How severe is CVE-2020-21523?

CVE-2020-21523 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-2020-21523 being actively exploited?

It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 3% (83rd percentile), an estimate of the probability of exploitation in the next 30 days.

What products are affected by CVE-2020-21523?

CVE-2020-21523 affects Halo. See the affected-products list for the exact vulnerable versions.

How do I fix CVE-2020-21523?

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-2020-21523 published?

CVE-2020-21523 was published on 2020-09-30 and last updated on 2026-06-17.

CVE-2020-21523

CVE-2020-21523 is a critical-severity vulnerability in Halo 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-74.

Key facts

Severity: Critical (CVSS 3.x base score 9.8)
CVSS v2: 10.0
EPSS exploit prediction: 3% (83rd percentile)
Actively exploited: Not listed in CISA KEV
Weakness: CWE-74
Affected product: Halo
Published: 2020-09-30
Last modified: 2026-06-17

Description

A Server-Side Freemarker template injection vulnerability in halo CMS v1.1.3 In the Edit Theme File function. The ftl file can be edited. This is the Freemarker template file. This file can cause arbitrary code execution when it is rendered in the background. exp: <#assign test="freemarker.template.utility.Execute"?new()> ${test("touch /tmp/freemarkerPwned")}

Frequently asked questions

What is CVE-2020-21523?: A Server-Side Freemarker template injection vulnerability in halo CMS v1.1.3 In the Edit Theme File function. The ftl file can be edited. This is the Freemarker template file. This file can cause arbitrary code execution when it is rendered in the background. exp: <#assign test="freemarker.template.utility.Execute"?new()> ${test("touch /tmp/freemarkerPwned")}
How severe is CVE-2020-21523?: CVE-2020-21523 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-2020-21523 being actively exploited?: It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 3% (83rd percentile), an estimate of the probability of exploitation in the next 30 days.
What products are affected by CVE-2020-21523?: CVE-2020-21523 affects Halo. See the affected-products list for the exact vulnerable versions.
How do I fix CVE-2020-21523?: 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-2020-21523 published?: CVE-2020-21523 was published on 2020-09-30 and last updated on 2026-06-17.

References

https://github.com/halo-dev/halo/issues/419

Affected products (1)

cpe:2.3:a:halo:halo:1.1.3:*:*:*:*:*:*:*

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