CVE-2026-6334Network attack vector

Cross-client OAuth token access via unbound codes

Published May 18, 2026 · Updated May 18, 2026

OAuth bypass in Mattermost 10.11.0 through 10.11.13 and 11.5.0 through 11.5.1 allows remote authenticated users to redeem cross-client codes. The OAuth token endpoint accepts an authorization code without verifying that the redeeming client matches the client for which the code was issued. Exploitation requires an authenticated OAuth client, a crafted token exchange, and user interaction; redemption exposes another client's authorized access within the token's scope.

CVSS severity3.1
Low
EPSS probability0.12%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

OAuth bypass in Mattermost 10.11.0 through 10.11.13 and 11.5.0 through 11.5.1 allows remote authenticated users to redeem cross-client codes. The OAuth token endpoint accepts an authorization code without verifying that the redeeming client matches the client for which the code was issued. Exploitation requires an authenticated OAuth client, a crafted token exchange, and user interaction; redemption exposes another client's authorized access within the token's scope.

The record

CVE
CVE-2026-6334
Published
May 18, 2026
Updated
May 18, 2026
Vendor
Mattermost
Product
Mattermost
Classifications
CWE-305, T1528
Attack vector
network
Privileges
authenticated

Timeline

How it unfolded

  1. May 18, 2026CVE publishedPublication date reported by the CVE source.
  2. May 18, 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?

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  1. Affected versionversion=10.11.0 <=10.11.13
  2. Affected versionversion=11.5.0 <=11.5.1

What conditions does exploitation require?

Attack vectornetwork
Required privilegesauthenticated

What is affected?

Mattermost · Mattermostversion=10.11.0 <=10.11.13; version=11.5.0 <=11.5.1

Published CVSS scores

3.8NIST NVDCVSS:3.1/AV:N/AC:L/PR:H/UI:N/S:U/C:L/I:L/A:N
3.1Mattermost, Inc.CVSS:3.1/AV:N/AC:H/PR:H/UI:R/S:U/C:L/I:L/A:N

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 9, 2026Sep 15, 2026
Latest reporting daySep 15, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-305Authentication Bypass by Primary Weakness
T1528Steal Application Access Token

Public exploit references

No public exploit references are available in this record.

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Technologies

Your stack

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Your stack

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