Mattermost Updates Bound Bulk Reactions Requests
Tech GuruQuick Answer
The authority published MMSA-2026-00771 for Mattermost 11.7, 11.8, 11.9, and 11.10 release streams. Mattermost corrected an unbounded post-ID array in the bulk reactions endpoint that allows an authenticated user to consume resources and cause denial of service. Affected deployments should be matched to the authority's exact scope and moved to the supported fixed release.
Confirm Scope And Apply The Supported Fix
What to do now: Inventory Mattermost 11.7, 11.8, 11.9, and 11.10 release streams, match the exact affected release or feature, apply the authority-supported fixed build or service remediation, validate the dependent workflow, and preserve evidence.
Last verified: 2026-09-22 UTC. Recheck the authoritative advisory and supported distribution channel before changing production.
Scope And Authority
| Product scope | Mattermost 11.7, 11.8, 11.9, and 11.10 release streams |
|---|---|
| Advisory | MMSA-2026-00771 |
| CVE | CVE-2026-95666 |
| Authoritative release date | 2026-09-22 13:11:13.798 UTC |
| Authority revision date | 2026-09-22 13:57:00.428 UTC |
| Affected versions | Mattermost 11.7.0–11.7.10, 11.8.0–11.8.5, 11.9.0–11.9.1, and 11.10.0–11.10.1. |
| Fixed version | Mattermost 11.7.11, 11.8.6, 11.9.2, 11.10.2, or 11.11.0 and later supported releases |
| CVSS base score | 4.3 (CVSS v3.1) |
| CVSS severity | Medium |
| Exploitation status | Not stated by the authority; no exploitation claim is inferred. |
What Changed
Mattermost corrected an unbounded post-ID array in the bulk reactions endpoint that allows an authenticated user to consume resources and cause denial of service.
What To Validate Now
- Inventory. Locate Mattermost 11.7, 11.8, 11.9, and 11.10 release streams deployments, versions, enabled features, exposure paths, owners, and dependent services.
- Establish applicability. Compare each deployment with the authority's affected-version statement: Mattermost 11.7.0–11.7.10, 11.8.0–11.8.5, 11.9.0–11.9.1, and 11.10.0–11.10.1. Do not infer applicability from product family or severity alone.
- Remediate. Apply Mattermost 11.7.11, 11.8.6, 11.9.2, 11.10.2, or 11.11.0 and later supported releases through the supported vendor or project channel, following the current advisory and change-control requirements.
- Validate. Confirm the resulting version or managed-service state, exercise representative functionality and recovery paths, monitor for regressions, and document exceptions.
- Retain evidence. Preserve asset and owner identifiers, the applicability decision, before-and-after versions, change approval, installation output, validation results, and any exception or rollback record.
Operational Cautions
Use the exact supported release stream and repository. Test dependencies, clusters, integrations, and rollback before broad deployment; a CVSS rating or reachable feature is not evidence of exploitation.
Evidence To Retain
- Exact product, release stream, package or application version, enabled feature, environment, and accountable owner.
- UTC timestamps, authority revision, approved change record, installer or package-manager output, and resulting version.
- Relevant logs and monitoring evidence, test results, exceptions, compensating controls, and rollback decisions.
Related TechGeeks Resources
Authoritative References
Correction policy: If the authority changes affected versions, fixed versions, severity, exploitation information, mitigations, or required action, TechGeeks will update this notice and its verification date.
