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CVE Record

CVE-2021-31876: Bitcoin Core 0.12.0 through 0.21.1 does not properly implement the replacement policy specified in BIP125,...

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.

UnknownCVSS not scoredNot KEV-listedUpdated
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Glexia's TakeAutomated analysismoderate

Security readout for executives and security teams

Plain-English summary

CVE-2021-31876 is a Bitcoin Core policy bug that can make some unconfirmed transaction replacements fail unexpectedly. For organizations running Bitcoin infrastructure or Lightning-related services, the concern is business disruption or potential fund-loss scenarios in systems that rely on correct replace-by-fee behavior.

Executive priority

Treat this as a targeted infrastructure risk for Bitcoin and Lightning operations, not a broad enterprise emergency. Prioritize if the organization processes cryptocurrency payments, runs routing/payment channels, or provides wallet, exchange, custody, or Lightning services.

Technical view

Bitcoin Core 0.12.0 through 0.21.1 incorrectly handles inherited BIP125 replaceability. A child transaction spending an unconfirmed opt-in parent should be replaceable, but PreChecks can reject replacement. Sources describe possible downstream denial of service or fund-loss impact, especially for Lightning network nodes.

Likely exposure

Exposure is most likely in organizations operating Bitcoin Core nodes from 0.12.0 through 0.21.1, or downstream services that depend on their mempool replacement policy. Lightning infrastructure and applications handling unconfirmed transaction chains deserve specific review.

Exploitation context

The source bundle does not show CISA KEV listing or cited evidence of active exploitation. The risk is protocol-behavior abuse against systems relying on BIP125-compatible replacement handling, not a conventional remote code execution issue.

Researcher notes

Evidence is limited to the CVE description and linked Bitcoin ecosystem references. No CVSS, CWE, concrete patch version, or active exploitation evidence is provided in the bundle. Avoid assuming exploit prevalence or remediation specifics beyond vendor guidance.

Mitigation direction

  • Inventory Bitcoin Core node versions across production and dependent environments.
  • Check Bitcoin Core guidance for the corrected release or recommended operational response.
  • Prioritize review of Lightning and payment systems relying on unconfirmed transaction replacement behavior.
  • Reduce reliance on unconfirmed transaction assumptions where operationally feasible.
  • Monitor Bitcoin Core security and operations channels for clarified remediation guidance.

Validation and detection

  • Confirm whether any Bitcoin Core node runs versions 0.12.0 through 0.21.1.
  • Identify services consuming mempool policy decisions from affected nodes.
  • Review Lightning node exposure to unconfirmed parent-child transaction workflows.
  • Check incident records for unexplained replacement failures or related service disruption.
  • Document compensating controls until vendor remediation is confirmed.
Prepared
Confidence
medium
Sources
6

Public sources used

Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.

Potential ATT&CK relevance

Conservative CVE-to-ATT&CK context

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CVE-2021-31876 mapping review

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Vulnerability profileCVE Program record
Severity
Unknown
CVSS
Not scored
Known Exploited
No
Published
Official CVE source material

CNA and ADP enrichment extracted from CVE v5

These fields come from the CVE record and ADP containers, not from Glexia's Take. They preserve time-varying source decisions such as CISA SSVC, KEV status, CVSS metrics, and provider references.

0CVSS vectors
0Timeline events
0ADP providers
6Source links

CVSS and timeline data

No CVSS vectors or timeline events were available in the normalized CVE source material.

Source materials

Affected products

Products and packages named in the record

VendorProductVersion / packageStatus
n/an/an/aListed
Weakness

CWE details

No CWE listed

CWE links open Glexia weakness intelligence pages with official CWE context, developer remediation guidance, and related CVE mappings.