Unrated

Improper communication channel restriction in Synology MailPlus Server

IdentifiersCVE-2026-13135CWE-923

CVE-2026-13135 is a moderate-severity vulnerability in Synology MailPlus Server on DiskStation Manager (DSM) caused by improper restriction of a communication channel to intended endpoints. The flaw allows a remote attacker to reach or access internal services that should not be exposed through the affected MailPlus Server deployment. Available reporting identifies the issue as ZDI-CAN-28485 and maps it to CWE-923. The vulnerability affects MailPlus Server deployments on DSM 7.3, 7.2.2, and 7.2.1 prior to the fixed releases.

For your environment

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ANALYST BRIEF

Impact, mitigation & remediation

What it means. What to do now. Patch path, mitigations, and the assume-compromise checklist.

Impact

What an attacker gets, and what they’ve been doing with it.

Successful exploitation allows a remote unauthenticated attacker to access internal services through the vulnerable MailPlus Server instance. Based on the reported CVSS characteristics, the primary impact is unauthorized exposure of otherwise internal-only functionality or information, with limited confidentiality impact and no confirmed direct integrity or availability impact for this specific CVE.

Mitigation

If you can’t patch tonight, do this now.

No vendor workaround or mitigation has been reported beyond applying the security updates. Where operationally feasible, reduce exposure by restricting external access to MailPlus services and limiting network paths to internal services until patching is completed.

Remediation

Patch, then assume compromise.

Upgrade Synology MailPlus Server to a fixed version. For DSM 7.3, upgrade to MailPlus Server 4.0.1-31663 or later. For DSM 7.2.2 and DSM 7.2.1, upgrade to MailPlus Server 4.0.1-21663 or later.

PUBLIC EXPLOITS

Exploits

No public exploits tracked yet. Mallory keeps watching.

VALID 0 / 0 TOTALView more in app

No public exploit code observed for this vulnerability.

EXPOSURE SURFACE

Affected products & vendors

Products and vendors Mallory has correlated with this vulnerability. Open in Mallory to drill down to specific CPE configurations and version ranges.

VendorProductType

SynologyMailplus Serverapplication

Vendor-confirmed product mapping. Mallory continuously reconciles this list against your asset inventory.

ACTIVITY FEED

Recent activity

4 sources tracked across advisories, community write-ups, and news. New activity surfaces here as Mallory finds it.

4 SOURCESView more in app

cyber security newsNews

Jun 30, 2026

Synology MailPlus Server Vulnerabilities Allow Attackers to Trigger DoS Attacks

A moderate-severity vulnerability in Synology MailPlus Server that can allow remote attackers to access internal services because of improper restrictions on communication channels.

synologyNews

Jun 26, 2026

Synology_SA_26_11 | Synology Inc.

A Synology MailPlus Server vulnerability that allows remote attackers to access internal services.

What this page doesn’t show

The version that knows your environment.

This page is what’s public. Mallory adds the parts that aren’t: which of your assets are affected, which adversaries are exploiting it right now, which detections to deploy, and what to do tonight.

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Exposure mapping

Query your assets running an affected version, and investigate the blast radius.

Threat actor evidence

Every observed campaign linking this CVE to a named adversary.

Associated malware

Malware families riding this exploit, with evidence and IOCs.

Detection signatures

YARA, Sigma, Snort, and vendor rules, auto-deployed to your SIEM.

Vendor-by-vendor mapping

Cross-references every affected SKU, including bundled OEM variants.

Social activity1