CVE-2026-25783Network attack vector

Request panic via empty User-Agent version token

Published Mar 16, 2026 · Updated Mar 16, 2026

Improper User-Agent validation in Mattermost Server allows remote authenticated users to trigger a request panic with a crafted header. getBrowserVersion takes the first strings.Fields result after a recognized client prefix without checking that a version token exists, so an empty suffix causes an out-of-range access. Any authenticated user able to submit the header can interrupt affected request processing; the published impact is limited to service availability.

CVSS severity4.3
Medium
EPSS probability0.29%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Improper User-Agent validation in Mattermost Server allows remote authenticated users to trigger a request panic with a crafted header. getBrowserVersion takes the first strings.Fields result after a recognized client prefix without checking that a version token exists, so an empty suffix causes an out-of-range access. Any authenticated user able to submit the header can interrupt affected request processing; the published impact is limited to service availability.

The record

CVE
CVE-2026-25783
Published
Mar 16, 2026
Updated
Mar 16, 2026
Vendor
Mattermost
Product
Mattermost
Classifications
CWE-1287, T1499.004
Attack vector
network
Privileges
authenticated

Timeline

How it unfolded

  1. Mar 16, 2026CVE publishedPublication date reported by the CVE source.
  2. Mar 16, 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 <=10.11.10
  2. Affected versionversion=11.2.0 <=11.2.2
  3. Affected versionversion=11.3.0 <=11.3.0

What conditions does exploitation require?

Attack vectornetwork
Required privilegesauthenticated

What is affected?

Mattermost · Mattermostversion=10.11.0 <=10.11.10; version=11.2.0 <=11.2.2; version=11.3.0 <=11.3.0

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

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-1287Improper Validation of Specified Type of Input
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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