CVE-2026-25783
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User-Agent Header Validation Flaw Causes Panic in Mattermost

Publication date: 2026-03-16

Last updated on: 2026-03-18

Assigner: Mattermost, Inc.

Description
Mattermost versions 11.3.x <= 11.3.0, 11.2.x <= 11.2.2, 10.11.x <= 10.11.10 fail to properly validate User-Agent header tokens which allows an authenticated attacker to cause a request panic via a specially crafted User-Agent header. Mattermost Advisory ID: MMSA-2026-00586
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Meta Information
Published
2026-03-16
Last Modified
2026-03-18
Generated
2026-06-16
AI Q&A
2026-03-16
EPSS Evaluated
2026-06-15
NVD
CVE-2026-25783
EUVD
EUVD-2026-12416
Affected Vendors & Products
Showing 3 associated CPEs
Vendor Product Version / Range
mattermost mattermost_server From 10.11.0 (inc) to 10.11.11 (exc)
mattermost mattermost_server From 11.2.0 (inc) to 11.2.3 (exc)
mattermost mattermost_server From 11.3.0 (inc) to 11.3.1 (exc)
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Exploitability
CWE
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KEV
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CWE ID Description
CWE-1287 The product receives input that is expected to be of a certain type, but it does not validate or incorrectly validates that the input is actually of the expected type.
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Executive Summary

This vulnerability exists in Mattermost versions 11.3.0 and earlier in the 11.3.x, 11.2.2 and earlier in the 11.2.x, and 10.11.10 and earlier in the 10.11.x series. It occurs because these versions fail to properly validate User-Agent header tokens. An authenticated attacker can exploit this by sending a specially crafted User-Agent header, which causes the application to panic during request processing.

Impact Analysis

The impact of this vulnerability is a denial of service condition. Because the specially crafted User-Agent header causes a request panic, it can disrupt normal operation of the Mattermost server, potentially leading to service interruptions or degraded availability.

Compliance Impact

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