CVE-2017-18909 Mattermost Server SAML implementation does not require encryption or signature verification as default

What is CVE-2017-18909?

CVE-2017-18909 is a high-severity vulnerability in Mattermost Mattermost Server, published on 19 Jun 2020. An attacker can exploit it over the internet without logging in, with no action from the victim.

Official description

An issue was discovered in Mattermost Server before 3.9.0 when SAML is used. Encryption and signature verification are not mandatory.

Weakness: CWE-295

Which versions are affected by CVE-2017-18909?

3 open source packages ship an affected version, including github.com/mattermost/mattermost-server. It affects 1 product.

Products include Mattermost Mattermost Server.

How do I fix CVE-2017-18909?

Upgrade github.com/mattermost/mattermost-server to 3.8.1-0.20170504181128-4f074fed0d65 or later.

Is CVE-2017-18909 being exploited?

There is no known exploitation of CVE-2017-18909 in the wild and no public exploit code.

Other vulnerabilities in Mattermost Mattermost Server

• CVE-2025-12419 Account takeover on OAuth/OpenID-enabled servers Critical 9.9
• CVE-2025-12421 Account Takeover via Code Exchange Endpoint Critical 9.9
• CVE-2025-4981 Path Traversal Leading to RCE by Any Authenticated Mattermost User Critical 9.9
• CVE-2025-20051 Arbitrary file read via block duplication in Mattermost Boards Critical 9.9
• CVE-2025-25279 Arbitrary file read in Mattermost Boards via import & export board archive Critical 9.9

All Mattermost Mattermost Server vulnerabilities

Sources

• CVE record
• NVD
• OSV
• GitHub advisory
• How this page is made

Also known as GHSA-r6j5-fqx9-7qv9, GO-2026-4478.