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CVE.org (no NVD analysis yet)· CNA Mattermost

CVE-2026-18799: Mattermost Server fails to restrict Slack import account binding, allowing team admins to forge posts as existing users

MediumCVSS 4.3 · v3.1—No exploit No fix recorded

Noroxi assessment

Priority: Monitor (action score 17/100)

  • The sources we track (CISA KEV, Exploit-DB, Metasploit, Nuclei, GitHub) show no public exploit code or exploitation in the wild.
  • Probability of exploitation in the next 30 days: 0.19% (FIRST EPSS).
  • Anyone who can reach it over the internet can trigger it. A low-privileged account is enough. No user action is required. No special conditions are required; it is repeatable.
  • According to the record, no fixed version is available yet.
  • Mentioned 1 time in Turkish national cyber security advisories (siberguvenlik.gov.tr).

Low priority for now. Update when a fix is released; follow the vendor's advisory.

Generated automatically from the record's data: CVSS (NVD and CNA), CISA KEV and SSVC, FIRST EPSS, exploit sources, fixed versions. How we calculate it

Primary sources: NVD · CVE.org · FIRST EPSS

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 restrict Slack import account binding to system administrator-initiated imports which allows an authenticated attacker with team admin privileges to force-add existing Mattermost users to their team and forge posts attributed to those users' identities via a crafted Slack export containing target users' email addresses.. Mattermost Advisory ID: MMSA-2026-00757

Published
Oct 16, 2026
Updated
Oct 29, 2026
EPSS
0.1% · 9th percentile

Report tools

JSON

Action score

17

Monitor

Low priority for now.

CVSS
17 / 40 · 4.3 / 10
CISA KEV
0 / 30 · Not listed
EPSS
0 / 30 · 0.1%

CISA SSVC decision

Exploitation
none
Automatable
no
Technical impact
partial

Vulnrichment: CISA's decision-tree inputs.

CNA vs NVD score

NVD
—
CNA · Mattermost
4.3
NVD has not scored this yet; the score shown is the CNA’s.

The score given by the assigning authority versus NVD’s independent score. A gap means the severity is contested.

Get told when this record changes

  • Not in KEV: no record of exploitation in the wild
  • No public exploit code on record
  • No fixed version yet

If any of these changes, we'll tell you in the daily email digest.

Follow CVE-2026-18799

When you'll hear from us

  • when it enters CISA KEV
  • when exploit code appears or is weaponized
  • when the record is updated (including a fixed version)
  • when EPSS rises 20 points in a week

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Noroxi analysis

What does it mean in your environment?

The assessment above is generated automatically from the record's data; it doesn't know your environment. Our team verifies on your systems whether this vulnerability is actually exploitable and which mitigation is enough.

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Affected systems

Is your version affected?

Pick the product and type the version you run. It's compared with this record's version ranges in your browser; the version isn't sent anywhere.

VendorProduct
mattermostmattermost server

Versions reported by the vendor

Affected version ranges reported by the assigning authority (Mattermost). Independent of NVD's CPE analysis and usually ahead of it.

  • Mattermost Mattermost

    • 11.11.0, 11.10.1, 11.7.10, 11.9.2, 11.8.6not affected
    • 11.10.0 and later · up to and including 11.10.0affected · semver
    • 11.7.0 and later · up to and including 11.7.9affected · semver
    • 11.9.0 and later · up to and including 11.9.1affected · semver
    • 11.8.0 and later · up to and including 11.8.5affected · semver
Affected versions · NVD (4)

NVD version ranges (for catalog products). Add the product to your stack with a version and matching uses these.

  • mattermost mattermost server11.7.0 and later · before 11.7.10
  • mattermost mattermost server11.8.0 and later · before 11.8.6
  • mattermost mattermost server11.9.0 and later · before 11.9.2
  • mattermost mattermost server11.10.0

Same product

mattermost: all records

Other highest-scoring records for the same primary product.

  • CVE-2026-4858Path traversal in integration action URL leading to arbitrary API execution via system admin’s auth token.
    39Monitor
  • CVE-2025-12421Account Takeover via Code Exchange Endpoint
    39Monitor
  • CVE-2025-12419Account takeover on OAuth/OpenID-enabled servers
    39Monitor

Published before and after this one in the same product

  • ← EarlierCVE-2026-18256Oct 16, 2026 · Private team metadata disclosure via the team search API
    17Monitor
  • ← EarlierCVE-2026-18560Oct 16, 2026 · Improper order of authorisations in Mattermost GetFile API leads to an authenticated user to determine a post’s quarantined status
    17Monitor
  • Later →CVE-2026-19076Oct 16, 2026 · Mattermost fails to reject OAuth application sessions in account type switch, allowing account takeover via bearer token
    19Monitor

Remediation

Which version to upgrade to

Fix versions compiled from the vendor, package registries and Microsoft. Verify the vendor's note before upgrading.

No fix version is recorded for this entry. Check the references for vendor advisories.

Exploit status

No known public exploit

No public exploit has been observed yet. That doesn't mean you're safe, only that the bar is a little higher.

Research context

For pentesters and researchers: attack profile, score disagreement, timeline, patch commits, credits, variant and chain candidates, bug bounty scope. All derived from existing data; no exploit code.

Timeline

From publication to today: proof of concept, Metasploit module, CISA KEV and fix record. Dates are as reported by the sources.

No dated events beyond publication.

EPSS, last 120 days

FIRST EPSS daily score; only changes of 0.01 or more are recorded (step chart).

Credits

All researchers

Finders, reporters and analysts named in the CNA record. Click a name for that researcher’s other records.

Variant candidates

Same product, same weakness class, within 18 months. If the patch missed the root cause, the sibling bug is here.

  • CVE-2025-58073365 days apartArbitrary Mattermost Team can be joined by manipulating the OAuth state
    32Monitor
  • CVE-2025-58075365 days apartArbitrary Mattermost Team can be joined by manipulating the SAML RelayState
    32Monitor
  • CVE-2026-882132 days apartPlaybooks run owner channel membership permission bypass
    28Monitor
  • CVE-2026-3117151 days apartInstance and webhook GitLab plugin commands were able to be run by non-admin users
    26Monitor
  • CVE-2026-5163151 days apartMissing authorization check in AI message rewrite endpoint allows access to private thread content
    26Monitor
  • CVE-2025-9076396 days apartMattermost Server exposes sensitive user credentials during shared channel membership synchronization
    26Monitor

Bug bounty scope

The product’s vendor appears in a public program. Matched by name; verify the scope text in the program.

National notice (Türkiye Cybersecurity Directorate / USOM)

Official security notices citing this record; remediation advice is on the agency's page.

All national notices →

Not on record for this CVE: patch commit or PR link.

Technical details

Attack conditions

  • Anyone who can reach it over the internet can trigger it.
  • A low-privileged account is enough.
  • No user action is required.
  • No special conditions are required; it is repeatable.

If successful

Confidentiality
none
Integrity
low · data or configuration can be modified
Availability
none
Attack vector
Network
Attack complexity
Low
Privileges required
Low
User interaction
None
Scope
Unchanged
Confidentiality impact
None
Integrity impact
Low
Availability impact
None

Weakness class (CWE)

CWE-862 · Missing Authorization

CVSS vector

CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:L/A:N

nvd-secondary

Attack context

MITRE CAPEC attack patterns and ATT&CK techniques for this weakness class (CWE). A starting point for detection rules and threat hunting.

References

Vendor advisories and official records. Exploit/PoC links are deliberately left out.

All records