How severe is CVE-2021-20837?

CVE-2021-20837 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-2021-20837 being actively exploited?

It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 88% (100th percentile), an estimate of the probability of exploitation in the next 30 days.

What products are affected by CVE-2021-20837?

CVE-2021-20837 primarily affects Sixapart Movable Type. In total, 5 product configurations (CPEs) are listed as vulnerable; see the affected-products list for the exact versions.

How do I fix CVE-2021-20837?

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-2021-20837 published?

CVE-2021-20837 was published on 2021-10-26 and last updated on 2026-06-17.

CVE-2021-20837

CVE-2021-20837 is a critical-severity vulnerability in Sixapart Movable Type with a CVSS 3.x base score of 9.8. Its EPSS exploit-prediction score of 88% places it in the 100th percentile, indicating an elevated likelihood of exploitation. The underlying weakness is classified as CWE-78.

Key facts

Severity: Critical (CVSS 3.x base score 9.8)
CVSS v2: 7.5
EPSS exploit prediction: 88% (100th percentile)
Actively exploited: Not listed in CISA KEV
Weakness: CWE-78
Affected product: Sixapart Movable Type
Published: 2021-10-26
Last modified: 2026-06-17

Description

Movable Type 7 r.5002 and earlier (Movable Type 7 Series), Movable Type 6.8.2 and earlier (Movable Type 6 Series), Movable Type Advanced 7 r.5002 and earlier (Movable Type Advanced 7 Series), Movable Type Advanced 6.8.2 and earlier (Movable Type Advanced 6 Series), Movable Type Premium 1.46 and earlier, and Movable Type Premium Advanced 1.46 and earlier allow remote attackers to execute arbitrary OS commands via unspecified vectors. Note that all versions of Movable Type 4.0 or later including unsupported (End-of-Life, EOL) versions are also affected by this vulnerability.

Frequently asked questions

What is CVE-2021-20837?: Movable Type 7 r.5002 and earlier (Movable Type 7 Series), Movable Type 6.8.2 and earlier (Movable Type 6 Series), Movable Type Advanced 7 r.5002 and earlier (Movable Type Advanced 7 Series), Movable Type Advanced 6.8.2 and earlier (Movable Type Advanced 6 Series), Movable Type Premium 1.46 and earlier, and Movable Type Premium Advanced 1.46 and earlier allow remote attackers to execute arbitrary OS commands via unspecified vectors. Note that all versions of Movable Type 4.0 or later including unsupported (End-of-Life, EOL) versions are also affected by this vulnerability.
How severe is CVE-2021-20837?: CVE-2021-20837 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-2021-20837 being actively exploited?: It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 88% (100th percentile), an estimate of the probability of exploitation in the next 30 days.
What products are affected by CVE-2021-20837?: CVE-2021-20837 primarily affects Sixapart Movable Type. In total, 5 product configurations (CPEs) are listed as vulnerable; see the affected-products list for the exact versions.
How do I fix CVE-2021-20837?: 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-2021-20837 published?: CVE-2021-20837 was published on 2021-10-26 and last updated on 2026-06-17.

References

Affected products (5)

cpe:2.3:a:sixapart:movable_type:*:*:*:*:premium:*:*:*
cpe:2.3:a:sixapart:movable_type:*:*:*:*:premium_advanced:*:*:*
cpe:2.3:a:sixapart:movable_type:*:*:*:*:*:*:*:*
cpe:2.3:a:sixapart:movable_type:*:*:*:*:-:*:*:*
cpe:2.3:a:sixapart:movable_type:*:*:*:*:advanced:*:*:*

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