CWE-93 CRLF Injection

Description

CRLF Injection

Reference: https://cwe.mitre.org/data/definitions/93.html

OWASP Category: A03:2021 – Injection

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Vulnerable Pattern

❌ Example 1: Vulnerable Pattern

// VULNERABLE: User input directly in header
response.setHeader("X-Custom", userInput);
// VULNERABLE: CRLF in redirect
response.sendRedirect(userUrl);

Why it's vulnerable: This pattern is vulnerable to CRLF Injection

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Deterministic Fix

✅ Secure Implementation: Secure Implementation

// SECURE: Remove CR/LF characters
String safeValue = userInput.replaceAll("[\\r\\n]", "");
response.setHeader("X-Custom", safeValue);

// For URLs, use URL encoding
String safeUrl = URLEncoder.encode(userUrl, StandardCharsets.UTF_8);

Why it's secure: Implements proper protection against CRLF Injection

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Detection Pattern

Look for these patterns in your codebase:

# Find setHeader with user input
grep -rn "setHeader.*getParameter\\|addHeader.*request" --include="*.java"

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Remediation Steps

1. Strip CR (\r) and LF (\n) from all header values

2. URL-encode user input used in redirects

3. Use framework-provided methods that auto-sanitize

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Key Imports

import java.net.URLEncoder;

import java.nio.charset.StandardCharsets;

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Verification

After remediation:

• Run SAST scanner to confirm vulnerability is resolved

• Review all instances of the vulnerable pattern

• Add unit tests that verify the secure implementation

• Check for similar patterns in related code

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Trigger Examples

Fix CWE-93 vulnerability
Resolve CRLF Injection issue
Secure this Java code against crlf injection
SAST reports CWE-93

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Common Vulnerable Locations

Layer	Files	Patterns

| Controller | *Controller.java | User input handling |

| Service | *Service.java | Business logic |

| Repository | *Repository.java | Data access |

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References

• CWE-93: CRLF Injection

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Source: Generated by Java CWE Security Skills Generator Last Updated: 2026-03-07