CVE-2021-31876

Name: CVE-2021-31876
Creator: NVD / NIST
Published: 2021-05-13T22:15:07.547+00:00
License: https://creativecommons.org/publicdomain/zero/1.0/

MEDIUMCVSS 6.5/10EPSS 1.61%

Last modified Jun 17, 2026

CVE-2021-31876 is a medium-severity vulnerability rated 6.5/10 on the CVSS scale. 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. EPSS estimates a 1.61% chance of exploitation in the next 30 days.

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.

Metrics

CVSS 3.1

6.5/10

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:L

EPSS Probability

1.61%

72.8th percentile

Probability of exploitation in the next 30 days. Learn more

Weakness Enumeration

CWE-863

Affected Software

Vendor	Product	Versions
Bitcoin	Bitcoin	>= 0.12.0, <= 0.21.1

References

https://bitcoinops.org/en/newsletters/2021/05/12/Vendor Advisory
https://bitcoinops.org/en/topics/replace-by-fee/Vendor Advisory
https://en.bitcoin.it/wiki/Common_Vulnerabilities_and_Exposures#CVE-2021-31876Third Party Advisory
https://github.com/bitcoin/bitcoinThird Party Advisory
https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2021-May/018893.htmlMailing List, Third Party Advisory
https://bitcoinops.org/en/newsletters/2021/05/12/Vendor Advisory
https://bitcoinops.org/en/topics/replace-by-fee/Vendor Advisory
https://en.bitcoin.it/wiki/Common_Vulnerabilities_and_Exposures#CVE-2021-31876Third Party Advisory
https://github.com/bitcoin/bitcoinThird Party Advisory
https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2021-May/018893.htmlMailing List, Third Party Advisory

Timeline

Published: May 13, 2021
Last Modified: Jun 17, 2026
Status: Modified

Frequently Asked Questions

What is CVE-2021-31876?

How severe is CVE-2021-31876?

CVE-2021-31876 has a CVSS score of 6.5/10 (MEDIUM severity). The EPSS model estimates a 1.61% probability of exploitation in the next 30 days.

How do I fix CVE-2021-31876?

Check the vendor references and advisories linked above for patched versions and mitigation guidance. You can also run a Strix scan to test if your systems are affected.

Are you affected by CVE-2021-31876?

Run a free Strix scan to check your systems for this vulnerability.

Scan your code now

Source: NVD / NIST