CVE-2026-16453— Mattermost SAML RelayState forgery allows unauthorized team membership
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 版本 11.10.x（≤ 11.10.0）、11.7.x（≤ 11.7.9）、11.9.x（≤ 11.9.1）和 11.8.x（≤ 11.8.5）未能验证 SAML 中继状态（RelayState）POST 参数的完整性，导致已认证的 SAML 用户可通过伪造 RelayState 中的 team_id 或 invite_id 字段，加入任意团队（包括仅限邀请的团队）。Mattermost 安全公告编号：MMSA-2026-00727。
Affected Version Matrix 9
| Vendor | Product | Version Range | Status |
|---|---|---|---|
| Mattermost | Mattermost | 11.10.0≤ 11.10.0 |
affected |
11.7.0≤ 11.7.9 |
affected | ||
11.9.0≤ 11.9.1 |
affected | ||
11.8.0≤ 11.8.5 |
affected | ||
11.11.0 |
unaffected | ||
11.10.1 |
unaffected | ||
11.7.10 |
unaffected | ||
11.9.2 |
unaffected | ||
| … +1 more rows | |||
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I. Basic Information for CVE-2026-16453
Vulnerability Information
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Vulnerability Title
Mattermost SAML RelayState forgery allows unauthorized team membership
Source: CVE Program / CVE List V5
Vulnerability Description
Mattermost versions 11.10.x <= 11.10.0, 11.7.x <= 11.7.9, 11.9.x <= 11.9.1, 11.8.x <= 11.8.5 fail to verify the integrity of the SAML RelayState POST parameter which allows an authenticated SAML user to join any team, including invite-only teams, via forging the team_id or invite_id fields in the RelayState.. Mattermost Advisory ID: MMSA-2026-00727
Source: CVE Program / CVE List V5
CVSS Information
CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:L/A:N
Source: CVE Program / CVE List V5
Vulnerability Type
对假设不可变Web参数的外部可控制
Source: CVE Program / CVE List V5
Affected Products
| Vendor | Product | Affected Versions | CPE | Subscribe |
|---|---|---|---|---|
| Mattermost | Mattermost | 11.10.0 ~ 11.10.0 | - |
|
II. Public POCs for CVE-2026-16453
| # | POC Description | Source Link | Shenlong Link |
|---|
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Same Patch Batch · Mattermost · 2026-10-16 · 15 CVEs total
| CVE-2026-19541 | 6.5 MEDIUM | Outgoing OAuth connection client secret exposure during connection validation |
| CVE-2026-18551 | 6.5 MEDIUM | Mattermost guest account privilege escalation via member roles API |
| CVE-2026-19621 | 6.5 MEDIUM | Mattermost leaks interactive-message integration secrets to non-admin users via post-delet |
| CVE-2026-16251 | 6.5 MEDIUM | Mattermost local image proxy allows server memory exhaustion via unbounded image response |
| CVE-2026-18795 | 6.4 MEDIUM | Missing server side validation allowed user to quarantine a DM and GM post |
| CVE-2026-18223 | 5.0 MEDIUM | Mattermost invitation tokens not bound to invited email address allowing unauthorized team |
| CVE-2026-19601 | 4.9 MEDIUM | Disclosure of AD/LDAP bind password via Test Connection |
| CVE-2026-19600 | 4.5 MEDIUM | Mattermost logs plaintext authentication tokens in debug-level server logs on failed auth |
| CVE-2026-19076 | 4.4 MEDIUM | Mattermost fails to reject OAuth application sessions in account type switch, allowing acc |
| CVE-2026-19603 | 4.3 MEDIUM | Private channel ID disclosure via user status endpoints |
| CVE-2026-19867 | 4.3 MEDIUM | Uncapped mention keys in user notification properties allow CPU exhaustion during post pro |
| CVE-2026-18560 | 4.3 MEDIUM | Improper order of authorisations in Mattermost GetFile API leads to an authenticated user |
| CVE-2026-18256 | 4.3 MEDIUM | Private team metadata disclosure via the team search API |
| CVE-2026-18799 | 4.3 MEDIUM | Mattermost Server fails to restrict Slack import account binding, allowing team admins to |
IV. Related Vulnerabilities
Same product: Mattermost
-
CVE-2026-18799
Mattermost Server fails to restrict Slack import account bin -
CVE-2026-16251
Mattermost local image proxy allows server memory exhaustion -
CVE-2026-18256
Private team metadata disclosure via the team search API -
CVE-2026-18223
Mattermost invitation tokens not bound to invited email addr -
CVE-2026-19076
Mattermost fails to reject OAuth application sessions in acc
Same vendor: Mattermost
-
CVE-2026-18799
Mattermost Server fails to restrict Slack import account bin -
CVE-2026-16251
Mattermost local image proxy allows server memory exhaustion -
CVE-2026-18256
Private team metadata disclosure via the team search API -
CVE-2026-18223
Mattermost invitation tokens not bound to invited email addr -
CVE-2026-19076
Mattermost fails to reject OAuth application sessions in acc
Same weakness: CWE-472
V. Comments for CVE-2026-16453
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