How severe is CVE-2017-18891?

CVE-2017-18891 has a CVSS score of 6.1/10, rated MEDIUM. Review your exposure and patch according to your risk tolerance.

Which packages are affected by CVE-2017-18891?

CVE-2017-18891 affects the following packages: github.com/mattermost/mattermost-server (Go), github.com/mattermost/mattermost-server (Go), github.com/mattermost/mattermost-server (Go). Ecosystems affected: Go.

How do I fix CVE-2017-18891?

Update github.com/mattermost/mattermost-server to 4.0.5 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2017-18891 is resolved across your whole dependency graph.

How do I detect CVE-2017-18891 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, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

How do I mitigate CVE-2017-18891 if there is no patch (or I can't update yet)?

Stop reflecting attacker-controlled destinations: resolve every redirect target against an allowlist of paths or hosts you own, prefer a server-side key over a full URL in the request, and reject absolute URLs entirely where the flow only ever needs a relative one.

Is CVE-2017-18891 actively exploited in the wild?

No public exploit code has been indexed for CVE-2017-18891 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-2017-18891?

CVE-2017-18891 has an EPSS (Exploit Prediction Scoring System) score of 0.7%, placing it in the 53th 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-2017-18891?

CVE-2017-18891 is classified as Open Redirect (CWE-601). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

When was CVE-2017-18891 published, and has it been updated?

CVE-2017-18891 was published on June 19, 2020 and was last updated on October 24, 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-2017-18891 — mattermost-server

MEDIUMFix: mattermost/mattermost#7378

CVE-2017-18891 is a medium-severity (CVSS 6.1) Open Redirect vulnerability in github.com/mattermost/mattermost-server. A fix is available for github.com/mattermost/mattermost-server — see the affected versions and patch details below.

Mattermost Server does not safeguard against phishing via error page links

Also known asGHSA-vrh2-rprg-rgc6GO-2026-4298

Published

Jun 19, 2020

Updated

Oct 24, 2026

Affected

3 pkgs

Patched

3 / 3

Exploits

None indexed

Exploitation data as of Oct 27, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

EPSS Exploitation Probability

via FIRST.org ↗

0.7%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs53th percentile — riskier than 53% 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

3 pkgs affected

🐹github.com/mattermost/mattermost-server🐹github.com/mattermost/mattermost-server🐹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

An issue was discovered in Mattermost Server before 4.2.0, 4.1.1, and 4.0.5. It allows Phishing because an error page can have a link.

Affected Packages

3 total 3 fixed

Ecosystem	Package	Vulnerable range	Fix
🐹Go	`github.com/mattermost/mattermost-server`	all versions	4.0.5`go get github.com/mattermost/mattermost-server@v4.0.5`
🐹Go	`github.com/mattermost/mattermost-server`	≥ 4.1.0&&< 4.1.1	4.1.1`go get github.com/mattermost/mattermost-server@v4.1.1`
🐹Go	`github.com/mattermost/mattermost-server`	≥ 4.2.0-rc1&&< 4.2.0	4.2.0`go get github.com/mattermost/mattermost-server@v4.2.0`

Affected Products

1 product · 6 configurations

Application

mattermost servermattermost

≥ 4.1.0 && < 4.1.1

1 version

4.2.0

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, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update github.com/mattermost/mattermost-server to 4.0.5 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2017-18891 is resolved across your whole dependency graph.
Workarounds
Stop reflecting attacker-controlled destinations: resolve every redirect target against an allowlist of paths or hosts you own, prefer a server-side key over a full URL in the request, and reject absolute URLs entirely where the flow only ever needs a relative one.

Frequently Asked Questions

An issue was discovered in Mattermost Server before 4.2.0, 4.1.1, and 4.0.5. It allows Phishing because an error page can have a link.

O3 Security · Impact-Aware SCA

Is CVE-2017-18891 in your dependencies?

Find it across Go, including transitive dependencies.

Scan my dependencies How O3 SCA works

CVSS Score

6.1/10CVSS v3 base

Vector breakdown

Attack Vector

Network

Attack Complexity

Low

Privileges Required

None

User Interaction

Required

Scope

Changed

Confidentiality

Low

Integrity

Low

Availability

None

Raw vector

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

CWE Classification

CWE-601Open Redirect

Weaponization

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

OpenSSF Scorecard

Package health score

mattermost

7.3/10

Dependency-Update-Tool10/10

Code-Review10/10

Maintained10/10

Branch-Protection4/10

Vulnerabilities0/10

Assessed Oct 27, 2026

Timeline

Disclosed

Jun 19, 2020

Last updated

Oct 24, 2026

Identifiers

CVE-2017-18891

GHSA-vrh2-rprg-rgc6

GO-2026-4298

References

Advisorymattermost.com · security updates Advisorynvd.nist.gov · CVE 2017 18891 Webgithub.com Webmattermost.com · security updates Packagegithub.com · mattermost

Related vulnerabilities

This CVE is medium severity; 6 of 8 other CVEs affecting mattermost-server are rated as high or higher (top: high).

CVE-2026-5139CVE-2026-8074CVE-2026-9162CVE-2026-6673CVE-2026-6062CVE-2026-8823CVE-2026-6046CVE-2026-7387

Provenance

Sources

OSV.devNVDFIRST.org (EPSS)

Last verifiedOct 24, 2026