# Improper Handling of Unicode Encoding (CWE-176)

The product does not properly handle when an input contains Unicode encoding.

- Prevalence: Mittel 3 Sprachen abgedeckt
- Impact: Mittel Review empfohlen
- Prevention: Dokumentiert 3 Fix-Beispiele

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

## Description

Unicode characters can have multiple encodings or representations. If an application does not properly handle Unicode, attackers may be able to bypass security filters or cause unexpected behavior using alternate encodings.

## Prevention

Präventionsstrategien für Improper Handling of Unicode basierend auf 3 Shoulder-Erkennungsregeln.

### Go

Normalize strings with NFKC before security-sensitive comparisons

### JavaScript

Normalize Unicode strings with NFKC before security-sensitive comparisons

### Python

Normalize Unicode strings with NFKC before comparison or security-critical operations

## Warning Signs

- [MEDIUM] missing Unicode normalization in security-sensitive string comparisons

## Consequences

- Schutzmechanismus umgehen
- Nicht autorisierten Code ausführen

## Mitigations

- Unicode-Eingaben vor der Verarbeitung in eine kanonische Form normalisieren
- Sicherheitsprüfungen erst nach der Unicode-Normalisierung anwenden
- Verwende Vergleichsfunktionen, die Unicode korrekt behandeln

## Detection

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

## Rules by Language

### Go (1 rules)

- **Unicode Normalization Security Issues** [MEDIUM]: Security-sensitive string comparison without Unicode normalization.
  - Remediation: Normalize strings with NFKC before security-sensitive comparisons.

```go
import "golang.org/x/text/unicode/norm"

func isAdmin(username string) bool {
    normalized := norm.NFKC.String(strings.ToLower(username))
    return normalized == "admin"
}
```

Learn more: https://shoulder.dev/learn/go/cwe-176/unicode-normalization

### Javascript (1 rules)

- **Unicode Normalization Security Issues** [MEDIUM]: Detects missing Unicode normalization in security-sensitive string comparisons.
Unicode allows multiple representations of visually identical characters, which
attackers can exploit to bypass input validation, authentication, or access control.

Common attack vectors:
- Homograph attacks (using lookalike characters): "аdmin" vs "admin" (Cyrillic 'а')
- Case folding differences: "ß" (German sharp s) becomes "SS" when uppercased
- Combining characters: "é" can be a single char or 'e' + combining a
  - Remediation: Normalize Unicode strings with NFKC before security-sensitive comparisons:

```javascript
app.post('/login', (req, res) => {
  const username = req.body.username.normalize('NFKC').toLowerCase();
  if (username === 'admin') {
    return res.send('Admin access');
  }
  res.send('User access');
});
```

Learn more: https://shoulder.dev/learn/javascript/cwe-176/unicode-normalization

### Typescript (1 rules)

- **Unicode Normalization Security Issues** [MEDIUM]: Detects missing Unicode normalization in security-sensitive string comparisons.
Unicode allows multiple representations of visually identical characters, which
attackers can exploit to bypass input validation, authentication, or access control.

Common attack vectors:
- Homograph attacks (using lookalike characters): "аdmin" vs "admin" (Cyrillic 'а')
- Case folding differences: "ß" (German sharp s) becomes "SS" when uppercased
- Combining characters: "é" can be a single char or 'e' + combining a
  - Remediation: Normalize Unicode strings with NFKC before security-sensitive comparisons:

```javascript
app.post('/login', (req, res) => {
  const username = req.body.username.normalize('NFKC').toLowerCase();
  if (username === 'admin') {
    return res.send('Admin access');
  }
  res.send('User access');
});
```

Learn more: https://shoulder.dev/learn/javascript/cwe-176/unicode-normalization

### Python (1 rules)

- **Unicode Normalization Issues** [MEDIUM]: Detects missing Unicode normalization leading to security bypasses.
  - Remediation: Normalize Unicode strings before comparison using unicodedata.normalize().

```python
import unicodedata

def normalize_username(username):
    return unicodedata.normalize('NFKC', username).lower()

if normalize_username(input_name) == normalize_username(stored_name):
    grant_access()
```

Learn more: https://shoulder.dev/learn/python/cwe-176/unicode-normalization