How severe is CVE-2018-21251?

CVE-2018-21251 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-2018-21251 being actively exploited?

It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 1% (67th percentile), an estimate of the probability of exploitation in the next 30 days.

What products are affected by CVE-2018-21251?

CVE-2018-21251 primarily affects Mattermost Mattermost Server. In total, 7 product configurations (CPEs) are listed as vulnerable; see the affected-products list for the exact versions.

How do I fix CVE-2018-21251?

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-2018-21251 published?

CVE-2018-21251 was published on 2020-06-19 and last updated on 2026-06-17.

CVE-2018-21251

CVE-2018-21251 is a critical-severity vulnerability in Mattermost Mattermost Server 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-862.

Key facts

Severity: Critical (CVSS 3.x base score 9.8)
CVSS v2: 7.5
EPSS exploit prediction: 1% (67th percentile)
Actively exploited: Not listed in CISA KEV
Weakness: CWE-862
Affected product: Mattermost Mattermost Server
Published: 2020-06-19
Last modified: 2026-06-17

Description

An issue was discovered in Mattermost Server before 5.2 and 5.1.1. Authorization could be bypassed if the channel name were not the same in the params and the body.

Frequently asked questions

What is CVE-2018-21251?: An issue was discovered in Mattermost Server before 5.2 and 5.1.1. Authorization could be bypassed if the channel name were not the same in the params and the body.
How severe is CVE-2018-21251?: CVE-2018-21251 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-2018-21251 being actively exploited?: It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 1% (67th percentile), an estimate of the probability of exploitation in the next 30 days.
What products are affected by CVE-2018-21251?: CVE-2018-21251 primarily affects Mattermost Mattermost Server. In total, 7 product configurations (CPEs) are listed as vulnerable; see the affected-products list for the exact versions.
How do I fix CVE-2018-21251?: 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-2018-21251 published?: CVE-2018-21251 was published on 2020-06-19 and last updated on 2026-06-17.

References

https://mattermost.com/security-updates/

Affected products (7)

cpe:2.3:a:mattermost:mattermost_server:*:*:*:*:*:*:*:*
cpe:2.3:a:mattermost:mattermost_server:5.2.0:rc1:*:*:*:*:*:*
cpe:2.3:a:mattermost:mattermost_server:5.2.0:rc2:*:*:*:*:*:*
cpe:2.3:a:mattermost:mattermost_server:5.2.0:rc3:*:*:*:*:*:*
cpe:2.3:a:mattermost:mattermost_server:5.2.0:rc4:*:*:*:*:*:*
cpe:2.3:a:mattermost:mattermost_server:5.2.0:rc5:*:*:*:*:*:*
cpe:2.3:a:mattermost:mattermost_server:5.2.0:rc6:*:*:*:*:*:*

More vulnerabilities in Mattermost Mattermost Server

CVE-2025-12421 — Critical (CVSS 9.9): Mattermost versions 11.0.x <= 11.0.2, 10.12.x <= 10.12.1, 10.11.x <= 10.11.4, 10.5.x <= 10.5.12 fail to to verify that…
CVE-2025-12419 — Critical (CVSS 9.9): Mattermost versions 10.12.x <= 10.12.1, 10.11.x <= 10.11.4, 10.5.x <= 10.5.12, 11.0.x <= 11.0.3 fail to properly…
CVE-2025-4981 — Critical (CVSS 9.9): Mattermost versions 10.5.x <= 10.5.5, 9.11.x <= 9.11.15, 10.8.x <= 10.8.0, 10.7.x <= 10.7.2, 10.6.x <= 10.6.5 fail to…
CVE-2025-25279 — Critical (CVSS 9.9): Mattermost versions 10.4.x <= 10.4.1, 9.11.x <= 9.11.7, 10.3.x <= 10.3.2, 10.2.x <= 10.2.2 fail to properly validate…
CVE-2025-20051 — Critical (CVSS 9.9): Mattermost versions 10.4.x <= 10.4.1, 9.11.x <= 9.11.7, 10.3.x <= 10.3.2, 10.2.x <= 10.2.2 fail to properly validate…
CVE-2017-18920 — Critical (CVSS 9.8): An issue was discovered in Mattermost Server before 3.6.2. The WebSocket feature does not follow the Same Origin Policy.

All CVEs affecting Mattermost Mattermost Server →

Other CWE-862 (Missing Authorization) vulnerabilities

CVE-2026-101000 — Critical (CVSS 10.0): A vulnerability was determined in Netcore NBR100V2 1.3.240614.030928. This affects the function uci.apply of the file…
CVE-2026-97360 — Critical (CVSS 10.0): HFS2 version 2.4.0 and earlier contains an unauthenticated arbitrary file access vulnerability that allows…
CVE-2026-65381 — Critical (CVSS 10.0): A validation issue existed in the entitlement verification. This issue was addressed with improved validation of the…
CVE-2026-81648 — Critical (CVSS 10.0): The CryptoPayment Gateway WordPress plugin from 1.2.1 to 1.2.2 does not apply an authorization check on one of its AJAX…
CVE-2026-77770 — Critical (CVSS 10.0): The miniOrange 2FA WordPress plugin before 6.3.1, miniOrange 2FA WordPress plugin before 19.3 does not require a…
CVE-2026-65667 — Critical (CVSS 10.0): Missing authorization in Microsoft Teams allows an unauthorized attacker to elevate privileges over a network.

Browse all CWE-862 (Missing Authorization) vulnerabilities →