# CVE-2021-31876: Business logic in n/a

> Bitcoin Core fails to enforce BIP125 replacement rules, allowing attackers to cause denial of service or loss of funds in downstream projects.

- **Severity:** Medium 6.5 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:L`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 13 May 2021
- **Updated:** 3 Aug 2024
- **Also known as:** GHSA-cm2c-8c7c-974f

## What is CVE-2021-31876?

CVE-2021-31876 is a medium-severity vulnerability in n/a, published on 13 May 2021. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

Bitcoin Core 0.12.0 through 0.21.1 does not properly implement the replacement policy specified in BIP125, which makes it easier for attackers to trigger a loss of funds, or a denial of service attack against downstream projects such as Lightning network nodes. An unconfirmed child transaction with nSequence = 0xff_ff_ff_ff, spending an unconfirmed parent with nSequence <= 0xff_ff_ff_fd, should be replaceable because there is inherited signaling by the child transaction. However, the actual PreChecks implementation does not enforce this. Instead, mempool rejects the replacement attempt of the unconfirmed child transaction.

Weakness: CWE-863 (Incorrect Authorization)

## Which versions are affected by CVE-2021-31876?

It affects 1 product.


Products: Bitcoin Bitcoin.

## How do I fix CVE-2021-31876?

No fix has been published for CVE-2021-31876 yet. Limit exposure of the affected software until one is available.

## Is CVE-2021-31876 being exploited?

There is no known exploitation of CVE-2021-31876 in the wild and no public exploit code.

## Other vulnerabilities in n/a

- [CVE-2021-3401](https://insights.sbomify.com/CVE-2021-3401/): Command injection in n/a (critical 9.8)
- [CVE-2017-9230](https://insights.sbomify.com/CVE-2017-9230/): Other in n/a (high 7.5)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2021-31876 and fix it if it is affected.

What it is: CVE-2021-31876 is a medium-severity vulnerability in n/a, published on 13 May 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-2021-31876 in the wild and no public exploit code.

Affected product: n/a. No package-level version data is available, so check the vendor advisory linked from the reference page.

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-2021-31876/ (Markdown: https://insights.sbomify.com/CVE-2021-31876/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-31876
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-31876
- OSV: https://osv.dev/vulnerability/CVE-2021-31876

---
From sbomify Insights: https://insights.sbomify.com/CVE-2021-31876/
Summaries are generated and may contain mistakes; the official description above is the record.