CVE-2026-9859

MEDIUMPre-NVD 6.56.5
EchelonGraph scoreMEDIUM confidence

“—” beside the score: it is equal to the CVSS score beside it.

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, GitHub Security Advisory, a secondary CVSS source
Trending — 3 sources updated this week
6.5EG
EchelonGraph verdictMonitorRemediate within your standard update timelines (the SSVC decision on this page).
  • Lower severity and no public exploit yet
CISA-KEV: Not listedEPSS PROB: 0.3%CVSS: 6.5Exploit: None knownExposed services: Not assessed

Update Mattermost to versions 11.9.0, 11.7.7, 10.11.22, 11.8.4 or higher (from the CVE record's solution, Mattermost, Inc.) — apply it.

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
6.5MEDIUMmedium confidence
EG Risk
29
EG Risk 29/100

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 (value × EchelonGraph’s weight)
Severity65% × 45%
The EG Score, 6.5 of 10.
Exploitation0.3% × 40%
FIRST EPSS: the 30-day exploitation probability when EchelonGraph scored this CVE.
Automatability0% × 15%
CISA's SSVC Automatable value: no (Vulnrichment).
SSVC (CISA's decision table, applied by EchelonGraph): Track at every mission impact level.
Exploitation none (CISA Vulnrichment) · Automatable no (CISA Vulnrichment) · Technical impact partial (CISA Vulnrichment). All three are CISA's SSVC values (Vulnrichment). EchelonGraph applied CISA's decision table to them; CISA publishes SSVC inputs and the table, not a decision for each CVE.
Action: Update Mattermost to versions 11.9.0, 11.7.7, 10.11.22, 11.8.4 or higher (from the CVE record's solution, Mattermost, Inc.). Apply it within your standard update timelines.
EPSS PROB
0.3%
EPSS %ILE
25th
KEV
Not listed

SSVC (CISA's decision table, applied by EchelonGraph)Track at every mission impact level.

Update Mattermost to versions 11.9.0, 11.7.7, 10.11.22, 11.8.4 or higher (from the CVE record's solution, Mattermost, Inc.). Apply it within your standard update timelines.

Exploitation none (CISA Vulnrichment) · Automatable no (CISA Vulnrichment) · Technical impact partial (CISA Vulnrichment). All three are CISA's SSVC values (Vulnrichment). EchelonGraph applied CISA's decision table to them; CISA publishes SSVC inputs and the table, not a decision for each CVE. Mission impact is CISA's Mission & Well-being decision point, and only you can judge it. CISA's table makes it high when Mission Prevalence is Essential — the vulnerable component "directly provides capabilities that constitute at least one MEF for at least one entity" (MEF: mission essential function) — or when Public Well-Being Impact is Irreversible: "multiple fatalities are likely", the cyber-physical system "is likely lost or destroyed", "extreme or serious externalities" are imposed on other parties, or social systems such as elections or the financial grid "are destabilized and potentially collapse". CISA's decision table

BOD 26-04 (CISA's remediation timeline, Table 1)60 days if the affected system is publicly exposed; Fix on system upgrade if it is not.

In CISA KEV no (CISA's KEV catalog) · Automatable no (CISA Vulnrichment) · Technical impact partial (CISA Vulnrichment). Whether your system is publicly exposed is yours to answer. BOD 26-04 binds US Federal Civilian Executive Branch agencies; for anyone else it is CISA's published timeline, a reference rather than an obligation. The directive · how EchelonGraph applies it

Published

August 17, 2026

Last Modified

August 19, 2026

Advisory Details (1)

Auto-updated Aug 17, 2026
Fix named in the CVE record's solution (Mattermost, Inc.).
Web page

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 16× in last 7d / 76× in last 30d)

Each row is a time one of our pipelines fetched, updated or re-scored this CVE. 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. An arrow marks a change the record states ("CVSS v3 → 9.1"); EPSS and GHSA rows show the values recorded at that time, not what changed (EPSS values are rounded; hover one for the exact figures). Most recent first.

Showing the most recent 100 of 150 refreshes on record for this CVE.

