How severe is CVE-2021-28428?

CVE-2021-28428 has a CVSS 3.x base score of 9.8, rated critical severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is high, integrity high, and availability high.

Is CVE-2021-28428 being actively exploited?

It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 1% (64th percentile), an estimate of the probability of exploitation in the next 30 days.

What products are affected by CVE-2021-28428?

CVE-2021-28428 primarily affects Horizontcms Project Horizontcms. In total, 11 product configurations (CPEs) are listed as vulnerable; see the affected-products list for the exact versions.

How do I fix CVE-2021-28428?

Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its critical severity, prioritise patching exposed systems.

When was CVE-2021-28428 published?

CVE-2021-28428 was published on 2022-04-05 and last updated on 2026-06-17.

CVE-2021-28428

CVE-2021-28428 is a critical-severity vulnerability in Horizontcms Project Horizontcms with a CVSS 3.x base score of 9.8. It is not currently listed as actively exploited by CISA, and its EPSS exploit-prediction score is low. The underlying weakness is classified as CWE-434.

Key facts

Severity: Critical (CVSS 3.x base score 9.8)
CVSS v2: 7.5
EPSS exploit prediction: 1% (64th percentile)
Actively exploited: Not listed in CISA KEV
Weakness: CWE-434
Affected product: Horizontcms Project Horizontcms
Published: 2022-04-05
Last modified: 2026-06-17

Description

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.

Frequently asked questions

What is CVE-2021-28428?: 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.
How severe is CVE-2021-28428?: CVE-2021-28428 has a CVSS 3.x base score of 9.8, rated critical severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is high, integrity high, and availability high.
Is CVE-2021-28428 being actively exploited?: It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 1% (64th percentile), an estimate of the probability of exploitation in the next 30 days.
What products are affected by CVE-2021-28428?: CVE-2021-28428 primarily affects Horizontcms Project Horizontcms. In total, 11 product configurations (CPEs) are listed as vulnerable; see the affected-products list for the exact versions.
How do I fix CVE-2021-28428?: Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its critical severity, prioritise patching exposed systems.
When was CVE-2021-28428 published?: CVE-2021-28428 was published on 2022-04-05 and last updated on 2026-06-17.

References

Affected products (11)

cpe:2.3:a:horizontcms_project:horizontcms:1.0.0:-:*:*:*:*:*:*
cpe:2.3:a:horizontcms_project:horizontcms:1.0.0:alpha:*:*:*:*:*:*
cpe:2.3:a:horizontcms_project:horizontcms:1.0.0:alpha2:*:*:*:*:*:*
cpe:2.3:a:horizontcms_project:horizontcms:1.0.0:alpha3:*:*:*:*:*:*
cpe:2.3:a:horizontcms_project:horizontcms:1.0.0:alpha4:*:*:*:*:*:*
cpe:2.3:a:horizontcms_project:horizontcms:1.0.0:alpha5:*:*:*:*:*:*
cpe:2.3:a:horizontcms_project:horizontcms:1.0.0:alpha6:*:*:*:*:*:*
cpe:2.3:a:horizontcms_project:horizontcms:1.0.0:alpha7:*:*:*:*:*:*
cpe:2.3:a:horizontcms_project:horizontcms:1.0.0:alpha8:*:*:*:*:*:*
cpe:2.3:a:horizontcms_project:horizontcms:1.0.0:beta:*:*:*:*:*:*
cpe:2.3:a:horizontcms_project:horizontcms:1.0.0:beta2:*:*:*:*:*:*

More vulnerabilities in Horizontcms Project Horizontcms

CVE-2020-28693 — High (CVSS 8.8): An unrestricted file upload issue in HorizontCMS 1.0.0-beta allows an authenticated remote attacker to upload PHP code…
CVE-2020-27387 — High (CVSS 8.8): An unrestricted file upload issue in HorizontCMS through 1.0.0-beta allows an authenticated remote attacker (with…
CVE-2022-25104 — High (CVSS 7.5): HorizontCMS v1.0.0-beta.2 was discovered to contain an arbitrary file download vulnerability via the component…

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