CVE-2026-4915Network attack vector

Server crash via nil webhook attachment

Published May 25, 2026 · Updated May 25, 2026

Improper exception handling in Mattermost Server allows remote authenticated users to terminate the process via an outgoing webhook response. ProcessMessageAttachments iterates the original attachment pointer slice after a helper creates a nil-filtered copy, then dereferences a null entry while processing its text fields. The affected branches are 10.11.0 through 10.11.14, 11.4.0 through 11.4.4, 11.5.0 through 11.5.3, and 11.6.0; an authenticated user must cause a configured outgoing webhook callback to return a null attachment, which crashes the server without reported data disclosure or modification.

CVSS severity6.5
Medium
EPSS probability0.28%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Improper exception handling in Mattermost Server allows remote authenticated users to terminate the process via an outgoing webhook response. ProcessMessageAttachments iterates the original attachment pointer slice after a helper creates a nil-filtered copy, then dereferences a null entry while processing its text fields. The affected branches are 10.11.0 through 10.11.14, 11.4.0 through 11.4.4, 11.5.0 through 11.5.3, and 11.6.0; an authenticated user must cause a configured outgoing webhook callback to return a null attachment, which crashes the server without reported data disclosure or modification.

The record

CVE
CVE-2026-4915
Published
May 25, 2026
Updated
May 25, 2026
Vendor
Mattermost
Product
Mattermost
Classifications
CWE-754, T1499.004
Attack vector
network
Privileges
authenticated

Timeline

How it unfolded

  1. May 25, 2026CVE publishedPublication date reported by the CVE source.
  2. May 25, 2026Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=10.11.0 <=10.11.14
  2. Affected versionversion=11.4.0 <=11.4.4
  3. Affected versionversion=11.5.0 <=11.5.3
  4. Affected versionversion=11.6.0 <=11.6.0

What conditions does exploitation require?

Attack vectornetwork
Required privilegesauthenticated

What is affected?

Mattermost · Mattermostversion=10.11.0 <=10.11.14; version=11.4.0 <=11.4.4; version=11.5.0 <=11.5.3; version=11.6.0 <=11.6.0

Published CVSS scores

6.5Mattermost, Inc.CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-754Improper Check for Unusual or Exceptional Conditions
T1499.004Application or System Exploitation

Public exploit references

No public exploit references are available in this record.

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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