CVE-2026-9859

MEDIUMPre-NVD 6.56.5—
EchelonGraph scoreHIGH confidence

Score 6.5 from GitHub Security Advisory published 2026-08-18. a secondary CVSS source baseline 6.5; sources differ by 0.0.

Triggered by: GitHub Security Advisory CVSS
Sources: epss, ghsa, secondary
Trending — 4 sources updated this week
6.5EG
EchelonGraph verdictMonitorLow exploitation likelihood right now — keep watching.
  • Lower severity and no public exploit yet
CISA-KEV: Not listedEPSS PROB: 0%CVSS: 6.5Exploit: None knownExposed: 0

No vendor fix yet — apply a workaround or compensating control (WAF / firewall / segmentation) and watch for a patch.

Mattermost versions 11.7.x <= 11.7.6, 10.11.x <= 10.11.21, 11.8.x <= 11.8.3 fail to enforce PermissionManageBoardRoles on the channelId field of the batch endpoint, which allows an authenticated board editor to relink any board they can edit to an arbitrary channel via a crafted PATCH request. Mattermost Advisory ID: MMSA-2026-00686

CVSS v3
6.5
EG Score
6.5(high)
EG Risk
29(Track)
EG Risk 29/100SSVC: Track

EG Risk is EchelonGraph's 0–100 priority score: it fuses intrinsic severity with real-world exploitation and automatability so you can rank equal-severity CVEs and fix the most dangerous first. Higher = act sooner. Distinct from the 0–10 EG Score (severity).

How it’s computed
Severity65% × 45%
Exploitation0% × 40%
Automatability0% × 15%
Action: Routine — remediate on your standard cadence.
EPSS PROB
0%
EPSS %ILE
17%
KEV
Not listed

Published

August 17, 2026

Last Modified

August 19, 2026

Advisory Details (1)

Auto-updated Aug 17, 2026
No patch confirmed yet.
generic

Security Updates

https://mattermost.com/security-updates

Vendor Advisories for CVE-2026-9859(1)

These vendors published their own advisory mentioning this CVE — often with vendor-specific remediation steps + affected product lists not in NVD.

Weakness Classification(1)

MITRE Common Weakness Enumeration — the root-cause categories this CVE belongs to.

Data Freshness Timeline

(refreshed 13× in last 7d / 13× in last 30d)

Each row is a source pipeline that fetched or updated this CVE on that date, with what changed. For example, "NVD update" means NVD published or revised its analysis for this CVE; "MITRE cvelistV5" means we ingested or refreshed it from the CNA feed. Most recent first.

  1. 2026-08-20 22:56 UTCEPSS rescore
  2. 2026-08-20 12:03 UTCEG score recompute
  3. 2026-08-20 12:03 UTCGHSA enrichment
  4. 2026-08-19 17:04 UTCEPSS rescore
  5. 2026-08-19 13:40 UTCGHSA enrichment
  6. 2026-08-18 15:23 UTCEG score recompute
  7. 2026-08-18 15:23 UTCGHSA enrichment
  8. 2026-08-18 14:49 UTCEG score recompute
  9. 2026-08-18 14:49 UTCGHSA enrichment
  10. 2026-08-18 13:49 UTCEPSS rescore
  11. 2026-08-17 22:21 UTCEG score recompute
  12. 2026-08-17 22:09 UTCEG score recompute
  13. 2026-08-17 22:08 UTCMITRE cvelistV5first tracked

Related CVEs(same CWE)

Frequently asked(5)

What is CVE-2026-9859?
CVE-2026-9859 is a medium vulnerability published on August 17, 2026. Mattermost versions 11.7.x <= 11.7.6, 10.11.x <= 10.11.21, 11.8.x <= 11.8.3 fail to enforce PermissionManageBoardRoles on the channelId field of the batch endpoint, which allows an authenticated board editor to relink any board they can edit to an arbitrary channel via a crafted PATCH request.…
When was CVE-2026-9859 disclosed?
CVE-2026-9859 was first published in the National Vulnerability Database on August 17, 2026, with the most recent update on August 19, 2026. EchelonGraph re-ingests CVE updates from NVD on a 2-hour cycle, so this page reflects the latest published state.
Is CVE-2026-9859 actively exploited?
CVE-2026-9859 is not currently on CISA's Known Exploited Vulnerabilities catalog. FIRST EPSS estimates a 0% probability of exploitation in the next 30 days, which ranks it in the top 83.0% of all scored CVEs.
What is the CVSS score of CVE-2026-9859?
CVE-2026-9859 has a CVSS v3 base score of 6.5 (NVD).
How do I remediate CVE-2026-9859?
Patch to the fixed version published by the affected vendor. Where vendor advisories exist for CVE-2026-9859, EchelonGraph cross-links them in the Vendor Advisories panel below — those typically contain the canonical remediation steps, fixed version numbers, and any vendor-specific mitigations.

Dependency Blast Radius

Explore the affected products and dependency analysis for CVE-2026-9859

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