CVE-2026-26233Network attack vector

Service crash via unthrottled parallel login requests

Published Mar 25, 2026 · Updated Mar 25, 2026

Denial of service in Mattermost Server allows remote attackers to crash and restart it with parallel HTTP/2 login requests. The login endpoint applies no rate limit, so one HTTP/2 packet carrying more than 100 parallel login requests consumes enough server resources to force a restart. Mattermost describes the attack as unauthenticated, although its CVSS assessment assigns low privileges; no data access or modification is reported.

CVSS severity4.3
Medium
EPSS probability0.30%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Denial of service in Mattermost Server allows remote attackers to crash and restart it with parallel HTTP/2 login requests. The login endpoint applies no rate limit, so one HTTP/2 packet carrying more than 100 parallel login requests consumes enough server resources to force a restart. Mattermost describes the attack as unauthenticated, although its CVSS assessment assigns low privileges; no data access or modification is reported.

The record

CVE
CVE-2026-26233
Published
Mar 25, 2026
Updated
Mar 25, 2026
Vendor
Mattermost
Product
Mattermost
Classifications
CWE-400, T1499.002
Attack vector
network
Privileges
Unavailable

Timeline

How it unfolded

  1. Mar 25, 2026CVE publishedPublication date reported by the CVE source.
  2. Mar 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 <=10.11.11
  2. Affected versionversion=11.2.0 <=11.2.3
  3. Affected versionversion=11.3.0 <=11.3.1
  4. Affected versionversion=11.4.0 <=11.4.0

What conditions does exploitation require?

Attack vectornetwork
Required privilegesUnavailable in this record.

What is affected?

Mattermost · Mattermostversion=10.11.0 <=10.11.11; version=11.2.0 <=11.2.3; version=11.3.0 <=11.3.1; version=11.4.0 <=11.4.0

Published CVSS scores

4.3Mattermost, Inc.CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L
6.5NIST NVDCVSS: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
PrivilegesUnavailable
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-400Uncontrolled Resource Consumption
T1499.002Service Exhaustion Flood

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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