How severe is CVE-2021-21271?

CVE-2021-21271 has a CVSS score of 6.5/10, rated MEDIUM. Review your exposure and patch according to your risk tolerance.

Which packages are affected by CVE-2021-21271?

CVE-2021-21271 affects the following packages: github.com/tendermint/tendermint (Go). Ecosystems affected: Go.

How do I fix CVE-2021-21271?

Update github.com/tendermint/tendermint to 0.34.3 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-21271 is resolved across your whole dependency graph.

How do I detect CVE-2021-21271 in my Go dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for github.com/tendermint/tendermint, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

How do I mitigate CVE-2021-21271 if there is no patch (or I can't update yet)?

Cap what an attacker can consume: apply request size, rate and timeout limits in front of the affected component, and run it with memory and CPU limits so exhaustion degrades one worker rather than the whole service.

Is CVE-2021-21271 actively exploited in the wild?

No public exploit code has been indexed for CVE-2021-21271 yet. This does not mean the vulnerability cannot be exploited — absence of public exploits does not imply safety. Apply the recommended fix and use O3 Security to monitor your exposure.

What is the EPSS score for CVE-2021-21271?

CVE-2021-21271 has an EPSS (Exploit Prediction Scoring System) score of 1.8%, placing it in the 78th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. This score indicates relatively lower exploitation probability, though the CVSS severity should still guide your patching priority.

What type of vulnerability is CVE-2021-21271?

CVE-2021-21271 is classified as Uncontrolled Resource Consumption (CWE-400). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

When was CVE-2021-21271 published, and has it been updated?

CVE-2021-21271 was published on January 26, 2021 and was last updated on October 20, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

🐹

🐹 Go

Not in CISA KEV

MEDIUM severity

CVE-2021-21271 — tendermint

MEDIUMFix: tendermint/tendermint@a2a6852

CVE-2021-21271 is a medium-severity (CVSS 6.5) Uncontrolled Resource Consumption vulnerability in github.com/tendermint/tendermint. A fix is available for github.com/tendermint/tendermint — see the affected versions and patch details below.

Tendermint Core vulnerable to Uncontrolled Resource Consumption

Also known asGHSA-p658-8693-mhvgGO-2022-1052

Published

Jan 26, 2021

Updated

Oct 20, 2026

Affected

1 pkg

Patched

1 / 1

Exploits

None indexed

Exploitation data as of Nov 7, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

EPSS Exploitation Probability

via FIRST.org ↗

1.8%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs78th percentile — riskier than 78% of all scored CVEsHighest risk

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

How urgent is this, really

Where this sits among everything scored

Real-World Exposure

1 pkg affected

🐹github.com/tendermint/tendermint

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects Go packages — download data is not available via public APIs for these ecosystems.

Description

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.

Impact

This is a denial-of-service vector which impacts networks running Tendermint Core v0.34.0 - v0.34.2.

Remediation

This problem has been patched in Tendermint Core v0.34.3. Networks running impacted versions of Tendermint Core should update immediately.

Workarounds

There are no workarounds, other than upgrading to a patched version of Tendermint Core.

Credits

Crypto.com (@cyril-crypto, @brianatcrypto, @tomtau and @yihuang) for finding and submitting this vulnerability
@melekes and @cmwaters for identifying the root cause and patching the problem

For more information

If you have any questions or comments about this advisory:

Open an issue in tendermint/tendermint
Email us at [email protected]

Affected Packages

1 total 1 fixed

Ecosystem	Package	Vulnerable range	Fix
🐹Go	`github.com/tendermint/tendermint`	≥ 0.34.0&&< 0.34.3	0.34.3`go get github.com/tendermint/tendermint@v0.34.3`

Affected Products

1 product · 1 configurations

Application

tenderminttendermint

≥ 0.34.0 && ≤ 0.34.2

range

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for github.com/tendermint/tendermint, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update github.com/tendermint/tendermint to 0.34.3 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-21271 is resolved across your whole dependency graph.
Workarounds
Cap what an attacker can consume: apply request size, rate and timeout limits in front of the affected component, and run it with memory and CPU limits so exhaustion degrades one worker rather than the whole service.

Frequently Asked Questions

### Description 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](https://github.com/tendermint/tendermint/pull/5219). 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