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GHSA-fcmm-54jp-7vf6
No affected components available
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.
Patches
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.
Workarounds
None.
References
A similar issue was presented in Geth's implementation and the fix can be found here.
The vulnerability can be exploited over the network without needing physical access. It is easy for an attacker to exploit this vulnerability. An attacker does not need any special privileges or access rights. No user interaction is needed for the attacker to exploit this vulnerability. The impact is confined to the system where the vulnerability exists. There is a high impact on the availability of the system.
Exploitation attempts have been detected. Elevated vigilance and prompt remediation are advised.
The exploit probability is very low. The vulnerability is unlikely to be exploited in the next 30 days.
We did not find any exploit available. Neither in GitHub repositories nor in the Exploit-Database.
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