CVE-2021-37859Network attack vector

Browser script execution via OAuth bypass

Published Aug 5, 2021 · Updated Aug 4, 2024

Reflected XSS in Mattermost 5.32.0 through 5.36.0 allows remote attackers to execute script in a user's browser through the OAuth flow. Mattermost describes the issue only as a bypass affecting OAuth-enabled instances and has not published the vulnerable endpoint, parameter, or failed neutralization operation. Exploitation requires OAuth to be enabled and user interaction; successful script execution can read or alter data available to the victim's browser session.

CVSS severity7.1
High
EPSS probability3.29%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Reflected XSS in Mattermost 5.32.0 through 5.36.0 allows remote attackers to execute script in a user's browser through the OAuth flow. Mattermost describes the issue only as a bypass affecting OAuth-enabled instances and has not published the vulnerable endpoint, parameter, or failed neutralization operation. Exploitation requires OAuth to be enabled and user interaction; successful script execution can read or alter data available to the victim's browser session.

The record

CVE
CVE-2021-37859
Published
Aug 5, 2021
Updated
Aug 4, 2024
Vendor
Mattermost
Product
Mattermost
Classifications
CWE-79, T1203
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Aug 5, 2021CVE publishedPublication date reported by the CVE source.
  2. Aug 4, 2024Record 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=5.32.0 <=5.36.0

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Mattermost · Mattermostversion=5.32.0 <=5.36.0

Published CVSS scores

6.1NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N
4.3NIST NVDAV:N/AC:M/Au:N/C:N/I:P/A:N
7.1Mattermost, Inc.CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:H/I:H/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
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-79Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')
T1203Exploitation for Client Execution

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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