GHSA-fg35-5rf6-qg3g CVE-2026-27656 GO-2026-5360
Mattermost allows attackers to take over arbitrary user accounts via overly permissive substring matching flaw
- Severity
- 5.7 / 10
- Medium · CVSS 3.1
- Exploit chance (EPSS)
- 0.18%
- Chance of exploitation in the next 30 days. Higher than 7.89% of scored vulnerabilities.
- Known exploited
- No
- Not in CISA's catalog
- Weakness
- CWE-303 Incorrect Implementation of Authentication Algorithm
- Fix
- Available
- 2 packages have a fixed version
Description
Mattermost versions 11.4.x <= 11.4.0, 11.3.x <= 11.3.1, 11.2.x <= 11.2.3, 10.11.x <= 10.11.11 fail to properly validate user identity in the OpenID {{IsSameUser()}} comparison logic, which allows an attacker to take over arbitrary user accounts via an overly permissive substring matching flaw in the user discovery flow. Mattermost Advisory ID: MMSA-2026-00590
Affected versions
Go
| Package | Vulnerable | Fixed in |
|---|---|---|
| github.com/mattermost/mattermost-server | >= 11.4.0-rc1, < 11.4.1 |
11.4.1 |
| github.com/mattermost/mattermost-server | >= 11.3.0-rc1, < 11.3.2 |
11.3.2 |
| github.com/mattermost/mattermost-server | >= 11.2.0-rc1, < 11.2.4 |
11.2.4 |
| github.com/mattermost/mattermost-server | >= 10.11.0-rc1, < 10.11.12 |
10.11.12 |
| github.com/mattermost/mattermost-server | >= 10.11.0-rc1+incompatible, < 10.11.12+incompatible>= 11.2.0-rc1+incompatible, < 11.2.4+incompatible>= 11.3.0-rc1+incompatible, < 11.3.2+incompatible>= 11.4.0-rc1+incompatible, < 11.4.1+incompatible |
10.11.12+incompatible11.2.4+incompatible11.3.2+incompatible11.4.1+incompatible |
| github.com/mattermost/mattermost-server/v5 | all versions |
No fix yet |
| github.com/mattermost/mattermost-server/v6 | all versions |
No fix yet |
| github.com/mattermost/mattermost/server/v8 | >= 8.0.0-20260105080200-d27a2195068d, < 8.0.0-20260217110922-b7d4a1f1f59b |
8.0.0-20260217110922-b7d4a1f1f59b |
How to fix
Move to the fixed version or later with your package manager, update the lockfile, and run your tests. If the package only arrives through another dependency, upgrade that dependency first. If none of its releases pulls a fixed version yet, force the version with your package manager's override.
github.com/mattermost/mattermost-server Go → 11.4.1
- go
go get github.com/mattermost/mattermost-server@v11.4.1 && go mod tidy
github.com/mattermost/mattermost/server/v8 Go → 8.0.0-20260217110922-b7d4a1f1f59b
- go
go get github.com/mattermost/mattermost/server/v8@v8.0.0-20260217110922-b7d4a1f1f59b && go mod tidy
Severity breakdown
CVSS 3.1 5.7
CVSS:3.1/AV:N/AC:H/PR:H/UI:R/S:U/C:H/I:H/A:N
- Attack vector
- Network
- Attack complexity
- High
- Privileges required
- High
- User interaction
- Required
- Scope
- Unchanged
- Confidentiality
- High
- Integrity
- High
- Availability
- None
Weakness
CWE-303: Incorrect Implementation of Authentication Algorithm
The requirements for the product dictate the use of an established authentication algorithm, but the implementation of the algorithm is incorrect.
References
Advisories
Packages
Other links
Timeline
- Published
- Last updated by the source
- Latest EPSS score
Related advisories
| Advisory | Severity | Why it's related |
|---|---|---|
| GHSA-xmfh-3ccg-c9fx Mattermost has an Incorrect Authorization issue | Low | Also affects github.com/mattermost/mattermost-server |
| GHSA-h998-hxxj-8q83 Mattermost doesn't invalidate cached authentication state for active WebSocket connections during global session revocation | Medium | Also affects github.com/mattermost/mattermost-server |
| GHSA-g5vr-6pgg-74qv Mattermost doesn't enforce bot-specific permission checks on the user active status endpoint | Low | Also affects github.com/mattermost/mattermost-server |
| GHSA-jqgv-39mg-7c2r Mattermost doesn't authenticate Atlassian Connect installed callbacks, allowing a remote unauthenticated attacker to inject a rogue sharedSecret | Medium | Also affects github.com/mattermost/mattermost-server |
| GHSA-mxq2-5jpg-7474 Mattermost doesn't validate channel ownership of an existing subscription before applying edits | Medium | Also affects github.com/mattermost/mattermost-server |
| GHSA-2g8v-grq3-hq2g Mattermost doesn't enforce administrator authorization on the {{setDefaultInstance}} call within the {{/gitlab connect}} command handler | Medium | Also affects github.com/mattermost/mattermost-server |
| GHSA-6hxm-w4hv-vgvw Mattermost doesn't require role-management authorization when setting the scheme_admin flag on group syncable link and patch endpoints | High | Also affects github.com/mattermost/mattermost-server |
| GHSA-9p44-r552-4wp9 Mattermost doesn't sanitize the Remote Cluster API response on PATCH operations | Medium | Also affects github.com/mattermost/mattermost-server |
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