CVE-2026-27656— Account Takeover via Substring Matching in OpenID Connect Authentication
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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: NVD (National Vulnerability Database)
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: NVD (National Vulnerability Database)
CVSS Information
CVSS:3.1/AV:N/AC:H/PR:H/UI:R/S:U/C:H/I:H/A:N
Source: NVD (National Vulnerability Database)
Vulnerability Type
认证算法的不正确实现
Source: NVD (National Vulnerability Database)
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
| # | POC Description | Source Link | Shenlong Link |
|---|
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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 |
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