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

> Attackers can cause denial of service in Frontier by exploiting slow even-modulus calculations in the modexp precompile due to unaccounted gas costs.

- **Severity:** High 7.5 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 22 Mar 2023
- **Updated:** 25 Feb 2025
- **Also known as:** GHSA-fcmm-54jp-7vf6

## What is CVE-2023-28431?

CVE-2023-28431 is a high-severity denial of service vulnerability in paritytech frontier, published on 22 Mar 2023. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official 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.

Weakness: CWE-682 (Incorrect Calculation)

## Which versions are affected by CVE-2023-28431?

2 open source packages ship an affected version, including pallet-evm-precompile-modexp. It affects 1 product.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `pallet-evm-precompile-modexp` | crates.io | All versions to 1.0.0 | None |
| `pallet-evm-precompile-modexp` | crates.io | All versions to 1.0.0 | None |

Products: Parity Frontier.

## How do I fix CVE-2023-28431?

No fix has been published for CVE-2023-28431 yet. Limit exposure of the affected software until one is available.

## Is CVE-2023-28431 being exploited?

There is no known exploitation of CVE-2023-28431 in the wild and no public exploit code.

## Other vulnerabilities in paritytech frontier

- [CVE-2023-45130](https://insights.sbomify.com/CVE-2023-45130/): Frontier opcode SUICIDE touches too many storage values on large contracts (high 7.5)
- [CVE-2022-36008](https://insights.sbomify.com/CVE-2022-36008/): Message length overflow in frontier (high 7.1)
- [CVE-2022-21685](https://insights.sbomify.com/CVE-2022-21685/): Integer underflow in Frontier (medium 6.5)
- [CVE-2022-39242](https://insights.sbomify.com/CVE-2022-39242/): Incorrect Calculation in Frontier leads to inflated Ethereum chain gas prices (medium 5.3)
- [CVE-2022-31111](https://insights.sbomify.com/CVE-2022-31111/): Discrepency in transfer value and actual value due to incorrect truncation in Frontier (medium 5.3)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2023-28431 and fix it if it is affected.

What it is: CVE-2023-28431 is a high-severity denial of service vulnerability in paritytech frontier, published on 22 Mar 2023. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2023-28431 in the wild and no public exploit code.

Affected packages and fixed versions:
- pallet-evm-precompile-modexp (crates.io): affected all versions up to 1.0.0; no fix listed
- pallet-evm-precompile-modexp (crates.io): affected all versions up to 1.0.0; no fix listed

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2023-28431/ (Markdown: https://insights.sbomify.com/CVE-2023-28431/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2023-28431
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2023-28431
- OSV: https://osv.dev/vulnerability/CVE-2023-28431

---
From sbomify Insights: https://insights.sbomify.com/CVE-2023-28431/
Summaries are generated and may contain mistakes; the official description above is the record.