CVE-2026-75025

Executive Summary

CVE-2026-75025 is a server-side request forgery-adjacent origin validation weakness in the Mattermost Desktop App versions up to and including 6.2.2.0. The application does not sufficiently restrict server-rendered content from accessing local or private network resources, allowing a threat actor to cause the desktop client to issue requests to internal or loopback endpoints. Successful exploitation enables a malicious or compromised Mattermost server, or content rendered within it, to perform limited reconnaissance of the victim's internal network. The CVSS base score is 4.7 (Medium), reflecting a network-vector, low-complexity flaw requiring user interaction, with scope change and limited confidentiality impact.

Technical Analysis

The vulnerability is classified as CWE-346, Origin Validation Error. In the Electron-based Mattermost Desktop client, the renderer process displays web content served by the configured Mattermost server. According to the NVD description, the client "did not sufficiently restrict server-rendered content from accessing local or private network resources." This means the origin boundary between untrusted server-supplied content and local/private network destinations was not enforced correctly, permitting server-rendered content to initiate requests toward loopback or RFC1918 address space.

The underlying class of issue is an origin validation failure: the application trusts content from the connected server and fails to apply network segmentation controls (such as request filtering, protocol restrictions, or origin allow-listing) to outbound requests originating from that content. When origin validation is insufficient, an attacker who controls or compromises the served content can direct the client to interact with resources it should not reach — for example, services bound to localhost or internal hosts reachable only from the user's machine.

The attack surface is the desktop client renderer component that renders server-provided HTML/JavaScript or embedded content. The vulnerability does not require authentication against the Mattermost server (PR:N in the vector) but does require user interaction (UI:R), because the victim must have the desktop client running and loading/rendering the malicious or attacker-influenced content.

Key CVSS characteristics:

• AV:N — the malicious content is delivered over the network from the server.
• AC:L — no special conditions are required beyond the normal client-server interaction.
• PR:N — no privileges on the server are required by the attacker in the modeled scenario.
• UI:R — a user must interact with (render/open) the crafted content in the desktop app.
• S:C — the vulnerable component (desktop renderer) impacts resources beyond its security scope (local/private network resources), hence scope change.
• C:L / I:N / A:N — impact is limited to information disclosure; no integrity or availability impact is modeled.

No specific function names, API endpoints, class names, or file paths are provided in the NVD description, so implementation-level detail beyond the above cannot be asserted. The CWE-346 classification and the description of restricted access to "local or private network resources" are the authoritative technical anchors.

Exploitation Scenarios

The attack chain is inferred from the vulnerability class (CWE-346 origin validation error enabling access to local/private resources). No public PoC, exploit code, or Nuclei template exists for this CVE at the time of writing.

1. Reconnaissance / target identification. An attacker identifies victims running the Mattermost Desktop App at a vulnerable version (<=6.2.2.0). Because the client connects to a Mattermost server, the realistic vector is an attacker who controls, compromises, or can inject content into a Mattermost server instance that the victim's desktop client connects to. This could be a malicious self-hosted server, a server subject to content injection, or another scenario in which server-rendered content is influenced by the adversary. The attacker needs the victim to use the desktop client against that server.

2. Triggering the flaw. The adversary supplies server-rendered content that causes the desktop client to issue requests to local or private network resources. Because the client does not sufficiently restrict such access, the origin validation failure permits these requests to be made from the renderer context. The user must interact with the content (UI:R) for the requests to occur — for example by loading the relevant view or channel content.

3. Execution method at the vulnerability-class level. The exploit is a client-side request forgery: crafted content causes the desktop client to emit HTTP(S) requests toward loopback or internal network addresses that should be outside the content's origin. At this abstraction level, no specific endpoint, header, or parameter names can be given, because none are present in the source description. The request generation is the payload; no code execution is involved.

4. Post-exploitation impact. The attacker can use the client as a proxy for limited reconnaissance of the victim's local and private network. Responses or side channels may reveal the existence and possibly characteristics of internal services bound to localhost or reachable from the victim host. The CVSS confidentiality impact is Low (C:L); there is no integrity or availability impact. This limits the effect to information disclosure and internal network mapping rather than data modification or denial of service.

Practical constraints: exploitation hinges on attacker influence over server-rendered content and on victim interaction; the scope is bounded by what the client can reach from the victim host. The absence of public exploitation artifacts means defenders are unlikely to observe widespread in-the-wild activity for this specific CVE.

Impact Assessment

Business impact is moderate and primarily confidentiality-focused. Affected systems are endpoints running the Mattermost Desktop App at versions through 6.2.2.0. The blast radius is the individual user's host and the local/private networks that host can reach: any internal services bound to loopback or RFC1918 ranges that respond to requests from the client may be discoverable.

Mapping to CVSS subscores:

• Scope change (S:C) means the weakness in the desktop renderer affects resources (local/private network resources) outside the component's normal security authority — this raises the effective severity relative to a same-scope confidentiality-only issue.
• Limited confidentiality impact (C:L) means only some information is exposed; there is no proven full compromise of internal services.
• No integrity (I:N) or availability (A:N) impact is modeled; the flaw does not enable modification of internal systems or disruption of service.
• User interaction (UI:R) reduces exploitability but does not eliminate it, since normal use of the client involves rendering server content.

Aggregate risk: Medium. The combination of network attack vector, low complexity, and scope change is offset by the need for user interaction and the restriction to low confidentiality impact. Enterprises with self-hosted Mattermost servers where content serving could be influenced, or where desktop users are highly targeted, should treat this as a priority endpoint-hardening item. Because no EPSS score is available and there is no CISA KEV listing, there is currently no evidence of active exploitation.

