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CVE-2026-5740· NVD / CVE Program· CNA Mattermost

Unauthenticated WebSocket binary frame causes denial of service in Mattermost Server

Noroxi assessment

Priority: Monitor (action score 30/100)

  • 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: 0.57% (FIRST EPSS).
  • Anyone who can reach it over the internet can trigger it. No account or password is required. No user action is required. No special conditions are required; it is repeatable.
  • According to CISA the attack is automatable: targets can be found by mass scanning.
  • Fixed version: github.com/mattermost/mattermost-server (Go) 10.11.15, 10.11.15+incompatible, 11.4.5; github.com/mattermost/mattermost/server/v8 (Go) 8.0.0-20260410202636-17939826efa2.

Low priority for now. Upgrade to github.com/mattermost/mattermost-server (Go) 10.11.15, 10.11.15+incompatible, 11.4.5; github.com/mattermost/mattermost/server/v8 (Go) 8.0.0-20260410202636-17939826efa2 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 11.6.x <= 11.6.0, 11.5.x <= 11.5.3, 11.4.x <= 11.4.4, 10.11.x <= 10.11.14 fail to properly validate msgpack-encoded WebSocket frames before memory allocation which allows an unauthenticated remote attacker to crash the server process and cause a full service outage for all users via a crafted binary WebSocket message sent to the public WebSocket endpoint.. Mattermost Advisory ID: MMSA-2026-00647

HighCVSS 7.5 · v3.1—No exploit Fix available
Published
May 22, 2026
Updated
Jul 23, 2026
EPSS
0.5% · 46th percentile

Report tools

JSON

Action score

30

Monitor

Low priority for now.

CVSS
30 / 40 · 7.5 / 10
CISA KEV
0 / 30 · Not listed
EPSS
0 / 30 · 0.5%

CISA SSVC decision

Exploitation
none
Automatable
yes
Technical impact
partial

Vulnrichment: CISA's decision-tree inputs.

CNA vs NVD score

NVD
—
CNA · Mattermost
7.5
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
  • A fix has been released

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

Follow CVE-2026-5740

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 10.11.15
  • mattermost mattermost server11.4.0 and later · before 11.4.5
  • mattermost mattermost server11.5.0 and later · before 11.5.4
  • mattermost mattermost server11.6.0 and later · before 11.6.1

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

    • 11.7.0, 11.6.1, 11.5.4, 11.4.5, 10.11.15not affected
    • 11.6.0 and later · up to and including 11.6.0affected · semver
    • 11.5.0 and later · up to and including 11.5.3affected · semver
    • 11.4.0 and later · up to and including 11.4.4affected · semver
    • 10.11.0 and later · up to and including 10.11.14affected · 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 · before 10.11.1510.11.15
Gogithub.com/mattermost/mattermost-serverfrom 10.11.0+incompatible · before 10.11.15+incompatible10.11.15+incompatible
Gogithub.com/mattermost/mattermost-serverfrom 11.4.0 · before 11.4.511.4.5
Gogithub.com/mattermost/mattermost-serverfrom 11.4.0+incompatible · before 11.4.5+incompatible11.4.5+incompatible
Gogithub.com/mattermost/mattermost-serverfrom 11.5.0 · before 11.5.411.5.4
Gogithub.com/mattermost/mattermost-serverfrom 11.5.0+incompatible · before 11.5.4+incompatible11.5.4+incompatible
Gogithub.com/mattermost/mattermost-serverfrom 11.6.0 · before 11.6.111.6.1
Gogithub.com/mattermost/mattermost-serverfrom 11.6.0+incompatible · before 11.6.1+incompatible11.6.1+incompatible
Gogithub.com/mattermost/mattermost-server/v5all versions—
Gogithub.com/mattermost/mattermost-server/v6all versions—
Gogithub.com/mattermost/mattermost/server/v8before 8.0.0-20260410202636-17939826efa28.0.0-20260410202636-17939826efa2

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
  • CVE-2025-4981Path Traversal Leading to RCE by Any Authenticated Mattermost User
    39Monitor
  • CVE-2023-6458Client side path traversal due to lack of route parameters validation
    39Monitor
  • CVE-2017-18920An issue was discovered in Mattermost Server before 3.6.2.
    39Monitor

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-server10.11.15Package registry (OSV)
Go:github.com/mattermost/mattermost-server10.11.15+incompatiblePackage registry (OSV)
Go:github.com/mattermost/mattermost-server11.4.5Package registry (OSV)
Go:github.com/mattermost/mattermost-server11.4.5+incompatiblePackage registry (OSV)
Go:github.com/mattermost/mattermost-server11.5.4Package registry (OSV)
Go:github.com/mattermost/mattermost-server11.5.4+incompatiblePackage registry (OSV)
Go:github.com/mattermost/mattermost-server11.6.1Package registry (OSV)
Go:github.com/mattermost/mattermost-server11.6.1+incompatiblePackage registry (OSV)
Go:github.com/mattermost/mattermost/server/v88.0.0-20260410202636-17939826efa2Package 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.

EPSS, last 120 days

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

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-2026-63404 days apartMemory Exhaustion via Malicious 7zip File Upload
    26Monitor
  • CVE-2026-96260123 days apartMattermost server missing request body size limit on plugin routes allows denial of service by an authenticated user
    26Monitor
  • CVE-2026-245667 days apartDenial of Service via Unbounded Memory Allocation in Integration Actions
    22Monitor
  • CVE-2026-2578067 days apartMemory Exhaustion via Malformed DOC File Upload
    17Monitor
  • CVE-2026-2624667 days apartMemory Exhaustion via Malformed PSD File Upload
    17Monitor

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.
  • No account or password is required.
  • No user action is required.
  • No special conditions are required; it is repeatable.

If successful

Confidentiality
none
Integrity
none
Availability
high · the service can be disrupted
Attack vector
Network
Attack complexity
Low
Privileges required
None
User interaction
None
Scope
Unchanged
Confidentiality impact
None
Integrity impact
None
Availability impact
High

Weakness class (CWE)

CWE-789 · Memory Allocation with Excessive Size Value

CVSS vector

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

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.

MITRE has no CAPEC/ATT&CK mapping for this CWE.

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