What is CVE-2021-21405?

CVE-2021-21405 is a medium-severity security vulnerability in github.com/filecoin-project/lotus (go), affecting versions < 1.5.0. It is fixed in 1.5.0.

How severe is CVE-2021-21405?

CVE-2021-21405 has a CVSS score of 5.9 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.

Which versions of github.com/filecoin-project/lotus are affected by CVE-2021-21405?

github.com/filecoin-project/lotus (go) versions < 1.5.0 is affected.

Is there a fix for CVE-2021-21405?

Yes. CVE-2021-21405 is fixed in 1.5.0. Upgrade to this version or later.

Is CVE-2021-21405 exploitable, and should I be worried?

Whether CVE-2021-21405 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk.

What actually determines whether CVE-2021-21405 is exploitable, and how bad it is?

Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.

How do I fix CVE-2021-21405?

Upgrade github.com/filecoin-project/lotus to 1.5.0 or later.

CVE-2021-21405 in github.com/filecoin-project/lotus

Q: How severe is CVE-2021-21405?

CVE-2021-21405 has a CVSS score of 5.9 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.

Q: Which versions of github.com/filecoin-project/lotus are affected by CVE-2021-21405?

github.com/filecoin-project/lotus (go) versions < 1.5.0 is affected.

Q: Is there a fix for CVE-2021-21405?

Yes. CVE-2021-21405 is fixed in 1.5.0. Upgrade to this version or later.

Q: Is CVE-2021-21405 exploitable, and should I be worried?

Whether CVE-2021-21405 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk.

Q: What actually determines whether CVE-2021-21405 is exploitable, and how bad it is?

Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.

Q: How do I fix CVE-2021-21405?

Upgrade github.com/filecoin-project/lotus to 1.5.0 or later.

Summary

BLS Signature "Malleability"

References

Original POC: https://gist.github.com/wadeAlexC/2490d522e81a796af9efcad1686e6754

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.

CVE-2021-21405 has a CVSS score of 5.9 (Medium). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (1.5.0); upgrading removes the vulnerable code path.

Affected versions

github.com/filecoin-project/lotus (< 1.5.0)

Security releases

github.com/filecoin-project/lotus → 1.5.0 (go)

Kodem intelligence

Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.

Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.

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Remediation advice

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

Frequently Asked Questions

What is CVE-2021-21405? CVE-2021-21405 is a medium-severity security vulnerability in github.com/filecoin-project/lotus (go), affecting versions < 1.5.0. It is fixed in 1.5.0.
How severe is CVE-2021-21405? CVE-2021-21405 has a CVSS score of 5.9 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
Which versions of github.com/filecoin-project/lotus are affected by CVE-2021-21405? github.com/filecoin-project/lotus (go) versions < 1.5.0 is affected.
Is there a fix for CVE-2021-21405? Yes. CVE-2021-21405 is fixed in 1.5.0. Upgrade to this version or later.
Is CVE-2021-21405 exploitable, and should I be worried? Whether CVE-2021-21405 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
What actually determines whether CVE-2021-21405 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
How do I fix CVE-2021-21405? Upgrade github.com/filecoin-project/lotus to 1.5.0 or later.