A repeat, a row identical to one the same pipeline wrote less than 10 seconds earlier, is not a separate refresh and is not counted. None are among the latest 100 rows. The 50 oldest rows on record are not shown and were not checked, so the counts can still include repeats.

  1. 2026-10-11 19:10 UTCEG score recompute
  2. 2026-10-11 16:24 UTCEG score recompute
  3. 2026-10-11 13:50 UTCEPSS rescoreEPSS 0.3% · 25th percentile
  4. 2026-10-11 13:37 UTCEG score recompute
  5. 2026-10-10 20:01 UTCEPSS rescoreEPSS 0.3% · 25th percentile
  6. 2026-10-09 20:08 UTCEPSS rescoreEPSS 0.3% · 25th percentile
  7. 2026-10-08 17:14 UTCEPSS rescoreEPSS 0.3% · 25th percentile
  8. 2026-10-07 23:48 UTCEPSS rescoreEPSS 0.3% · 25th percentile
  9. 2026-10-07 11:24 UTCEG score recompute
  10. 2026-10-07 11:24 UTCGHSA enrichmentGHSA-27x7-qjjm-4f83 · GitHub CVSS 6.5
  11. 2026-10-06 15:19 UTCEPSS rescoreEPSS 0.3% · 25th percentile
  12. 2026-10-06 12:57 UTCEG score recompute
  13. 2026-10-06 12:57 UTCGHSA enrichmentGHSA-27x7-qjjm-4f83 · GitHub CVSS 6.5
  14. 2026-10-05 17:44 UTCEPSS rescoreEPSS 0.3% · 25th percentile
  15. 2026-10-05 14:38 UTCEG score recompute
  16. 2026-10-05 14:38 UTCGHSA enrichmentGHSA-27x7-qjjm-4f83 · GitHub CVSS 6.5
  17. 2026-10-04 23:23 UTCEPSS rescoreEPSS 0.3% · 25th percentile
  18. 2026-10-04 16:24 UTCGHSA enrichmentGHSA-27x7-qjjm-4f83 · GitHub CVSS 6.5
  19. 2026-10-03 17:45 UTCEG score recompute
  20. 2026-10-03 17:45 UTCGHSA enrichmentGHSA-27x7-qjjm-4f83 · GitHub CVSS 6.5
  21. 2026-10-03 14:26 UTCEPSS rescoreEPSS 0.3% · 25th percentile
  22. 2026-10-02 17:57 UTCEPSS rescoreEPSS 0.3% · 25th percentile
  23. 2026-10-02 06:56 UTCEG score recompute
  24. 2026-10-02 06:56 UTCGHSA enrichmentGHSA-27x7-qjjm-4f83 · GitHub CVSS 6.5
  25. 2026-10-01 19:51 UTCEPSS rescoreEPSS 0.3% · 25th percentile
Show 75 more
  1. 2026-10-01 08:11 UTCEG score recompute
  2. 2026-10-01 08:11 UTCGHSA enrichmentGHSA-27x7-qjjm-4f83 · GitHub CVSS 6.5
  3. 2026-09-30 15:04 UTCEPSS rescoreEPSS 0.3% · 25th percentile
  4. 2026-09-30 09:56 UTCEG score recompute
  5. 2026-09-30 09:56 UTCGHSA enrichmentGHSA-27x7-qjjm-4f83 · GitHub CVSS 6.5
  6. 2026-09-29 15:24 UTCEPSS rescoreEPSS 0.3% · 24th percentile
  7. 2026-09-29 04:40 UTCEG score recompute
  8. 2026-09-29 04:40 UTCGHSA enrichmentGHSA-27x7-qjjm-4f83 · GitHub CVSS 6.5
  9. 2026-09-28 13:52 UTCEPSS rescoreEPSS 0.3% · 24th percentile
  10. 2026-09-28 06:17 UTCEG score recompute
  11. 2026-09-28 06:17 UTCGHSA enrichmentGHSA-27x7-qjjm-4f83 · GitHub CVSS 6.5
  12. 2026-09-27 13:49 UTCEPSS rescoreEPSS 0.3% · 24th percentile
  13. 2026-09-27 08:03 UTCEG score recompute
