Mattermost: External Control of Assumed-Immutable Web Parameter
CVE-2026-16453 is a high-severity external control of assumed-immutable web parameter vulnerability in Mattermost, with a CVSS 3.1 base score of 7.1 out of 10. It can be exploited remotely over the network by an attacker with a low-privileged account, without any action from a user. Affected Mattermost versions: 11.10.0 and earlier; 11.7.0 through 11.7.9; 11.9.0 through 11.9.1, and others listed below. The record does not name a fixed version, so check the vendor advisory. There is no confirmed exploitation so far. EPSS estimates a 0.19% chance of exploitation within 30 days, which is low compared with other CVEs.
High severity with no evidence of exploitation so far.
What to do
- Update Mattermost Server to 11.7.10 or 11.8.6 or 11.9.2, whichever matches your release line, or a later version.
- Read the advisory on mattermost.com for workarounds and exact fixed builds.
Affected products and versions
| Vendor | Product | Affected versions | Fixed in |
|---|---|---|---|
| Mattermost | Mattermost | 11.10.0 and earlier; 11.7.0 through 11.7.9; 11.9.0 through 11.9.1; 11.8.0 through 11.8.5 | Not stated |
| Mattermost | Mattermost Server | 11.7.0 up to, but not including, 11.7.10; 11.8.0 up to, but not including, 11.8.6; 11.9.0 up to, but not including, 11.9.2; 11.10.0 | 11.7.10, 11.8.6, 11.9.2 |
Version data comes from the organisation that assigned the CVE. Confirm against the vendor advisory before deciding you are unaffected.
How the severity score breaks down
- Attack vector
- NetworkCan be exploited remotely over a network, including the internet.
- Attack complexity
- LowNo special conditions are needed; the attack works reliably.
- Privileges required
- LowA basic user account is enough.
- User interaction
- NoneNo action by a victim is required.
- Scope
- UnchangedImpact stays within the vulnerable component.
- Confidentiality
- HighAll data handled by the component can be read.
- Integrity
- LowLimited modification of data is possible.
- Availability
- NoneAvailability is not affected.
Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:L/A:N. Scored by CNA (mattermost.com).
Official 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: the CVE record as published by its numbering authority, via NVD and CVE.org.
The underlying weakness: External Control of Assumed-Immutable Web Parameter
The web application does not sufficiently verify inputs that are assumed to be immutable but are actually externally controllable, such as hidden form fields.
How developers prevent this class of flaw
- Inputs should be decoded and canonicalized to the application's current internal representation before being validated (CWE-180). Make sure that the application does not decode the same input twice (CWE-174). Such errors could be used to bypass allowlist validation schemes by introducing dangerous inputs after they have been checked.
More External Control of Assumed-Immutable Web Parameter vulnerabilities (CWE-472)
Timeline
- CVE record published.
- CISA published its SSVC assessment (exploitation: none, automatable: no, technical impact: partial).
- Record last changed materially (score, affected products, status or references).
References
Advisories
Other vulnerabilities in Mattermost
- 8.0 High severity
- 9.3 Critical severity
- 7.1 High severity
Recent External Control of Assumed-Immutable Web Parameter vulnerabilities
- 8.1 High severity
- 8.3 High severity
- 8.7 High severity
- 8.7 High severity
- 7.7 High severity
- 7.5 High severity
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