The Even Number DoS: Cooking a Blockchain with Bad Math
Jan 4, 2026·7 min read·2 visits
Executive Summary (TL;DR)
A math function in Frontier's Ethereum layer is secretly 20x slower for even numbers than odd ones, but the gas fee was the same. Attackers could spam cheap transactions using even numbers to trigger this slow path, causing a Denial of Service and potentially halting the blockchain.
CVE-2023-28431 is a critical Denial of Service vulnerability in Frontier, an Ethereum compatibility layer for Substrate. The issue stems from a gross miscalculation in transaction costs for a cryptographic precompile. A specific mathematical operation, modular exponentiation, is dramatically slower when using even numbers as a modulus due to an underlying library's implementation. Frontier failed to charge extra for this slow path, allowing an attacker to submit cheap transactions that consume massive amounts of computational power, effectively grinding the entire network to a halt for pennies on the dollar.
Fix Analysis (1)
Technical Appendix
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:HAffected Systems
Affected Versions Detail
| Product | Affected Versions | Fixed Version |
|---|---|---|
Frontier paritytech | All versions before the inclusion of commit 5af12e94d7dfc8a0208a290643a800f55de7b219 | Not specified, but patched in commit 5af12e94d7dfc8a0208a290643a800f55de7b219 |
| Attribute | Detail |
|---|---|
| CWE ID | CWE-682: Incorrect Calculation |
| Attack Vector | Network |
| Attack Complexity | Low |
| Privileges Required | None |
| CVSS v3.1 Score | 7.5 (High) |
| EPSS Score | 0.25% (0.00249) |
| Impact | Denial of Service |
| Exploit Status | Proof-of-Concept |
MITRE ATT&CK Mapping
The software performs a calculation that results in an incorrect value. In this case, the gas cost calculation did not accurately reflect the computational resources required for modular exponentiation operations with even moduli, creating an economic imbalance that leads to a Denial of Service condition.