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CVE-2026-27656— Account Takeover via Substring Matching in OpenID Connect Authentication

Quick assessment

Affected
Mattermost Mattermost
Exploitation
No confirmed in-the-wild exploitation; assess based on exposure
Recommended action
Check the vendor advisory and references for a fixed version. If immediate upgrade is impossible, restrict exposure and increase monitoring.

Mattermost是美国Mattermost公司的一个开源协作平台。 Mattermost 11.4.0及之前的11.4.x版本、11.3.1及之前的11.3.x版本、11.2.3及之前的11.2.x版本和10.11.11及之前的10.11.x版本存在安全漏洞，该漏洞源于OpenID IsSameUser比较逻辑中用户身份验证不当，可能导致攻击者通过用户发现流程中的子字符串匹配缺陷接管任意用户账户。

CVSS 5.7 · Medium EPSS 0.18% · P8

Possible ATT&CK Techniques 1 AI

T1078 · Valid Accounts
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I. Basic Information for CVE-2026-27656

Vulnerability Information
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Vulnerability Title
Account Takeover via Substring Matching in OpenID Connect Authentication
Source: CVE Program / CVE List V5
Vulnerability Description
Mattermost versions 11.4.x <= 11.4.0, 11.3.x <= 11.3.1, 11.2.x <= 11.2.3, 10.11.x <= 10.11.11 fail to properly validate user identity in the OpenID {{IsSameUser()}} comparison logic, which allows an attacker to take over arbitrary user accounts via an overly permissive substring matching flaw in the user discovery flow.. Mattermost Advisory ID: MMSA-2026-00590
Source: CVE Program / CVE List V5
CVSS Information
CVSS:3.1/AV:N/AC:H/PR:H/UI:R/S:U/C:H/I:H/A:N
Source: CVE Program / CVE List V5
Vulnerability Type
认证算法的不正确实现
Source: CVE Program / CVE List V5
Vulnerability Title
Mattermost 安全漏洞
Source: CNNVD (China National Vulnerability Database)
Vulnerability Description
Mattermost是美国Mattermost公司的一个开源协作平台。 Mattermost 11.4.0及之前的11.4.x版本、11.3.1及之前的11.3.x版本、11.2.3及之前的11.2.x版本和10.11.11及之前的10.11.x版本存在安全漏洞，该漏洞源于OpenID IsSameUser比较逻辑中用户身份验证不当，可能导致攻击者通过用户发现流程中的子字符串匹配缺陷接管任意用户账户。
Source: CNNVD (China National Vulnerability Database)
CVSS Information
N/A
Source: CNNVD (China National Vulnerability Database)
Vulnerability Type
N/A
Source: CNNVD (China National Vulnerability Database)

Affected Products

Vendor Product Affected Versions CPE Subscribe
Mattermost Mattermost 11.4.0 ~ 11.4.0 -

II. Public POCs for CVE-2026-27656

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III. Intelligence Information for CVE-2026-27656

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Same Patch Batch · Mattermost · 2026-03-25 · 4 CVEs total

CVE-2026-27659 4.6 MEDIUM CSRF vulnerability in UpdateAccessControlPolicyActiveStatus endpoint
CVE-2026-26233 4.3 MEDIUM Denial of Service via HTTP/2 single packet attack on login endpoint
CVE-2026-20719 4.3 MEDIUM DoS via URL Previews Rendering Malicious SVGs

IV. Related Vulnerabilities

Same product: Mattermost
Same vendor: Mattermost
Same weakness: CWE-303

V. Comments for CVE-2026-27656

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