CVE-2021-21271: Tendermint Core is an open source Byzantine Fault Tolerant (BFT) middleware that takes a state transition machine - written in any programming languag...

Description

Tendermint Core is an open source Byzantine Fault Tolerant (BFT) middleware that takes a state transition machine - written in any programming language - and securely replicates it on many machines. Tendermint Core v0.34.0 introduced a new way of handling evidence of misbehavior. As part of this, we added a new Timestamp field to Evidence structs. This timestamp would be calculated using the same algorithm that is used when a block is created and proposed. (This algorithm relies on the timestamp of the last commit from this specific block.) In Tendermint Core v0.34.0-v0.34.2, the consensus reactor is responsible for forming DuplicateVoteEvidence whenever double signs are observed. However, the current block is still “in flight” when it is being formed by the consensus reactor. It hasn’t been finalized through network consensus yet. This means that different nodes in the network may observe different “last commits” when assigning a timestamp to DuplicateVoteEvidence. In turn, different nodes could form DuplicateVoteEvidence objects at the same height but with different timestamps. One DuplicateVoteEvidence object (with one timestamp) will then eventually get finalized in the block, but this means that any DuplicateVoteEvidence with a different timestamp is considered invalid. Any node that formed invalid DuplicateVoteEvidence will continue to propose invalid evidence; its peers may see this, and choose to disconnect from this node. This bug means that double signs are DoS vectors in Tendermint Core v0.34.0-v0.34.2. Tendermint Core v0.34.3 is a security release which fixes this bug. As of v0.34.3, DuplicateVoteEvidence is no longer formed by the consensus reactor; rather, the consensus reactor passes the Votes themselves into the EvidencePool, which is now responsible for forming DuplicateVoteEvidence. The EvidencePool has timestamp info that should be consistent across the network, which means that DuplicateVoteEvidence formed in this reactor should have consistent timestamps. This release changes the API between the consensus and evidence reactors.

Metrics

CVSS 3.1

6.5/10

CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H

EPSS Probability

1.74%

74.9th percentile

Probability of exploitation in the next 30 days. Learn more

Weakness Enumeration

CWE-400

Affected Software

Vendor	Product	Versions
Tendermint	Tendermint	>= 0.34.0, <= 0.34.2

References

https://github.com/tendermint/tendermint/blob/v0.34.3/CHANGELOG.md#v0.34.3Release Notes, Third Party Advisory
https://github.com/tendermint/tendermint/commit/a2a6852ab99e4a0f9e79f0ea8c1726e262e25c76Patch, Third Party Advisory
https://github.com/tendermint/tendermint/security/advisories/GHSA-p658-8693-mhvgThird Party Advisory
https://github.com/tendermint/tendermint/blob/v0.34.3/CHANGELOG.md#v0.34.3Release Notes, Third Party Advisory
https://github.com/tendermint/tendermint/commit/a2a6852ab99e4a0f9e79f0ea8c1726e262e25c76Patch, Third Party Advisory
https://github.com/tendermint/tendermint/security/advisories/GHSA-p658-8693-mhvgThird Party Advisory

Timeline

Published: Jan 26, 2021
Last Modified: Jun 17, 2026
Status: Modified

Frequently Asked Questions

What is CVE-2021-21271?

Tendermint Core is an open source Byzantine Fault Tolerant (BFT) middleware that takes a state transition machine - written in any programming language - and securely replicates it on many machines. Tendermint Core v0.34.0 introduced a new way of handling evidence of misbehavior. As part of this, we added a new Timestamp field to Evidence structs. This timestamp would be calculated using the same algorithm that is used when a block is created and proposed. (This algorithm relies on the timestamp of the last commit from this specific block.) In Tendermint Core v0.34.0-v0.34.2, the consensus reactor is responsible for forming DuplicateVoteEvidence whenever double signs are observed. However, the current block is still “in flight” when it is being formed by the consensus reactor. It hasn’t been finalized through network consensus yet. This means that different nodes in the network may observe different “last commits” when assigning a timestamp to DuplicateVoteEvidence. In turn, different nodes could form DuplicateVoteEvidence objects at the same height but with different timestamps. One DuplicateVoteEvidence object (with one timestamp) will then eventually get finalized in the block, but this means that any DuplicateVoteEvidence with a different timestamp is considered invalid. Any node that formed invalid DuplicateVoteEvidence will continue to propose invalid evidence; its peers may see this, and choose to disconnect from this node. This bug means that double signs are DoS vectors in Tendermint Core v0.34.0-v0.34.2. Tendermint Core v0.34.3 is a security release which fixes this bug. As of v0.34.3, DuplicateVoteEvidence is no longer formed by the consensus reactor; rather, the consensus reactor passes the Votes themselves into the EvidencePool, which is now responsible for forming DuplicateVoteEvidence. The EvidencePool has timestamp info that should be consistent across the network, which means that DuplicateVoteEvidence formed in this reactor should have consistent timestamps. This release changes the API between the consensus and evidence reactors.

How severe is CVE-2021-21271?

CVE-2021-21271 has a CVSS score of 6.5/10 (MEDIUM severity). The EPSS model estimates a 1.74% probability of exploitation in the next 30 days.

How do I fix CVE-2021-21271?

Check the vendor references and advisories linked above for patched versions and mitigation guidance. You can also run a Strix scan to test if your systems are affected.