Vulnerability Monitor

The vendors, products, and vulnerabilities you care about

CVE-2021-31876

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.

Security Impact Summary

This vulnerability carries a MEDIUM severity rating with a CVSS v3.1 score of 6.5, indicating it can be exploited remotely over the network with relatively low complexity without requiring user interaction and does not require pre-existing privileges . The vulnerability impacts limited integrity, and limited availability for affected systems. Impacting 1 product from bitcoin organizations running these solutions should prioritize assessment and patching.

Historical Context

Reported in 2021, this vulnerability emerged during an era marked by increased sophistication in supply chain attacks, cloud infrastructure vulnerabilities, and software-as-a-service (SaaS) security challenges. Security practices during this period emphasized zero-trust architectures, container security, and API protection.

Published

2021-05-13T22:15:07.547

Last Modified

2024-11-21T06:06:24.633

Status

Modified

Source

[email protected]

Severity

CVSSv3.1: 6.5 (MEDIUM)

CVSSv2 Vector

AV:N/AC:L/Au:N/C:N/I:P/A:P

Access Vector: NETWORK
Access Complexity: LOW
Authentication: NONE
Confidentiality Impact: NONE
Integrity Impact: PARTIAL
Availability Impact: PARTIAL

Exploitability Score

10.0

Impact Score

4.9

Weaknesses

Type: Primary
CWE-863

Affected Vendors & Products

Type	Vendor	Product	Version/Range	Vulnerable?
Application	bitcoin	bitcoin	≤ 0.21.1	Yes

References

https://bitcoinops.org/en/newsletters/2021/05/12/ Vendor Advisory ([email protected])
https://bitcoinops.org/en/topics/replace-by-fee/ Vendor Advisory ([email protected])
https://en.bitcoin.it/wiki/Common_Vulnerabilities_and_Exposures#CVE-2021-31876 Third Party Advisory ([email protected])
https://github.com/bitcoin/bitcoin Third Party Advisory ([email protected])
https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2021-May/018893.html Mailing List, Third Party Advisory ([email protected])
https://bitcoinops.org/en/newsletters/2021/05/12/ Vendor Advisory (af854a3a-2127-422b-91ae-364da2661108)
https://bitcoinops.org/en/topics/replace-by-fee/ Vendor Advisory (af854a3a-2127-422b-91ae-364da2661108)
https://en.bitcoin.it/wiki/Common_Vulnerabilities_and_Exposures#CVE-2021-31876 Third Party Advisory (af854a3a-2127-422b-91ae-364da2661108)
https://github.com/bitcoin/bitcoin Third Party Advisory (af854a3a-2127-422b-91ae-364da2661108)
https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2021-May/018893.html Mailing List, Third Party Advisory (af854a3a-2127-422b-91ae-364da2661108)

How SecUtils Interprets This CVE

SecUtils normalizes and enriches National Vulnerability Database (NVD) records by standardizing vendor and product identifiers, aggregating vulnerability metadata from both NVD and MITRE sources, and providing structured context for security teams. For bitcoin's affected products, we extract Common Platform Enumeration (CPE) data, Common Weakness Enumeration (CWE) classifications, CVSS severity metrics, and reference data to enable rapid vulnerability prioritization and asset correlation. This record contains no exploit code, proof-of-concept instructions, or attack methodologies—only defensive intelligence necessary for patch management, risk assessment, and security operations.