CVE-2021-26737

Tunnel shutdown via RPC client-validation race

Published Oct 23, 2023 · Updated Sep 11, 2024

Origin validation error in Zscaler Client Connector for macOS before 3.6 allows local users to shut down the Zscaler tunnel. The local RPC service trusts a client identity derived from a process ID that can be reused during a race, accepting an unauthorized client. Low-privileged local access and winning the race are required; successful exploitation interrupts the protected traffic tunnel without reported data exposure or modification.

CVSS severity4.7
Medium
EPSS probability0.11%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
Coverage for this vulnerability

See if you're affected

Explore vulnerability checks for your environment with Hinoki.

Book a demo

Summary

What happened

Origin validation error in Zscaler Client Connector for macOS before 3.6 allows local users to shut down the Zscaler tunnel. The local RPC service trusts a client identity derived from a process ID that can be reused during a race, accepting an unauthorized client. Low-privileged local access and winning the race are required; successful exploitation interrupts the protected traffic tunnel without reported data exposure or modification.

The record

CVE
CVE-2021-26737
Published
Oct 23, 2023
Updated
Sep 11, 2024
Vendor
Zscaler
Product
Zscaler Client Connector
Classifications
CWE-346, CAPEC-26, T1489
Attack vector
local
Privileges
authenticated

Timeline

How it unfolded

  1. Oct 23, 2023CVE publishedPublication date reported by the CVE source.
  2. Sep 11, 2024Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

Compare your product and version with the public record. A matching version still requires validation against your environment.

Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=0 <3.6

What conditions does exploitation require?

Attack vectorlocal
Required privilegesauthenticated

What is affected?

Zscaler · Zscaler Client Connectorversion=0 <3.6

Published CVSS scores

5.5ZscalerCVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
4.7NIST NVDCVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectorlocal
Privilegesauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-346Origin Validation Error
CAPEC-26Leveraging Race Conditions
T1489Service Stop

Public exploit references

No public exploit references are available in this record.

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

See the directory against your own environment.

Your stack

Check the software in your environment

Book a demo to see how Hinoki identifies affected software and validates exploitability in your environment.

Book a demo