# Improper Neutralization of CRLF Sequences in HTTP Headers ('HTTP Response Splitting') (CWE-113)

The product receives data from an HTTP agent/component, and it places this data in HTTP response headers without neutralizing CRLF sequences.

**Stack:** Go

- Prevalence: Medium 3 languages covered
- Impact: High 2 high-severity rules
- Prevention: Documented 3 fix examples

**OWASP:** Injection (A03:2021-Injection) - #3

## Description

An attacker can inject CRLF sequences into HTTP headers to create additional headers or response body content. This can lead to cache poisoning, cross-site scripting, or other attacks.

## Prevention

Prevention strategies for HTTP Response Splitting based on 1 Shoulder detection rules.

### Go

Strip CRLF characters from user input before setting HTTP headers

## Warning Signs

- [MEDIUM] user input flowing to HTTP headers without CRLF sanitization

## Consequences

- Execute Unauthorized Code
- Bypass Protection Mechanism
- Modify Application Data

## Mitigations

- Never directly include user input in HTTP response headers
- Sanitize all user input that might be included in headers
- Use framework-provided methods for setting headers that handle encoding

## Detection

- Total rules: 3
- Languages: go, javascript, typescript, python

## Rules by Language

### Go (1 rules)

- **HTTP Header Injection** [MEDIUM]: Detects user input flowing to HTTP headers without CRLF sanitization.
  - Remediation: Remove CRLF characters from user input before setting headers.

```go
value := strings.ReplaceAll(userInput, "\r", "")
value = strings.ReplaceAll(value, "\n", "")
w.Header().Set("X-Custom", value)
```

Learn more: https://shoulder.dev/learn/go/cwe-113/header-injection