CVE-2026-2455: Mattermost Server SSRF Vulnerability

CVE-2026-2455 Overview

CVE-2026-2455 is a Server-Side Request Forgery (SSRF) vulnerability in Mattermost Server that allows authenticated attackers to bypass reserved IP address validation by using IPv4-mapped IPv6 address literals. The vulnerability exists because Mattermost fails to properly canonicalize IPv4-mapped IPv6 addresses (such as [::ffff:127.0.0.1]) before performing reserved IP validation checks, enabling attackers to access internal services that should be protected.

Critical Impact

Attackers can bypass network segmentation controls to access internal services, potentially exposing sensitive configuration data, internal APIs, and backend systems not intended to be publicly accessible.

Affected Products

• Mattermost Server versions 11.3.x up to and including 11.3.0
• Mattermost Server versions 11.2.x up to and including 11.2.2
• Mattermost Server versions 10.11.x up to and including 10.11.10

Discovery Timeline

• 2026-03-16 - CVE-2026-2455 published to NVD
• 2026-03-18 - Last updated in NVD database

Technical Details for CVE-2026-2455

Vulnerability Analysis

This vulnerability falls under CWE-918 (Server-Side Request Forgery), a class of web application flaws that allow attackers to induce the server-side application to make HTTP requests to arbitrary destinations. In this case, the SSRF bypass is achieved through an IP address parsing inconsistency.

Mattermost implements reserved IP address validation to prevent SSRF attacks targeting internal network resources such as 127.0.0.1, 10.0.0.0/8, 192.168.0.0/16, and other RFC 1918 private address ranges. However, the validation logic fails to account for IPv4-mapped IPv6 addresses, which are a legitimate IPv6 representation format defined in RFC 4291.

When an attacker supplies an IPv4-mapped IPv6 address like [::ffff:127.0.0.1] instead of the direct IPv4 equivalent 127.0.0.1, the reserved IP validation check is bypassed because the address format doesn't match the expected IPv4 patterns. However, when the actual HTTP request is made, the networking stack properly resolves the IPv4-mapped IPv6 address to its IPv4 equivalent, successfully connecting to the internal resource.

Root Cause

The root cause is insufficient input canonicalization in the IP address validation routine. The validation logic checks for reserved IPv4 address patterns but does not first normalize IPv4-mapped IPv6 addresses to their IPv4 equivalents. This creates a discrepancy between the validation phase and the request execution phase, where different representations of the same address are handled inconsistently.

Attack Vector

The attack is network-based and requires low-privilege authenticated access to the Mattermost platform. An attacker can exploit this vulnerability by crafting requests that include IPv4-mapped IPv6 address literals in URL parameters or webhook configurations. The attack does not require user interaction and can be executed directly through the Mattermost API or user interface where external URL inputs are accepted.

For example, instead of attempting to access http://127.0.0.1:8080/admin (which would be blocked), an attacker would use http://[::ffff:127.0.0.1]:8080/admin to reach the same internal endpoint while bypassing the reserved IP validation.

Detection Methods for CVE-2026-2455

Indicators of Compromise

• Outbound HTTP requests from Mattermost Server to IPv4-mapped IPv6 addresses in server logs
• Unusual access patterns to internal services originating from the Mattermost application
• Webhook or integration configurations containing IPv6 literals with the ::ffff: prefix
• Error logs indicating connections to internal IP addresses via IPv6 format

Detection Strategies

• Implement network-level monitoring for outbound connections from Mattermost servers to internal IP ranges
• Configure Web Application Firewall (WAF) rules to detect and block IPv4-mapped IPv6 address patterns in user-supplied input
• Enable verbose logging for Mattermost webhook and integration features to capture URL destinations
• Deploy network segmentation alerts for unexpected traffic between DMZ and internal network zones

Monitoring Recommendations

• Review Mattermost application logs for outbound requests containing ::ffff: patterns
• Monitor internal service access logs for requests originating from Mattermost server IPs
• Establish baseline network behavior and alert on deviations in outbound connection patterns
• Audit existing webhook and integration configurations for suspicious URL patterns

How to Mitigate CVE-2026-2455

Immediate Actions Required

• Upgrade Mattermost Server to the latest patched version immediately
• Audit all existing webhook and integration configurations for IPv6 address literals
• Implement network-level egress filtering to block SSRF attempts
• Review access logs for evidence of exploitation attempts

Patch Information

Mattermost has released security updates to address this vulnerability. Administrators should upgrade to the following minimum versions:

• Version 11.3.1 or later for the 11.3.x branch
• Version 11.2.3 or later for the 11.2.x branch
• Version 10.11.11 or later for the 10.11.x branch

For complete patch details and download links, refer to the Mattermost Security Updates page. The advisory tracking ID is MMSA-2026-00585.

Workarounds

• Configure network-level egress filtering to restrict Mattermost server outbound connections to only required external destinations
• Implement WAF rules to block requests containing IPv4-mapped IPv6 address patterns (::ffff:)
• Disable or restrict webhook and integration features to trusted administrators only until patching is complete
• Deploy network segmentation to limit potential impact if SSRF exploitation occurs

# Example: iptables rule to block outbound connections to localhost via IPv6-mapped addresses
# Apply on the Mattermost server host
ip6tables -A OUTPUT -d ::ffff:127.0.0.0/104 -j DROP
ip6tables -A OUTPUT -d ::ffff:10.0.0.0/104 -j DROP
ip6tables -A OUTPUT -d ::ffff:192.168.0.0/112 -j DROP
ip6tables -A OUTPUT -d ::ffff:172.16.0.0/108 -j DROP