CVE-2021-4300: A vulnerability has been found in ghostlander Halcyon and classified as critical

Q: How do I fix CVE-2021-4300?

halcyon: upgrade to 1.1.1.0. Exact commands for each OS are on this page.

Q: Is CVE-2021-4300 being exploited?

CVE-2021-4300 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

Q: What does CVE-2021-4300 affect?

CVE-2021-4300 affects halcyon. Severity is medium (CVSS 6.3). A vulnerability has been found in ghostlander Halcyon and classified as critical. Affected by this vulnerability is the function CBlock::AddToBlockIndex of the file src/main.cpp of the component Block Verification. The manipulation leads to

Q: How urgent is CVE-2021-4300?

Priority 35/100. No elevated risk signals yet.

Routine

MEDIUM 6.3UPGRADE

CVE-2021-4300 is a medium-severity flaw (CVSS 6.3) affecting halcyon. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

A vulnerability has been found in ghostlander Halcyon and classified as critical. Affected by this vulnerability is the function CBlock::AddToBlockIndex of the file src/main.cpp of the component Block Verification. The manipulation leads to improper access controls. The attack can be launched remotely. Upgrading to version 1.1.1.0-hal is able to address this issue. The identifier of the patch is 0675b25ae9cc10b5fdc8ea3a32c642979762d45e. It is rec

Why this priority

No elevated risk signals yet

A fixed release or distro patch exists.

Risk facts

CVSS score	6.3 (Medium)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	Low
User interaction	None
EPSS exploit chance	0.7% (top 47%)
CISA exploited list	No
Weakness	CWE-284
Published	2023-01-04
Last modified	2026-06-17

Publisher (CNA): VulDB · Vendor: halcyon project

Last updated 2026-06-17 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2023-01-04 22:15 UTCPublished CVE record published
2026-06-17 04:19 UTCUpdated Record last modified

halcyon project halcyon UPGRADE

Affected: < 1.1.1.0

Fixed in: 1.1.1.0

Safe to downgrade to: no unaffected older release

1. Check installed version

apt-cache policy halcyon || rpm -q halcyon || halcyon --version

2. UPGRADE to 1.1.1.0 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade halcyon

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade halcyon

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2021-4300?

halcyon: upgrade to 1.1.1.0. Exact commands for each OS are on this page.

Is CVE-2021-4300 being exploited?

CVE-2021-4300 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2021-4300 affect?

CVE-2021-4300 affects halcyon. Severity is medium (CVSS 6.3). A vulnerability has been found in ghostlander Halcyon and classified as critical. Affected by this vulnerability is the function CBlock::AddToBlockIndex of the file src/main.cpp of the component Block Verification. The manipulation leads to

How urgent is CVE-2021-4300?

Priority 35/100. No elevated risk signals yet.

References

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