CVE-2021-28428: File upload vulnerability in HorizontCMS before 1.0.0-beta.3 via uploading a .htaccess and *.hello files using

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CRITICAL 9.8REMOTEDOWNGRADE

CVE-2021-28428 is a critical-severity flaw (CVSS 9.8) affecting horizontcms. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

File upload vulnerability in HorizontCMS before 1.0.0-beta.3 via uploading a .htaccess and *.hello files using the Media Files upload functionality. The original file upload vulnerability (CVE-2020-27387) was remediated by restricting the PHP extensions; however, we confirmed that the filter was bypassed via uploading an arbitrary .htaccess and *.hello files in order to execute PHP code to gain RCE.

Why this priority

Critical severity
Remotely exploitable, no login, no user action

No fix is published yet: follow the downgrade or mitigation steps.

Risk facts

CVSS score	9.8 (Critical)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	None
EPSS exploit chance	1.2% (top 31%)
CISA exploited list	No
Weakness	CWE-434
Published	2022-04-05
Last modified	2026-06-17

Publisher (CNA): MITRE · Vendor: horizontcms project

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

Am I affected?

Timeline

2022-04-05 16:15 UTCPublished CVE record published
2026-06-17 03:46 UTCUpdated Record last modified

horizontcms project horizontcms DOWNGRADE

Affected: 1.0.0 · 1.0.0 alpha · 1.0.0 alpha2 · 1.0.0 alpha3 · 1.0.0 alpha4 · 1.0.0 alpha5 · 1.0.0 alpha6 · 1.0.0 alpha7

Fixed in: no fixed release known

Safe to downgrade to: a release before 1.0.0

1. Check installed version

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

2. No fixed release: DOWNGRADE to a release before 1.0.0 (Debian / Ubuntu)

apt-cache madison horizontcms sudo apt install horizontcms=<VERSION_BEFORE_1.0.0> sudo apt-mark hold horizontcms

2. No fixed release: DOWNGRADE (RHEL / Amazon Linux / Fedora)

dnf --showduplicates list horizontcms sudo dnf downgrade horizontcms-<VERSION_BEFORE_1.0.0> sudo dnf versionlock add horizontcms

Downgrade risk

Downgrading can break dependent software and may reopen older bugs. Read the distro advisory first and test on a non-production host.

After downgrade

Restart the service, and keep the package held until a fixed release ships.

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-2021-28428?

horizontcms: downgrade or apply the listed mitigation. Exact commands for each OS are on this page.

Is CVE-2021-28428 being exploited?

CVE-2021-28428 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-2021-28428 affect?

CVE-2021-28428 affects horizontcms. Severity is critical (CVSS 9.8). File upload vulnerability in HorizontCMS before 1.0.0-beta.3 via uploading a .htaccess and *.hello files using the Media Files upload functionality. The original file upload vulnerability (CVE-2020-27387) was remediated by restricting the P

How urgent is CVE-2021-28428?

Priority 57/100. Critical severity; Remotely exploitable, no login, no user action

Software pages

PHP vulnerabilities

Related CVEs

CVE-2022-25104 HIGH 7.5 DOWNGRADE HorizontCMS v1.0.0-beta.2 was discovered to contain an arbitrary file download vulnerability via the component
CVE-2020-28693 HIGH 8.8 DOWNGRADE An unrestricted file upload issue in HorizontCMS 1.0.0-beta allows an authenticated remote attacker to upload
CVE-2020-27387 HIGH 8.8 DOWNGRADE An unrestricted file upload issue in HorizontCMS through 1.0.0-beta allows an authenticated remote attacker (w

References

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