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NVD / CVE Program· CNA Mattermost

CVE-2026-2456: Denial of Service via Unbounded Memory Allocation in Integration Actions

Noroxi assessment

Priority: Monitor (action score 22/100)

  • 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: 0.17% (FIRST EPSS).
  • Anyone who can reach it over the internet can trigger it. A low-privileged account is enough. 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) 10.11.11, 10.11.11+incompatible, 11.2.3; github.com/mattermost/mattermost/server/v8 (Go) 8.0.0-20260127165411-fe3052073dc6; govulncheck-vulndb (openSUSE:Leap 15.6) 0.0.20260326T203309-150000.1.155.2.

Low priority for now. Upgrade to github.com/mattermost/mattermost-server (Go) 10.11.11, 10.11.11+incompatible, 11.2.3; github.com/mattermost/mattermost/server/v8 (Go) 8.0.0-20260127165411-fe3052073dc6; govulncheck-vulndb (openSUSE:Leap 15.6) 0.0.20260326T203309-150000.1.155.2 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 11.3.x <= 11.3.0, 11.2.x <= 11.2.2, 10.11.x <= 10.11.10 Mattermost fails to limit the size of responses from integration action endpoints, which allows an authenticated attacker to cause server memory exhaustion and denial of service via a malicious integration server that returns an arbitrarily large response when a user clicks an interactive message button.. Mattermost Advisory ID: MMSA-2026-00571

MediumCVSS 5.7 · v3.1—No exploit Fix available
Published
Mar 16, 2026
Updated
Jun 17, 2026
EPSS
0.1% · 6th percentile

Report tools

JSON

Action score

22

Monitor

Low priority for now.

CVSS
22 / 40 · 5.7 / 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
5.7
CNA · Mattermost
5.3
0.4 point gap

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

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-2026-2456

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 10.11.11
  • mattermost mattermost server11.2.0 and later · before 11.2.3
  • mattermost mattermost server11.3.0 and later · before 11.3.1

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

    • 11.4.0, 11.3.1, 11.2.3, 10.11.11not affected
    • 11.3.0 and later · up to and including 11.3.0affected · semver
    • 11.2.0 and later · up to and including 11.2.2affected · semver
    • 10.11.0 and later · up to and including 10.11.10affected · 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 10.11.1110.11.11
Gogithub.com/mattermost/mattermost-serverfrom 10.11.0-rc1+incompatible · before 10.11.11+incompatible10.11.11+incompatible
Gogithub.com/mattermost/mattermost-serverfrom 11.2.0-rc1 · before 11.2.311.2.3
Gogithub.com/mattermost/mattermost-serverfrom 11.2.0-rc1+incompatible · before 11.2.3+incompatible11.2.3+incompatible
Gogithub.com/mattermost/mattermost-serverfrom 11.3.0-rc1 · before 11.3.111.3.1
Gogithub.com/mattermost/mattermost-serverfrom 11.3.0-rc1+incompatible · before 11.3.1+incompatible11.3.1+incompatible
Gogithub.com/mattermost/mattermost-serverbefore 5.3.2-0.20260127165411-fe3052073dc65.3.2-0.20260127165411-fe3052073dc6
Gogithub.com/mattermost/mattermost-server/v5from 0—
Gogithub.com/mattermost/mattermost-server/v6from 0—
Gogithub.com/mattermost/mattermost/server/v8before 8.0.0-20260127165411-fe3052073dc68.0.0-20260127165411-fe3052073dc6
openSUSE:Leap 15.6govulncheck-vulndbbefore 0.0.20260326T203309-150000.1.155.20.0.20260326T203309-150000.1.155.2

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-2026-25783Mar 16, 2026 · Denial of service via malformed User-Agent header in getBrowserVersion
    17Monitor
  • ← EarlierCVE-2026-26246Mar 16, 2026 · Memory Exhaustion via Malformed PSD File Upload
    17Monitor
  • Later →CVE-2026-2457Mar 16, 2026 · WebSocket Message Spoofing via Permalink Embed Manipulation
    17Monitor

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-server10.11.11Package registry (OSV)
Go:github.com/mattermost/mattermost-server10.11.11+incompatiblePackage registry (OSV)
Go:github.com/mattermost/mattermost-server11.2.3Package registry (OSV)
Go:github.com/mattermost/mattermost-server11.2.3+incompatiblePackage registry (OSV)
Go:github.com/mattermost/mattermost-server11.3.1Package registry (OSV)
Go:github.com/mattermost/mattermost-server11.3.1+incompatiblePackage registry (OSV)
Go:github.com/mattermost/mattermost-server5.3.2-0.20260127165411-fe3052073dc6Package registry (OSV)
Go:github.com/mattermost/mattermost/server/v88.0.0-20260127165411-fe3052073dc6Package registry (OSV)
opensuse:govulncheck-vulndb0.0.20260326T203309-150000.1.155.2 · openSUSE:Leap 15.6Package 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-2026-574067 days apartUnauthenticated WebSocket binary frame causes denial of service in Mattermost Server
    30Monitor
  • CVE-2026-634063 days apartMemory Exhaustion via Malicious 7zip File Upload
    26Monitor
  • CVE-2026-96260190 days apartMattermost server missing request body size limit on plugin routes allows denial of service by an authenticated user
    26Monitor
  • CVE-2026-93849224 days apartMattermost allows authenticated users to trigger excessive memory allocation via crafted msgpack binary WebSocket frames
    26Monitor
  • CVE-2026-257800 days apartMemory Exhaustion via Malformed DOC File Upload
    17Monitor
  • CVE-2026-262460 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.
  • A low-privileged account is enough.
  • A user must open a link or visit a page.
  • 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
Low
User interaction
Required
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:L/UI:R/S:U/C:N/I:N/A:H

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.

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