Integer Underflow

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Pronunciation

[in-tuh-jer un-der-floh]

Analogy

Integer underflow is like a countdown timer going below zero and wrapping to the highest value.

Definition

A vulnerability where subtracting below zero wraps the value to the maximum of the data type.

Key Points Intro

Underflows happen via fixed‑width subtraction through:

Key Points

Wrapping behavior: Negative results modulo 2ⁿ.

Unchecked operations: Pre‑0.8 Solidity allowed wrap.

Balance manipulation: Can inflate values unexpectedly.

Mitigation: Built‑in checks or SafeMath subtraction.

Example

``` uint8 x = 0; x -= 1; // x becomes 255 in older ```

Technical Deep Dive

`SUB` opcode wraps on underflow. ≥0.8 inserts a check that reverts on underflow. Developers can use `unchecked` blocks to skip checks when safe.

Security Warning

Underflow can be exploited to set large balances or bypass limits—always enable compiler checks.

Caveat

Unchecked contexts reintroduce underflow risk; audit `unchecked` usage carefully.

Related Terms

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