How severe is CVE-2026-3495?

CVE-2026-3495 has a CVSS score of 3.8/10, rated LOW. Review your exposure and patch according to your risk tolerance.

Which packages are affected by CVE-2026-3495?

CVE-2026-3495 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 (Go). Ecosystems affected: Go.

How do I fix CVE-2026-3495?

Update github.com/mattermost/mattermost/server/v8 to 10.11.14 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-3495 is resolved across your whole dependency graph.

How do I detect CVE-2026-3495 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-2026-3495 if there is no patch (or I can't update yet)?

Escape or sanitise the affected output on the server side rather than relying on client-side filtering, and add a Content-Security-Policy that blocks inline script execution so injected markup cannot run even if it reaches the page.

Is CVE-2026-3495 actively exploited in the wild?

No public exploit code has been indexed for CVE-2026-3495 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-2026-3495?

CVE-2026-3495 has an EPSS (Exploit Prediction Scoring System) score of 0.2%, placing it in the 14th 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-2026-3495?

CVE-2026-3495 is classified as Cross-site Scripting (XSS) (CWE-79). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation. This is a high-impact weakness class that often enables remote code execution or data exposure.

When was CVE-2026-3495 published, and has it been updated?

CVE-2026-3495 was published on May 18, 2026 and was last updated on August 12, 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

LOW severity

CVE-2026-3495 — v8

LOWFix: mattermost/mattermost@5a1ea95

CVE-2026-3495 is a low-severity (CVSS 3.8) Cross-site Scripting (XSS) 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.

Unescaped variables during error page composition

Also known asGHSA-jx93-pf6x-874rGO-2026-5484

Published

May 18, 2026

Updated

Aug 12, 2026

Affected

4 pkgs

Patched

4 / 4

Exploits

None indexed

Exploitation data as of Sep 29, 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-2026-3495.

EPSS Exploitation Probability

via FIRST.org ↗

0.2%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs14th percentile — riskier than 14% of all scored CVEsHighest risk

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

How urgent is this, really

CVE-2026-3495 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.

Where this sits among everything scored

Of 380,526 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Counts from FIRST.org, log-scaled.

Real-World Exposure

4 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

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 11.5.x <= 11.5.1, 10.11.x <= 10.11.13 fail to escape some variables that could contain malicious content during error page composition which allows an attacker with access to edit some site configuration to execute some malicious code via injecting some JS as part of those values.. Mattermost Advisory ID: MMSA-2026-00622

Affected Packages

4 total 4 fixed

Ecosystem	Package	Vulnerable range	Fix
🐹Go	`github.com/mattermost/mattermost/server/v8`	≥ 10.11.0&&< 10.11.14	10.11.14`go get github.com/mattermost/mattermost/server/v8@v10.11.14`
🐹Go	`github.com/mattermost/mattermost/server/v8`	≥ 11.5.0&&< 11.5.2	11.5.2`go get github.com/mattermost/mattermost/server/v8@v11.5.2`
🐹Go	`github.com/mattermost/mattermost/server/v8`	all versions	8.0.0-20260310115442-5a1ea95044d`go get github.com/mattermost/mattermost/server/v8@v8.0.0-20260310115442-5a1ea95044d`
🐹Go	`github.com/mattermost/mattermost-server`	all versions	5.3.2-0.20260310115442-5a1ea95044dc`go get github.com/mattermost/mattermost-server@v5.3.2-0.20260310115442-5a1ea95044dc`

Affected Products

1 product · 2 configurations

Application

mattermost servermattermost

≥ 11.5.0 && < 11.5.2

range

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 10.11.14 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-3495 is resolved across your whole dependency graph.
Workarounds
Escape or sanitise the affected output on the server side rather than relying on client-side filtering, and add a Content-Security-Policy that blocks inline script execution so injected markup cannot run even if it reaches the page.

Frequently Asked Questions

O3 Security · Impact-Aware SCA

Is CVE-2026-3495 in your dependencies?

Find it across Go, including transitive dependencies.

Scan my dependencies How O3 SCA works

CVSS Score

3.8/10CVSS v3 base

Vector breakdown

Attack Vector

Network

Attack Complexity

Low

Privileges Required

High

User Interaction

None

Scope

Unchanged

Confidentiality

Low

Integrity

Low

Availability

None

Raw vector

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

CWE Classification

CWE-79Cross-site Scripting (XSS)

Weaponization

No known public exploit tooling found.

OpenSSF Scorecard

Package health score

mattermost

7.3/10

Maintained10/10

Code-Review10/10

Dependency-Update-Tool10/10

Vulnerabilities0/10

Branch-Protection4/10

Assessed Sep 29, 2026

Timeline

Disclosed

May 18, 2026

Last updated

Aug 12, 2026

Identifiers

CVE-2026-3495

GHSA-jx93-pf6x-874r

GO-2026-5484

References

Advisorygithub.com Advisorymattermost.com Advisorynvd.nist.gov Webgithub.com Packagegithub.com

Related vulnerabilities

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

CVE-2026-6046CVE-2026-7387CVE-2026-6961CVE-2026-7184CVE-2026-6689CVE-2026-6739CVE-2026-3433CVE-2026-4915

Provenance

Sources

OSV.devNVDFIRST.org (EPSS)

Last verifiedAug 12, 2026