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Mattermost fails to canonicalize IPv4-mapped IPv6 addresses before reserved IP validation

Moderate severity GitHub Reviewed Published Mar 16, 2026 to the GitHub Advisory Database • Updated Mar 18, 2026

Package

gomod github.com/mattermost/mattermost-server (Go)

Affected versions

< 5.3.2-0.20260129133647-5d787969c2d5
>= 10.11.0-rc1, < 10.11.11
>= 11.2.0-rc1, < 11.2.3
>= 11.3.0-rc1, < 11.3.1

Patched versions

5.3.2-0.20260129133647-5d787969c2d5
10.11.11
11.2.3
11.3.1
gomod github.com/mattermost/mattermost/server/v8 (Go)
< 8.0.0-20260129133647-5d787969c2d5
8.0.0-20260129133647-5d787969c2d5

Description

Mattermost versions 11.3.x <= 11.3.0, 11.2.x <= 11.2.2, 10.11.x <= 10.11.10 fail to canonicalize IPv4-mapped IPv6 addresses before reserved IP validation which allows an attacker to perform SSRF attacks against internal services via IPv4-mapped IPv6 literals (e.g., [::ffff:127.0.0.1]).. Mattermost Advisory ID: MMSA-2026-00585

References

Published by the National Vulnerability Database Mar 16, 2026
Published to the GitHub Advisory Database Mar 16, 2026
Reviewed Mar 18, 2026
Last updated Mar 18, 2026

Severity

Moderate

CVSS overall score

This score calculates overall vulnerability severity from 0 to 10 and is based on the Common Vulnerability Scoring System (CVSS).
/ 10

CVSS v3 base metrics

Attack vector
Network
Attack complexity
Low
Privileges required
Low
User interaction
None
Scope
Unchanged
Confidentiality
Low
Integrity
None
Availability
None

CVSS v3 base metrics

Attack vector: More severe the more the remote (logically and physically) an attacker can be in order to exploit the vulnerability.
Attack complexity: More severe for the least complex attacks.
Privileges required: More severe if no privileges are required.
User interaction: More severe when no user interaction is required.
Scope: More severe when a scope change occurs, e.g. one vulnerable component impacts resources in components beyond its security scope.
Confidentiality: More severe when loss of data confidentiality is highest, measuring the level of data access available to an unauthorized user.
Integrity: More severe when loss of data integrity is the highest, measuring the consequence of data modification possible by an unauthorized user.
Availability: More severe when the loss of impacted component availability is highest.
CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:N/A:N

EPSS score

Exploit Prediction Scoring System (EPSS)

This score estimates the probability of this vulnerability being exploited within the next 30 days. Data provided by FIRST.
(9th percentile)

Weaknesses

Server-Side Request Forgery (SSRF)

The web server receives a URL or similar request from an upstream component and retrieves the contents of this URL, but it does not sufficiently ensure that the request is being sent to the expected destination. Learn more on MITRE.

CVE ID

CVE-2026-2455

GHSA ID

GHSA-gqv7-j2j8-qmwq

Source code

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