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. O3 Security confirms whether CVE-2021-21405 is actually reachable in your code before you act, and blocks exploitation at runtime until you patch.
BLS Signature "Malleability"
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
Lotus is an Implementation of the Filecoin protocol written in Go. 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. 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.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐹Go | github.com/filecoin-project/lotus | all versions | 1.5.0 |
Research 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. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.
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
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 pinpoints whether CVE-2021-21405 is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.
Tailored to CVE-2021-21405. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is CVE-2021-21405 in your dependencies?
O3 detects CVE-2021-21405 across Go dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.