# CVE-2021-21405: BLS Signature "Malleability"

> BLS signature malleability allows creating duplicate block CIDs, potentially causing valid blocks to be rejected and punishing miners.

- **Severity:** Medium 5.9 (`CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:H/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 15 Apr 2021
- **Updated:** 3 Aug 2024
- **Also known as:** GHSA-4g52-pqcj-phvh, GO-2022-0905

## What is CVE-2021-21405?

CVE-2021-21405 is a medium-severity vulnerability in filecoin-project lotus, published on 15 Apr 2021. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official 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.

Weakness: CWE-347 (Improper Verification of Cryptographic Signature)

## Which versions are affected by CVE-2021-21405?

3 open source packages ship an affected version, including github.com/filecoin-project/lotus. It affects 1 product.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `github.com/filecoin-project/lotus` | Go | All versions | 1.5.0 |
| `github.com/filecoin-project/lotus` | Go | All versions | 1.5.0 |
| `github.com/filecoin-project/lotus` | Go | All versions | 1.5.0 |

Products: Filecoin Lotus.

## How do I fix CVE-2021-21405?

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

## Is CVE-2021-21405 being exploited?

There is no known exploitation of CVE-2021-21405 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2021-21405 and fix it if it is affected.

What it is: CVE-2021-21405 is a medium-severity vulnerability in filecoin-project lotus, published on 15 Apr 2021. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2021-21405 in the wild and no public exploit code.

Affected packages and fixed versions:
- github.com/filecoin-project/lotus (Go): affected all versions; fixed in 1.5.0
- github.com/filecoin-project/lotus (Go): affected all versions; fixed in 1.5.0
- github.com/filecoin-project/lotus (Go): affected all versions; fixed in 1.5.0

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2021-21405/ (Markdown: https://insights.sbomify.com/CVE-2021-21405/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-21405
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-21405
- OSV: https://osv.dev/vulnerability/CVE-2021-21405

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From sbomify Insights: https://insights.sbomify.com/CVE-2021-21405/
Summaries are generated and may contain mistakes; the official description above is the record.