without stripping \r\n
• Salesforce Code Analyzer (PMD) runs clean with no XSS or injection findings
• Stored user data rendered in Visualforce uses encoding appropriate for its output context

Salesforce-Specific Gotchas

Non-obvious platform behaviors that cause real production problems:

1. {!field} is NOT auto-encoded in JavaScript contexts — The single most common XSS vector in Visualforce. Every Apex expression inside a <script> block needs {!JSENCODE(...)} explicitly.
2. String.escapeSingleQuotes() does not fully prevent SOQL injection — It handles the common case but does not cover all edge cases. Always use bind variables as the primary defense.
3. LWC Locker Service does not sanitize Apex-returned HTML — Locker Service protects the DOM from cross-component access, not from XSS in Apex-returned data rendered via innerHTML.

Output Artifacts

Related Skills

• soql-security — CRUD/FLS enforcement in SOQL queries (sharing model, not injection)
• secure-coding-review-checklist — AppExchange security review and Checkmarx patterns