# Improper Output Neutralization for Logs (CWE-117)

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

- 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 4 Shoulder detection rules.

### Go

Strip newlines and control characters from user input before logging

### Node.js

Strip newline characters from user input before writing to log files

Sanitize user input by stripping CRLF characters before writing to logs

### Python

Use structured logging with separate fields for user data instead of string interpolation

## Warning Signs

- [MEDIUM] unsanitized user input flowing into log statements, enabling log forging attacks
- [MEDIUM] user input flowing to logging functions without sanitization
- [MEDIUM] user input flowing directly into log messages 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

### Go (1 rules)

- **Log Injection / Log Forging** [MEDIUM]: Detects unsanitized user input flowing into log statements, enabling log forging attacks.
  - Remediation: Remove newlines and control characters from user input before logging.

```go
sanitized := strings.ReplaceAll(userInput, "\n", "")
sanitized = strings.ReplaceAll(sanitized, "\r", "")
log.Printf("User action: %s", sanitized)
```

Learn more: https://shoulder.dev/learn/go/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

### Python (1 rules)

- **Log Injection / Log Forging** [MEDIUM]: Detects user input flowing directly into log messages without sanitization.
  - Remediation: Use structured logging with separate fields for user data.

```python
logging.info("Login attempt", extra={'username': username})
```

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