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Noroxi
NVD / CVE Program· CNA Mattermost

CVE-2026-4915: Server panic via outgoing webhook responses

Noroxi assessment

Priority: Monitor (action score 26/100)

  • 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: 0.27% (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.
  • 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) 8.0.0-20260407102538-faa7d75b4ea0.

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) 8.0.0-20260407102538-faa7d75b4ea0 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 filter nil elements from outgoing webhook attachment payloads before processing, which allows an authenticated user to cause a denial of service (server process termination) via a crafted webhook callback response containing a null attachment entry.. Mattermost Advisory ID: MMSA-2026-00641

MediumCVSS 6.5 · v3.1—No exploit Fix available
Published
May 25, 2026
Updated
Jul 23, 2026
EPSS
0.2% · 20th percentile

Report tools

JSON

Action score

26

Monitor

Low priority for now.

CVSS
26 / 40 · 6.5 / 10
CISA KEV
0 / 30 · Not listed
EPSS
0 / 30 · 0.2%

CISA SSVC decision

Exploitation
none
Automatable
no
Technical impact
partial

Vulnrichment: CISA's decision-tree inputs.

CNA vs NVD score

NVD
—
CNA · Mattermost
6.5
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-2026-4915

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/v5from 0—
Gogithub.com/mattermost/mattermost-server/v6from 0—
Gogithub.com/mattermost/mattermost/server/v8before 8.0.0-20260407102538-faa7d75b4ea08.0.0-20260407102538-faa7d75b4ea0

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-2026-5755May 22, 2026 · Denial of service via crafted TIFF file upload
    26Monitor
  • ← EarlierCVE-2026-28735May 22, 2026 · GitHub OAuth Scope Validation
    21Monitor
  • Later →CVE-2026-3433Jun 12, 2026 · Mattermost fails to scope role_updated websocket events to authorized team and channel members
    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-server10.11.15Package 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/v88.0.0-20260407102538-faa7d75b4ea0Package 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-2026-2071961 days apartDoS via URL Previews Rendering Malicious SVGs
    30Monitor
  • CVE-2026-405410 days apartSVG content served through Mattermost image proxy despite Content-Type restrictions causes client-side denial of service
    26Monitor
  • CVE-2026-10556112 days apartUnauthenticated webhook request with null notification entry could crash the Microsoft Calendar plugin.
    21Monitor
  • CVE-2025-0503465 days apartLeaked User IDs and Metadata of Deleted DMs
    21Monitor
  • CVE-2025-22445501 days apartMisleading UI for undefined admin console settings in Calls causes security confusion
    21Monitor
  • CVE-2026-13417112 days apartBoards plugin denial of service via unvalidated block fields.properties
    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.
  • 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
Low
User interaction
None
Scope
Unchanged
Confidentiality impact
None
Integrity impact
None
Availability impact
High

Weakness class (CWE)

CWE-754 · Improper Check for Unusual or Exceptional Conditions

CVSS vector

CVSS:3.1/AV:N/AC:L/PR:L/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