  14. 2026-09-27 08:03 UTCGHSA enrichmentGHSA-27x7-qjjm-4f83 · GitHub CVSS 6.5
  15. 2026-09-26 15:59 UTCEPSS rescoreEPSS 0.3% · 24th percentile
  16. 2026-09-25 01:50 UTCEG score recompute
  17. 2026-09-25 01:50 UTCGHSA enrichmentGHSA-27x7-qjjm-4f83 · GitHub CVSS 6.5
  18. 2026-09-24 14:04 UTCEPSS rescoreEPSS 0.3% · 24th percentile
  19. 2026-09-23 20:26 UTCEG score recompute
  20. 2026-09-23 20:26 UTCGHSA enrichmentGHSA-27x7-qjjm-4f83 · GitHub CVSS 6.5
  21. 2026-09-23 17:54 UTCEPSS rescoreEPSS 0.3% · 17th percentile
  22. 2026-09-22 20:37 UTCEG score recompute
  23. 2026-09-22 20:37 UTCGHSA enrichmentGHSA-27x7-qjjm-4f83 · GitHub CVSS 6.5
  24. 2026-09-22 16:01 UTCEPSS rescoreEPSS 0.3% · 17th percentile
  25. 2026-09-21 21:09 UTCEPSS rescoreEPSS 0.3% · 17th percentile
  26. 2026-09-21 18:36 UTCEG score recompute
  27. 2026-09-21 18:36 UTCGHSA enrichmentGHSA-27x7-qjjm-4f83 · GitHub CVSS 6.5
  28. 2026-09-20 20:16 UTCEPSS rescoreEPSS 0.3% · 17th percentile
  29. 2026-09-20 18:38 UTCGHSA enrichmentGHSA-27x7-qjjm-4f83 · GitHub CVSS 6.5
  30. 2026-09-19 20:08 UTCEG score recompute
  31. 2026-09-19 20:08 UTCGHSA enrichmentGHSA-27x7-qjjm-4f83 · GitHub CVSS 6.5
  32. 2026-09-18 21:54 UTCEG score recompute
  33. 2026-09-18 21:54 UTCGHSA enrichmentGHSA-27x7-qjjm-4f83 · GitHub CVSS 6.5
  34. 2026-09-18 19:28 UTCEPSS rescoreEPSS 0.3% · 17th percentile
  35. 2026-09-17 21:53 UTCEG score recompute
  36. 2026-09-17 21:53 UTCGHSA enrichmentGHSA-27x7-qjjm-4f83 · GitHub CVSS 6.5
  37. 2026-09-17 19:32 UTCEPSS rescoreEPSS 0.3% · 17th percentile
  38. 2026-09-16 23:01 UTCEG score recompute
  39. 2026-09-16 23:00 UTCGHSA enrichmentGHSA-27x7-qjjm-4f83 · GitHub CVSS 6.5
  40. 2026-09-16 14:09 UTCEPSS rescoreEPSS 0.3% · 17th percentile
  41. 2026-09-16 05:15 UTCEPSS rescoreEPSS 0.3% · 16th percentile
  42. 2026-09-15 19:11 UTCEG score recompute
  43. 2026-09-15 19:11 UTCGHSA enrichmentGHSA-27x7-qjjm-4f83 · GitHub CVSS 6.5
  44. 2026-09-15 03:12 UTCEPSS rescoreEPSS 0.3% · 16th percentile
  45. 2026-09-14 19:49 UTCGHSA enrichmentGHSA-27x7-qjjm-4f83 · GitHub CVSS 6.5
  46. 2026-09-13 20:13 UTCEG score recompute
  47. 2026-09-13 20:13 UTCGHSA enrichmentGHSA-27x7-qjjm-4f83 · GitHub CVSS 6.5
  48. 2026-09-13 16:48 UTCEPSS rescoreEPSS 0.3% · 16th percentile
  49. 2026-09-12 21:59 UTCEG score recompute
  50. 2026-09-12 21:59 UTCGHSA enrichmentGHSA-27x7-qjjm-4f83 · GitHub CVSS 6.5
  51. 2026-09-12 15:02 UTCEPSS rescoreEPSS 0.3% · 16th percentile
  52. 2026-09-11 23:45 UTCEG score recompute
  53. 2026-09-11 23:45 UTCGHSA enrichmentGHSA-27x7-qjjm-4f83 · GitHub CVSS 6.5
  54. 2026-09-11 14:53 UTCEPSS rescoreEPSS 0.3% · 16th percentile
