CVE-2025-20621Network attack vector

Authenticated webapp crash via unvalidated attachment fields

Published Jan 16, 2025 · Updated Jan 16, 2025

Improper input-type validation in affected Mattermost server releases allows remote authenticated users to crash the webapp with a crafted channel post. The attachment-rendering path attempts to cast post attachment field values to String without validating their types. An authenticated channel member can send the malformed post; rendering it interrupts the webapp without altering server data.

CVSS severity6.5
Medium
EPSS probability0.46%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Improper input-type validation in affected Mattermost server releases allows remote authenticated users to crash the webapp with a crafted channel post. The attachment-rendering path attempts to cast post attachment field values to String without validating their types. An authenticated channel member can send the malformed post; rendering it interrupts the webapp without altering server data.

The record

CVE
CVE-2025-20621
Published
Jan 16, 2025
Updated
Jan 16, 2025
Vendor
Mattermost
Product
Mattermost
Classifications
CWE-1287, T1499.004
Attack vector
network
Privileges
authenticated

Timeline

How it unfolded

  1. Jan 16, 2025CVE publishedPublication date reported by the CVE source.
  2. Jan 16, 2025Record 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=10.0.0 <=10.0.3
  2. Affected versionversion=10.1.0 <=10.1.3
  3. Affected versionversion=10.2.0 <=10.2.0
  4. Affected versionversion=9.11.0 <=9.11.5

What conditions does exploitation require?

Attack vectornetwork
Required privilegesauthenticated

What is affected?

Mattermost · Mattermostversion=10.0.0 <=10.0.3; version=10.1.0 <=10.1.3; version=10.2.0 <=10.2.0; version=9.11.0 <=9.11.5

Published CVSS scores

6.5Mattermost, Inc.CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
7.5NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/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
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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