Detection Guidance

Sigma Rule

title: Potential Mattermost Desktop Origin Validation Abuse (CVE-2026-75025)
id: 8f3a2c1e-6b4d-4f9a-9c2e-7d1b5a0e3c11
status: experimental
description: Detects Mattermost Desktop client process reaching loopback or private network destinations, consistent with CWE-346 origin validation abuse.
references:
  - https://nvd.nist.gov/vuln/detail/CVE-2026-75025
author: valtersit.com
date: 2026/01/01
tags:
  - attack.t1190
  - cve.2026-75025
logsource:
  category: network_connection
  product: windows
detection:
  selection_proc:
    Image|endswith:
      - '\Mattermost.exe'
  selection_dst:
    DestinationIp|cidr:
      - '127.0.0.0/8'
      - '10.0.0.0/8'
      - '172.16.0.0/12'
      - '192.168.0.0/16'
  condition: selection_proc and selection_dst
falsepositives:
  - Legitimate Mattermost client communication with self-hosted servers on private networks
level: low

YARA Rule

rule CVE_2026_75025_Mattermost_Desktop_Origin_Validation
{
    meta:
        description = "Identifies Mattermost Desktop App builds in the vulnerable version range and local/private network request artifacts"
        author = "valtersit.com"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2026-75025"
        cve = "CVE-2026-75025"
        date = "2026-01-01"
    strings:
        $prod = "Mattermost Desktop" ascii wide nocase
        $vuln = "6.2.2.0" ascii wide
        $lo = "127.0.0.1" ascii wide
        $priv = "192.168." ascii wide
    condition:
        uint16(0) == 0x5A4D and $prod and $vuln and any of ($lo, $priv)
}

Suricata IDS Rule

alert http any any -> $HOME_NET any (msg:"CVE-2026-75025 Mattermost Desktop App access to private-range resource"; flow:established,to_server; content:"Mattermost"; http_header; pcre:"/Host\x3a\s(?:127\.|10\.|192\.168\.|172\.(?:1[6-9]|2[0-9]|3[01])\.)/Hi"; reference:cve,2026-75025; reference:url,nvd.nist.gov/vuln/detail/CVE-2026-75025; classtype:attempted-recon; sid:202675025; rev:1;)

Elastic Detection

network where process.name : "Mattermost.exe" and
  destination.ip : ("127.0.0.0/8" or "10.0.0.0/8" or "172.16.0.0/12" or "192.168.0.0/16")

Splunk SPL Query

index=network OR index=proxy
| search process_name="Mattermost*" OR user_agent="*Mattermost*"
| regex dest_ip="^(127\.|10\.|192\.168\.|172\.(1[6-9]|2[0-9]|3[01])\.)"
| stats count min(_time) as first_seen max(_time) as last_seen values(dest_ip) as dest_ips by src_ip, process_name
| where count > 0

Wazuh OSSEC Rule

<group name="mattermost,cve-2026-75025,">
  <rule id="2026750" level="4">
    <decoded_as>json</decoded_as>
    <field name="process.name">Mattermost.exe</field>
    <regex field="destination.ip">^(127\.|10\.|192\.168\.|172\.(1[6-9]|2[0-9]|3[01])\.)</regex>
    <description>CVE-2026-75025: Mattermost Desktop client accessed a local/private network resource (possible origin validation abuse).</description>
    <mitre>
      <id>T1190</id>
    </mitre>
    <group>cve_2026_75025,</group>
  </rule>
</group>

Mitigation & Patch Guidance

Patch confidence: 0/4 (UNPATCHED — no patch evidence from any tracked source). No OSV packages, no affected-version list, no vendor fix, and no workaround are recorded. The only vendor-provided artifact is advisory MMSA-2026-00698. The description states versions "<=6.2.2.0" are affected, implying a fixed release exists above 6.2.2.0, but no patch reference is available in the tracked data.

1. Immediate mitigations:

• Update the Mattermost Desktop App to a release above 6.2.2.0 as soon as a fixed version is published and validated; monitor the Mattermost security advisories page for MMSA-2026-00698.
• Restrict which Mattermost servers desktop clients are permitted to connect to; treat connecting to untrusted or third-party Mattermost servers as high risk until a fix is applied.
• Apply host-based egress filtering so the desktop client cannot reach loopback or unexpected private-range destinations, and segment endpoint networks to limit reachable internal services.
• Ensure internal services are not exposed on loopback interfaces reachable by desktop applications, and require authentication/authorization on all internal services.

2. Patch/upgrade steps:

• Track https://mattermost.com/security-updates/ and the advisory ID MMSA-2026-00698 for the fixed version.
• When available, deploy the fixed Mattermost Desktop App through your software distribution tooling and verify the installed version exceeds 6.2.2.0.
• Re-verify after upgrade that the client no longer generates requests to local/private network resources when rendering server content.

3. Long-term hardening:

• Enforce strict origin and network access controls in desktop clients through application-level policy where supported.
• Adopt allow-listing for desktop client server connections and require corporate-managed server configurations.
• Continuously monitor desktop client network telemetry for outbound connections to loopback/RFC1918 ranges per the detection guidance above.
• Track CWE-346 origin validation issues in your client software inventory as a recurring risk class.

References

• NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-75025
• Mattermost Advisory MMSA-2026-00698 (referenced in the NVD description): https://mattermost.com/security-updates/
• CWE-346 Origin Validation Error: https://cwe.mitre.org/data/definitions/346.html
• CISA KEV catalog (CVE-2026-75025 not listed): https://www.cisa.gov/known-exploited-vulnerabilities-catalog
• FIRST EPSS (CVE-2026-75025 not yet scored): https://www.first.org/epss/

Threat actor / exploit-in-the-wild evidence: none available. No PoC, exploit code, or Nuclei template is associated with this CVE, and it is not in the CISA KEV catalog. No attribution or campaign data is present in the source material.