Trend Micro's Zero Day Initiative published advisory ZDI-26-401 on July 8, 2026, revealing a zero-day vulnerability in the AnyDesk remote access software that remains unpatched 15 months after initial reporting. The flaw allows an attacker with low-privileged code execution to trigger a denial-of-service condition by abusing the Send Support Information feature. AnyDesk support, in a response dated December 11, 2025, closed the case classifying it as "out of their scope."
- ZDI-26-401 is an unpatched zero-day vulnerability in AnyDesk as of the coordinated disclosure on July 8, 2026.
- The attack requires local access with low-privileged code execution and exploits NTFS junctions to abuse the Send Support Information service.
- The documented impact is denial-of-service, with arbitrary file creation on the target filesystem.
- The vendor declared the issue out of scope after six interactions with ZDI; no fix is available.
The Mechanism: How a Junction Compromises the Service
The flaw resides specifically in AnyDesk's Send Support Information function. The service, which operates with elevated privileges, follows NTFS junctions without validating the destination path. A process with limited permissions can create a junction that redirects the service's write operations to arbitrary filesystem paths.
As the ZDI advisory documents, "by creating a junction, an attacker can abuse the service to create arbitrary files." The result is a denial-of-service condition: overwriting system files or exhausting resources renders the target installation unusable. Administrative access is not required, only "the ability to execute low-privileged code on the target system."
The link-following technique via junctions represents a known vulnerability class on Windows platforms. The Send Support Information service, designed to collect and transfer diagnostic data, implements no security controls on the resolved path. This omission allows an attacker to divert legitimate service operations to attacker-controlled destinations.
A Timeline of a Broken Dialogue
ZDI opened the case on March 30, 2025. Interactions with AnyDesk spanned over nine months, with six documented contacts. On December 11, 2025, vendor support communicated that the issue was "out of their scope." Coordinated public disclosure occurred on June 26, 2026 per ZDI's internal timeline, with final advisory publication on July 8, 2026.
The vendorPatchUrl in the advisory points to the advisory URL itself: explicit confirmation that no patch exists. No CVE was assigned, nor was a CVSS score provided. Specific affected AnyDesk versions are not listed in the document.
The exceptional duration of the disclosure cycle — over 15 months from reporting to publication — reflects the failure to converge between researcher and vendor. ZDI followed standard coordinated disclosure procedure, but the out-of-scope closure precluded any possibility of collaborative remediation.
Why the Out-of-Scope Classification Raises a Broader Problem
"The vendor's support team communicated that the issue was out of their scope" — ZDI Advisory ZDI-26-401, timeline 2025-12-11
AnyDesk's decision fits a pattern observed in vulnerability management: vendors drawing responsibility boundaries that exclude attack scenarios not directly involving their own servers. In the case of ZDI-26-401, the flaw is in the client installed on millions of enterprise endpoints, not in remote infrastructure. The distinction between "our scope" and "user problem" leaves installations exposed.
AnyDesk is deployed in enterprise environments for remote support, help desk, and access to critical systems. An attacker who has already compromised an endpoint with malware can trigger the DoS, halting the machine's operability. The absence of an official fix transforms the vulnerability from theoretical to manageable only through mitigations external to the product.
The classification as a zero-day in ZDI's publication list — with the "(0Day)" prefix in the title — confirms the status of an unpatched vulnerability. This labeling is reserved for cases where the vendor has not released a patch at the time of coordinated disclosure.
What to Do Now
Organizations running AnyDesk must contend with an unpatchable local zero-day outside the vendor's normal update cycle. The ZDI advisory indicates limiting interaction with the product as the only documented mitigation.
Specific actions for ZDI-26-401 include: mapping endpoints running AnyDesk and verifying which users hold sufficient privileges to create NTFS junctions; evaluating disabling the Send Support Information feature where not essential for operations; monitoring system logs for anomalous file creation operations by the AnyDesk service; reviewing local access policies to reduce the attack surface from low-privileged accounts.
Risk assessment further requires identifying which endpoints run the software and with what privileges the Send Support Information service operates. Priority should go to critical systems and those exposed to local compromise scenarios.
Limits and Technical Context
The dossier does not specify affected versions nor provide operational guidance from the vendor. The source does not document whether public exploits or in-the-wild attacks exist. The presumed operating system is Windows, given the reference to NTFS junctions, but the advisory does not state this explicitly.
It is unclear whether Trend Micro Vision One or other ecosystem solutions provide dedicated detection for this technique. The advisory mentions no detection products or automated countermeasures.
For organizations deploying AnyDesk, the zero-day introduces an unpatchable local vector outside the vendor's normal update cycle. Risk assessment requires mapping which endpoints run the software and with what privileges the Send Support Information service operates.
Information is based on the cited source and current as of publication.
Sources
- http://www.zerodayinitiative.com/advisories/ZDI-26-401/
- http://www.zerodayinitiative.com/advisories/published/