  55. 2026-09-11 09:38 UTCEPSS rescoreEPSS 0.3% · 16th percentile
  56. 2026-09-11 00:42 UTCEG score recompute
  57. 2026-09-11 00:42 UTCGHSA enrichmentGHSA-27x7-qjjm-4f83 · GitHub CVSS 6.5
  58. 2026-09-10 09:35 UTCEPSS rescoreEPSS 0.3% · 16th percentile
  59. 2026-09-10 01:53 UTCGHSA enrichmentGHSA-27x7-qjjm-4f83 · GitHub CVSS 6.5
  60. 2026-09-09 03:35 UTCEG score recompute
  61. 2026-09-09 03:35 UTCGHSA enrichmentGHSA-27x7-qjjm-4f83 · GitHub CVSS 6.5
  62. 2026-09-08 22:01 UTCEPSS rescoreEPSS 0.3% · 16th percentile
  63. 2026-09-07 16:10 UTCEG score recompute
  64. 2026-09-07 16:10 UTCGHSA enrichmentGHSA-27x7-qjjm-4f83 · GitHub CVSS 6.5
  65. 2026-09-06 15:44 UTCEG score recompute
  66. 2026-09-06 15:44 UTCGHSA enrichmentGHSA-27x7-qjjm-4f83 · GitHub CVSS 6.5
  67. 2026-09-06 13:48 UTCEPSS rescoreEPSS 0.3% · 16th percentile
  68. 2026-09-05 17:29 UTCEG score recompute
  69. 2026-09-05 17:29 UTCGHSA enrichmentGHSA-27x7-qjjm-4f83 · GitHub CVSS 6.5
  70. 2026-09-05 15:31 UTCEPSS rescoreEPSS 0.3% · 16th percentile
  71. 2026-09-04 19:03 UTCEG score recompute
  72. 2026-09-04 19:03 UTCGHSA enrichmentGHSA-27x7-qjjm-4f83 · GitHub CVSS 6.5
  73. 2026-09-04 05:07 UTCEPSS rescoreEPSS 0.3% · 16th percentile
  74. 2026-09-03 20:26 UTCGHSA enrichmentGHSA-27x7-qjjm-4f83 · GitHub CVSS 6.5
  75. 2026-09-02 22:06 UTCEG score recompute

Related CVEs(same CWE)

Frequently asked(5)

What is CVE-2026-9859?
CVE-2026-9859 is a medium-severity vulnerability; its CVE record was 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 the CVE record for CVE-2026-9859 published?
The CVE record for CVE-2026-9859 was published on August 17, 2026. That is the record's publication date; the vulnerability itself may have been made public earlier. EchelonGraph's copy of the record carries a last-modified date of August 19, 2026; the record at its source may have been updated since.
Is CVE-2026-9859 actively exploited?
CVE-2026-9859 is not currently on CISA's Known Exploited Vulnerabilities catalog. FIRST EPSS estimates a 0.3% probability of exploitation in the next 30 days (25th percentile of EPSS-scored CVEs).
What is the CVSS score of CVE-2026-9859?
CVE-2026-9859 has a CVSS base score of 6.5 (a secondary CVSS source that NVD displays; NVD's own analysis pending).
How do I remediate CVE-2026-9859?
The CVE record's solution for CVE-2026-9859, written by its CNA Mattermost, Inc., names the fix: "Update Mattermost to versions 11.9.0, 11.7.7, 10.11.22, 11.8.4 or higher." The vendor advisories EchelonGraph has for CVE-2026-9859 are linked in the Vendor Advisories panel on this page.

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