How severe is CVE-2025-20621?

CVE-2025-20621 has a CVSS score of 6.5/10, rated MEDIUM. Review your exposure and patch according to your risk tolerance.

Which packages are affected by CVE-2025-20621?

CVE-2025-20621 affects the following packages: github.com/mattermost/mattermost/server/v8 (Go), github.com/mattermost/mattermost/server/v8 (Go), github.com/mattermost/mattermost/server/v8 (Go), github.com/mattermost/mattermost/server/v8 (Go), github.com/mattermost/mattermost/server/v8 (Go). Ecosystems affected: Go.

How do I fix CVE-2025-20621?

Update github.com/mattermost/mattermost/server/v8 to 10.2.1 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2025-20621 is resolved across your whole dependency graph.

How do I detect CVE-2025-20621 in my Go dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for github.com/mattermost/mattermost/server/v8, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

How do I mitigate CVE-2025-20621 if there is no patch (or I can't update yet)?

If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

Is CVE-2025-20621 actively exploited in the wild?

No public exploit code has been indexed for CVE-2025-20621 yet. This does not mean the vulnerability cannot be exploited — absence of public exploits does not imply safety. Apply the recommended fix and use O3 Security to monitor your exposure.

What is the EPSS score for CVE-2025-20621?

CVE-2025-20621 has an EPSS (Exploit Prediction Scoring System) score of 0.5%, placing it in the 41th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. This score indicates relatively lower exploitation probability, though the CVSS severity should still guide your patching priority.

What type of vulnerability is CVE-2025-20621?

CVE-2025-20621 is classified as CWE-1287 (CWE-1287). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

When was CVE-2025-20621 published, and has it been updated?

CVE-2025-20621 was published on January 16, 2025 and was last updated on October 28, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

🐹

🐹 Go

Not in CISA KEV

MEDIUM severity

CVE-2025-20621 — server

MEDIUM

CVE-2025-20621 is a medium-severity (CVSS 6.5) CWE-1287 vulnerability in github.com/mattermost/mattermost/server/v8. A fix is available for github.com/mattermost/mattermost/server/v8 — see the affected versions and patch details below.

Webapp crash via object that can't be cast to String in Attachment Field

Also known asGHSA-w6xh-c82w-h997GO-2025-3407

Published

Jan 16, 2025

Updated

Oct 28, 2026

Affected

5 pkgs

Patched

5 / 5

Exploits

None indexed

Exploitation data as of Nov 3, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

Exploitation Status

No confirmed exploitation observed yet

CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.

Exploitation and automatability from CISA’s SSVC triage for CVE-2025-20621.

EPSS Exploitation Probability

via FIRST.org ↗

0.5%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs41th percentile — riskier than 41% of all scored CVEsHighest risk

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

How urgent is this, really

Where this sits among everything scored

Real-World Exposure

5 pkgs affected

🐹github.com/mattermost/mattermost/server/v8🐹github.com/mattermost/mattermost/server/v8🐹github.com/mattermost/mattermost/server/v8🐹github.com/mattermost/mattermost/server/v8🐹github.com/mattermost/mattermost/server/v8

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects Go packages — download data is not available via public APIs for these ecosystems.

Description

Mattermost versions 10.2.x <= 10.2.0, 9.11.x <= 9.11.5, 10.0.x <= 10.0.3, 10.1.x <= 10.1.3 fail to properly handle posts with attachments containing fields that cannot be cast to a String, which allows an attacker to cause the webapp to crash via creating and sending such a post to a channel.

Affected Packages

5 total 5 fixed

Ecosystem	Package	Vulnerable range	Fix
🐹Go	`github.com/mattermost/mattermost/server/v8`	≥ 10.2.0&&< 10.2.1	10.2.1`go get github.com/mattermost/mattermost/server/v8@v10.2.1`
🐹Go	`github.com/mattermost/mattermost/server/v8`	≥ 10.1.0&&< 10.1.4	10.1.4`go get github.com/mattermost/mattermost/server/v8@v10.1.4`
🐹Go	`github.com/mattermost/mattermost/server/v8`	≥ 10.0.0&&< 10.0.4	10.0.4`go get github.com/mattermost/mattermost/server/v8@v10.0.4`
🐹Go	`github.com/mattermost/mattermost/server/v8`	≥ 9.11.0&&< 9.11.6	9.11.6`go get github.com/mattermost/mattermost/server/v8@v9.11.6`
🐹Go	`github.com/mattermost/mattermost/server/v8`	all versions	8.0.0-20241127161322-25ff7a3779a5`go get github.com/mattermost/mattermost/server/v8@v8.0.0-20241127161322-25ff7a3779a5`

Affected Products

1 product · 4 configurations

Application

mattermost servermattermost

≥ 10.1.0 && < 10.1.4

1 version

10.2.0

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for github.com/mattermost/mattermost/server/v8, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update github.com/mattermost/mattermost/server/v8 to 10.2.1 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2025-20621 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

Frequently Asked Questions

O3 Security · Impact-Aware SCA

Is CVE-2025-20621 in your dependencies?

Find it across Go, including transitive dependencies.

Scan my dependencies How O3 SCA works

CVSS Score

6.5/10CVSS v3 base

Vector breakdown

Attack Vector

Network

Attack Complexity

Low

Privileges Required

Low

User Interaction

None

Scope

Unchanged

Confidentiality

None

Integrity

None

Availability

High

Raw vector

CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H

CWE Classification

CWE-1287CWE-1287

Weaponization

No Nuclei template, Metasploit module or public proof-of-concept found.

Timeline

Disclosed

Jan 16, 2025

Last updated

Oct 28, 2026

Identifiers

CVE-2025-20621

GHSA-w6xh-c82w-h997

GO-2025-3407

References

Advisorygithub.com · CVE 2025 20621.json Advisorynvd.nist.gov · CVE 2025 20621 Webmattermost.com · security updates Packagegithub.com · mattermost

Related vulnerabilities

This CVE is medium severity; 7 of 8 other CVEs affecting v8 are rated as high or higher (top: high).

CVE-2026-8823CVE-2026-6046CVE-2026-7387CVE-2026-6961CVE-2026-7184CVE-2026-6689CVE-2026-6739CVE-2026-3433

Provenance

Sources

OSV.devNVDFIRST.org (EPSS)

Last verifiedOct 28, 2026

CVE-2025-20621 — server

Exploitation Status

No confirmed exploitation observed yet

EPSS Exploitation Probability

How urgent is this, really

Where this sits among everything scored

Real-World Exposure

Description

Affected Packages

Affected Products

Detection & mitigation playbook

Detect

Fix

Workarounds

Frequently Asked Questions

Is CVE-2025-20621 in your dependencies?