CVE-2021-21405 is a high-severity (CVSS 7.5) CWE-347 vulnerability in github.com/filecoin-project/lotus. 1 public exploit reference exists, so weaponization risk is real. A fix is available for github.com/filecoin-project/lotus — see the affected versions and patch details below.
BLS Signature "Malleability"
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
How urgent is this, really
CVE-2021-21405 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.
Where this sits among everything scored
Of 385,738 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Counts from FIRST.org, log-scaled.
Real-World Exposure
github.com/filecoin-project/lotusReal-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
Impact
- BLS signature validation in lotus uses blst library method VerifyCompressed. This method accepts signatures in 2 forms - "serialized", and "compressed", meaning that BLS signatures can be provided as either of 2 unique byte arrays.
- Lotus block validation functions perform a uniqueness check on provided blocks. Two blocks are considered distinct if the CIDs of their blockheader do not match. The CID method for blockheader includes the BlockSig of the block.
The result of these issues is that it would be possible to punish miners for valid blocks, as there are two different valid block CIDs available for each block, even though this must be unique.
Patches
By switching from the go based blst bindings over to the bindings in filecoin-ffi, the code paths now ensure that all signatures are compressed by size and the way they are deserialized.
This happened in https://github.com/filecoin-project/lotus/pull/5393
References
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐹Go | github.com/filecoin-project/lotus | all versions | 1.5.0go get github.com/filecoin-project/lotus@v1.5.0 |
Affected Products
lotusfilecoinResearch use only. For defensive security, authorized penetration testing, and academic research only. Never execute exploit code against systems without explicit written authorization.
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for github.com/filecoin-project/lotus, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update github.com/filecoin-project/lotus to 1.5.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-21405 is resolved across your whole dependency graph.
Workarounds
Do not deserialise data from untrusted sources: where the format allows it, restrict deserialisation to an explicit allowlist of expected types, and prefer a data-only format (JSON, Protobuf) over one that can reconstruct arbitrary objects until you can upgrade.
Frequently Asked Questions
Is CVE-2021-21405 in your dependencies?
Find it across Go, including transitive dependencies.