Summary
Frontier's modexp precompile is slow for even modulus
Workarounds
None.
References
A similar issue was presented in Geth's implementation and the fix can be found here.
Impact
Frontier's modexp precompile uses num-bigint crate under the hood. In the implementation, 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.
CVE-2023-28431 has a CVSS score of 7.5 (High). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. No fixed version is listed yet, so configuration controls and monitoring matter more in the interim.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
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No fixes for num-bigint is currently available, and thus this advisory will be first 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 PR 1017.
The recommendations are as follows:
- If you anticipate malicious validators, it's recommended to issue an emergency runtime upgrade as soon as possible.
- If you do not anticipate malicious validators, it's recommended to issue a normal runtime upgrade, as Substrate has builtin timeout protection when validators are building blocks.
Frequently Asked Questions
- What is CVE-2023-28431? CVE-2023-28431 is a high-severity security vulnerability in pallet-evm-precompile-modexp (rust), affecting versions <= 1.0.0. No fixed version is listed yet.
- How severe is CVE-2023-28431? CVE-2023-28431 has a CVSS score of 7.5 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
- Which versions of pallet-evm-precompile-modexp are affected by CVE-2023-28431? pallet-evm-precompile-modexp (rust) versions <= 1.0.0 is affected.
- Is there a fix for CVE-2023-28431? No fixed version is listed for CVE-2023-28431 yet. Monitor the advisory for updates and apply mitigations in the interim.
- Is CVE-2023-28431 exploitable, and should I be worried? Whether CVE-2023-28431 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2023-28431 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.