# Cwe 191 Integer Underflow

> Use this skill when you need to remediate CWE-191 (Integer Underflow) vulnerabilities in Java code. Triggers on SAST findings, security reviews, or when fixing integer underflow issues.

- Skill: `developerscoffee/cwe-191-integer-underflow` (Agent Skill)
- Install (CLI): `npx skillmds@latest add developerscoffee/cwe-191-integer-underflow`
- Raw SKILL.md: https://api.skillmd.com/api/skills/developerscoffee/cwe-191-integer-underflow/raw
- Safety review: pending (external: skill-scanner PASS, skillspector PASS)
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Security
- License: MIT
- Author: DevelopersCoffee (https://skillmd.com/u/developerscoffee)
- Updated: 2026-09-10
- Page: https://skillmd.com/skills/developerscoffee/cwe-191-integer-underflow

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# CWE-191 Integer Underflow

## Description

Integer Underflow

Reference:
https://cwe.mitre.org/data/definitions/191.html


**OWASP Category**: A03:2021 – Injection


---

## Vulnerable Pattern


### ❌ Example 1: Vulnerable Pattern

```java
// VULNERABLE: Possible underflow
int balance = user.getBalance();
int withdrawal = request.getAmount();
user.setBalance(balance - withdrawal); // Could underflow
```


**Why it's vulnerable:** This pattern is vulnerable to Integer Underflow




---

## Deterministic Fix


### ✅ Secure Implementation: Secure Implementation

```java
// SECURE: Check for underflow
int balance = user.getBalance();
int withdrawal = request.getAmount();
if (withdrawal > balance) {
    throw new IllegalArgumentException("Insufficient funds");
}
// Or use Math.subtractExact for automatic overflow detection
user.setBalance(Math.subtractExact(balance, withdrawal));
```


**Why it's secure:** Implements proper protection against Integer Underflow




---

## Detection Pattern

Look for these patterns in your codebase:


```bash
# Find subtraction operations
grep -rn "balance.*-\\|index.*-" --include="*.java"
```



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## Remediation Steps


1. Validate input values before arithmetic

2. Use Math.subtractExact() for automatic overflow/underflow detection

3. Check bounds before array index calculations


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## Key Imports

```java

import java.lang.Math;

```

---

## 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


---

## Trigger Examples

```
Fix CWE-191 vulnerability
Resolve Integer Underflow issue
Secure this Java code against integer underflow
SAST reports CWE-191
```

---

## Common Vulnerable Locations

| Layer | Files | Patterns |
|-------|-------|----------|

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

| Service | `*Service.java` | Business logic |

| Repository | `*Repository.java` | Data access |


---

## References


- [CWE-191: Integer Underflow](https://cwe.mitre.org/data/definitions/191.html)


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**Source**: Generated by [Java CWE Security Skills Generator](https://github.com/DevelopersCoffee/java-cwe-security-skills)
**Last Updated**: 2026-03-07