CVE-2023-28431: Frontier's modexp precompile is slow for even modulus

Severity: high (CVSS 7.5 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.9% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2023-03-22 · updated 2026-06-17
Weakness: CWE-682

Description

Frontier is an Ethereum compatibility layer for Substrate. Frontier's `modexp` precompile uses `num-bigint` crate under the hood. In the implementation prior to pull request 1017, the cases for modulus being even and modulus being odd are treated separately. Odd modulus uses the fast Montgomery multiplication, and even modulus uses the slow plain power algorithm. This gas cost discrepancy was not accounted for in the `modexp` precompile, leading to possible denial of service attacks. No fixes for `num-bigint` are currently available, and thus this issue is fixed in the short term by raising the gas costs for even modulus, and in the long term fixing it in `num-bigint` or switching to another modexp implementation. The short-term fix for Frontier is deployed at pull request 1017. There are no known workarounds aside from applying the fix.

Affected products

paritytech frontier: before 2023-03-15 — fixed in 2023-03-15

How to fix

Upgrade to a fixed version: paritytech frontier 2023-03-15. Check the vendor advisory for mitigations.

References

Get alerted when CVE-2023-28431 or the next vulnerability affects your stack