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NVD / CVE Program· CNA Mattermost

CVE-2026-27656: Account Takeover via Substring Matching in OpenID Connect Authentication

Noroxi assessment

Priority: Monitor (action score 24/100)

  • 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: 0.18% (FIRST EPSS).
  • Anyone who can reach it over the internet can trigger it. Administrator privileges are required. A user must open a link or visit a page. No special conditions are required; it is repeatable.
  • Fixed version: github.com/mattermost/mattermost-server (Go) 10.11.12, 10.11.12+incompatible, 11.2.4; github.com/mattermost/mattermost/server/v8 (Go) 8.0.0-20260217110922-b7d4a1f1f59b.

Low priority for now. Upgrade to github.com/mattermost/mattermost-server (Go) 10.11.12, 10.11.12+incompatible, 11.2.4; github.com/mattermost/mattermost/server/v8 (Go) 8.0.0-20260217110922-b7d4a1f1f59b or later.

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 11.4.x <= 11.4.0, 11.3.x <= 11.3.1, 11.2.x <= 11.2.3, 10.11.x <= 10.11.11 fail to properly validate user identity in the OpenID {{IsSameUser()}} comparison logic, which allows an attacker to take over arbitrary user accounts via an overly permissive substring matching flaw in the user discovery flow.. Mattermost Advisory ID: MMSA-2026-00590

MediumCVSS 6.1 · v3.1—No exploit Fix available
Published
Mar 25, 2026
Updated
Jun 17, 2026
EPSS
0.1% · 7th percentile

Report tools

JSON

Action score

24

Monitor

Low priority for now.

CVSS
24 / 40 · 6.1 / 10
CISA KEV
0 / 30 · Not listed
EPSS
0 / 30 · 0.1%

CISA SSVC decision

Exploitation
none
Automatable
no
Technical impact
total

Vulnrichment: CISA's decision-tree inputs.

CNA vs NVD score

NVD
6.1
CNA · Mattermost
5.7
0.4 point gap

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

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
  • A fix has been released

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

Follow CVE-2026-27656

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

Affected versions

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

  • mattermost mattermost server10.11.0 and later · before 10.11.12
  • mattermost mattermost server11.2.0 and later · before 11.2.4
  • mattermost mattermost server11.3.0 and later · before 11.3.2
  • mattermost mattermost server11.4.0 and later · before 11.4.1

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

    • 11.5.0, 11.4.1, 11.3.2, 11.2.4, 10.11.12not affected
    • 11.4.0 and later · up to and including 11.4.0affected · semver
    • 11.3.0 and later · up to and including 11.3.1affected · semver
    • 11.2.0 and later · up to and including 11.2.3affected · semver
    • 10.11.0 and later · up to and including 10.11.11affected · semver

Package-level exposure

OSV and GitHub Advisory data: ecosystem, package and range. SBOM matching uses this table.

EcosystemPackageAffected rangeFix
Gogithub.com/mattermost/mattermost-serverfrom 10.11.0-rc1 · before 10.11.1210.11.12
Gogithub.com/mattermost/mattermost-serverfrom 10.11.0-rc1+incompatible · before 10.11.12+incompatible10.11.12+incompatible
Gogithub.com/mattermost/mattermost-serverfrom 11.2.0-rc1 · before 11.2.411.2.4
Gogithub.com/mattermost/mattermost-serverfrom 11.2.0-rc1+incompatible · before 11.2.4+incompatible11.2.4+incompatible
Gogithub.com/mattermost/mattermost-serverfrom 11.3.0-rc1 · before 11.3.211.3.2
Gogithub.com/mattermost/mattermost-serverfrom 11.3.0-rc1+incompatible · before 11.3.2+incompatible11.3.2+incompatible
Gogithub.com/mattermost/mattermost-serverfrom 11.4.0-rc1 · before 11.4.111.4.1
Gogithub.com/mattermost/mattermost-serverfrom 11.4.0-rc1+incompatible · before 11.4.1+incompatible11.4.1+incompatible
Gogithub.com/mattermost/mattermost-server/v5from 0—
Gogithub.com/mattermost/mattermost-server/v6from 0—
Gogithub.com/mattermost/mattermost/server/v8from 8.0.0-20260105080200-d27a2195068d · before 8.0.0-20260217110922-b7d4a1f1f59b8.0.0-20260217110922-b7d4a1f1f59b

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-2026-20719Mar 25, 2026 · DoS via URL Previews Rendering Malicious SVGs
    30Monitor
  • ← EarlierCVE-2026-26233Mar 25, 2026 · Denial of Service via HTTP/2 single packet attack on login endpoint
    26Monitor
  • Later →CVE-2026-27659Mar 25, 2026 · CSRF vulnerability in UpdateAccessControlPolicyActiveStatus endpoint
    18Monitor

Remediation

Which version to upgrade to

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

Product / packageFixed versionSource
Go:github.com/mattermost/mattermost-server10.11.12Package registry (OSV)
Go:github.com/mattermost/mattermost-server10.11.12+incompatiblePackage registry (OSV)
Go:github.com/mattermost/mattermost-server11.2.4Package registry (OSV)
Go:github.com/mattermost/mattermost-server11.2.4+incompatiblePackage registry (OSV)
Go:github.com/mattermost/mattermost-server11.3.2Package registry (OSV)
Go:github.com/mattermost/mattermost-server11.3.2+incompatiblePackage registry (OSV)
Go:github.com/mattermost/mattermost-server11.4.1Package registry (OSV)
Go:github.com/mattermost/mattermost-server11.4.1+incompatiblePackage registry (OSV)
Go:github.com/mattermost/mattermost/server/v88.0.0-20260217110922-b7d4a1f1f59bPackage registry (OSV)

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.

No history yet (it accumulates with the daily EPSS job).

Patch and commit links

Commit, PR and diff links among the references. A starting point for patch-diffing and variant hunting; fixes, not exploits.

No commit or PR link among the references.

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-2025-12419118 days apartAccount takeover on OAuth/OpenID-enabled servers
    39Monitor
  • CVE-2025-12421118 days apartAccount Takeover via Code Exchange Endpoint
    39Monitor
  • CVE-2025-1427393 days apartMattermost Jira plugin user spoofing enables Jira request forgery.
    33Monitor
  • CVE-2025-3230299 days apartBypass of System Admin User Deactivation Controls for Personal Access Tokens in Mattermost Server
    21Monitor
  • CVE-2025-2475345 days apartUnauthorized Bot Login Using Credentials
    21Monitor
  • CVE-2024-36250501 days apartMattermost versions 9.11.x <= 9.11.2, and 9.5.x <= 9.5.10 fail to protect the mfa code against replay attacks, which allows an attacker to r
    19Monitor

Chain candidates

An authentication bypass and a privilege-requiring bug in the same product, published close together: combined they may become an unauthenticated path.

—

Bug bounty scope

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

Technical details

Attack conditions

  • Anyone who can reach it over the internet can trigger it.
  • Administrator privileges are required.
  • A user must open a link or visit a page.
  • No special conditions are required; it is repeatable.

If successful

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

Weakness class (CWE)

CWE-303 · Incorrect Implementation of Authentication Algorithm

CVSS vector

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

nvd-primary

Attack context

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

ATT&CK techniques

—

Change log

  1. Fix✗ → ✓

For records you follow, these changes also arrive as notifications. →

References

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

All records