HOMEVULNERABILITIESCVE-2021-31876
MEDIUM

CVE-2021-31876

CWE-863Published: May 13, 2021· Updated: Jun 17, 2026

6.5
CVSS v3.1

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.

NVD Source

Technical Analysis

CVE-2021-31876 can be exploited remotely over the network without requiring physical or adjacent access, significantly expanding the attack surface for threat actors.

The vulnerability requires no privileges and no user interaction, making it a prime target for automated exploitation campaigns and worm-like propagation.

CVSS v3.1 Vector Breakdown

Exploitability
Attack VectorNetwork
Attack ComplexityLow
Privileges Req.None
User InteractionNone
ScopeUnchanged
Impact
ConfidentialityNone
IntegrityLow
AvailabilityLow
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:L

Affected Vendors & Products

bitcoin1 product(s)
bitcoin
Source: NVD CPE · 1 total CPE entries

Exploit & PoC Resources

NO KNOWN EXPLOITNo public exploit confirmed at this time
Exploit-DB
Offensive Security exploit database
GitHub PoC Search
Community PoC repositories
Rapid7 / Metasploit
Metasploit module search
PacketStorm
Security advisories & exploits
NVD Detail
Official NIST entry
MITRE CVE
CVE Program entry
External links open in a new tab. Always verify in a controlled environment before use.

Official Patches & Advisories

ADVISORYhttps://bitcoinops.org/en/newsletters/2021/05/12/
ADVISORYhttps://bitcoinops.org/en/topics/replace-by-fee/
ADVISORYhttps://en.bitcoin.it/wiki/Common_Vulnerabilities_and_Exposures#CVE-2021-31876
ADVISORYhttps://github.com/bitcoin/bitcoin
ADVISORYhttps://lists.linuxfoundation.org/pipermail/bitcoin-dev/2021-May/018893.html
ADVISORYhttps://bitcoinops.org/en/newsletters/2021/05/12/
ADVISORYhttps://bitcoinops.org/en/topics/replace-by-fee/
ADVISORYhttps://en.bitcoin.it/wiki/Common_Vulnerabilities_and_Exposures#CVE-2021-31876
ADVISORYhttps://github.com/bitcoin/bitcoin
ADVISORYhttps://lists.linuxfoundation.org/pipermail/bitcoin-dev/2021-May/018893.html

All References (10)

→
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

Quick Facts

CVE IDCVE-2021-31876
CVSS Score6.5 / 10
SeverityMEDIUM
WeaknessCWE-863
CISA KEVNo
Affected1 vendor(s)
PublishedMay 13, 2021

Related CVEs (CWE-863)

CVE-2026-48286
10.0
CVE-2026-48321
10.0
CVE-2020-13300
10.0
CVE-2026-48303
10.0
CVE-2023-4617
10.0
CVE-2026-44330
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Recommended Actions

  • →Apply vendor patches immediately
  • →Monitor CVE-2021-31876 in threat intel feeds
  • →Review IDS/IPS signatures for exploitation attempts
Data sourced from NVD (NIST), CISA KEV, and EPSS (FIRST). Analysis generated by CTIWatch. CVE data is provided under the NVD usage policy.
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