# Improper Output Neutralization for Logs (CWE-117)

The product does not neutralize or incorrectly neutralizes output that is written to logs.

**Stack:** JavaScript

- Prevalence: Medium 3 languages covered
- Impact: Medium Review recommended
- Prevention: Documented 4 fix examples

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

## Description

Log injection attacks occur when user input is written to log files without proper sanitization. This can allow attackers to forge log entries, inject malicious content, or exploit log analysis tools.

## Prevention

Prevention strategies for Log Injection based on 2 Shoulder detection rules.

### JavaScript

Strip newline characters from user input before writing to log files

Sanitize user input by stripping CRLF characters before writing to logs

## Warning Signs

- [MEDIUM] user input flowing to logging functions without sanitization
- [LOW] user input flowing to persistent log files without sanitization

## Consequences

- Modify Application Data
- Hide Activities
- Execute Unauthorized Code

## Mitigations

- Validate and sanitize all input before writing to logs
- Use structured logging formats that separate data from log syntax
- Encode special characters when writing user-controlled data to logs

## Detection

- Total rules: 4
- Languages: go, javascript, typescript, python

## Rules by Language

### Javascript (2 rules)

- **Log Injection** [LOW]: Detects user input flowing to persistent log files without sanitization.
  - Remediation: Sanitize user input by removing newline characters before logging.

```javascript
const safe = userInput.replace(/[\r\n]/g, '');
logger.info(safe);
```

Learn more: https://shoulder.dev/learn/javascript/cwe-117/log-injection
- **Log Injection** [MEDIUM]: Detects user input flowing to logging functions without sanitization.
Allows log forgery via newline injection.
  - Remediation: Sanitize user input before logging to prevent log forgery:

```javascript
const sanitize = (str) => str.replace(/[\r\n]/g, '').substring(0, 200);
logger.info('Login attempt', { username: sanitize(req.body.username) });
```

Learn more: https://shoulder.dev/learn/javascript/cwe-117/log-injection

### Typescript (1 rules)

- **Log Injection** [MEDIUM]: Detects user input flowing to logging functions without sanitization.
Allows log forgery via newline injection.
  - Remediation: Sanitize user input before logging to prevent log forgery:

```javascript
const sanitize = (str) => str.replace(/[\r\n]/g, '').substring(0, 200);
logger.info('Login attempt', { username: sanitize(req.body.username) });
```

Learn more: https://shoulder.dev/learn/javascript/cwe-117/log-injection