CVE-2026-66306: Skype for Business Information Disclosure

CVE-2026-66306 Overview

CVE-2026-66306 is an information disclosure vulnerability in Microsoft Skype for Business. The flaw stems from the generation of error messages that contain sensitive information [CWE-209]. An unauthorized attacker can trigger these messages over a network to obtain data that should remain confidential. Exploitation requires user interaction, according to the CVSS vector.

Microsoft published the advisory on September 8, 2026. The vulnerability affects the confidentiality of Skype for Business deployments but does not impact integrity or availability. No public proof-of-concept code has been observed, and the issue is not listed in the CISA Known Exploited Vulnerabilities catalog.

Critical Impact
An unauthenticated network attacker can extract sensitive information from Skype for Business through crafted requests that trigger verbose error responses.

Affected Products

Microsoft Skype for Business (per Microsoft Security Response Center advisory)
Refer to the Microsoft Security Update CVE-2026-66306 for the full list of affected builds
On-premises Skype for Business Server deployments exposed to network traffic

Discovery Timeline

2026-09-08 - CVE CVE-2026-66306 published to NVD
2026-09-08 - Last updated in NVD database

Technical Details for CVE-2026-66306

Vulnerability Analysis

The vulnerability falls under CWE-209: Generation of Error Message Containing Sensitive Information. Skype for Business returns error responses that expose internal data an unauthorized user should not see. This data can include stack traces, internal identifiers, configuration values, or user attributes surfaced during exception handling.

An attacker sends crafted requests to a Skype for Business network endpoint. The server processes the request, encounters an error condition, and includes sensitive context in the response body or headers. The attacker parses these responses to build reconnaissance data about the environment.

The attack vector is network-based and requires no privileges. Exploitation requires user interaction, which typically means an authenticated user must open a message, click a link, or load content that causes the client or server to emit the sensitive error. This raises the practical bar but does not eliminate risk in phishing scenarios.

Root Cause

The root cause is improper handling of exceptions in code paths that produce user-facing error messages. Diagnostic details intended for developers are returned to remote callers rather than logged internally. Microsoft's advisory contains the authoritative technical description.

Attack Vector

The attacker delivers input over the network to Skype for Business components. When combined with the required user interaction, the server or client surfaces verbose error output. The disclosed content can then be used to plan follow-on attacks, such as targeted authentication attempts or lateral movement.

No verified public exploit code is available. Consult the Microsoft Security Update CVE-2026-66306 for exploitation prerequisites and affected build ranges.

Detection Methods for CVE-2026-66306

Indicators of Compromise

Unusual volumes of malformed requests to Skype for Business web endpoints from single sources.
Server responses containing stack traces, internal hostnames, or file paths returned to external clients.
Client-side error dialogs or logs referencing internal server components after clicking links from untrusted senders.

Detection Strategies

Inspect HTTP responses from Skype for Business front-end servers for verbose error content and flag responses exceeding expected size for error codes.
Correlate error response bursts with authentication events and inbound message metadata to identify probing patterns.
Enable verbose IIS and Skype for Business Server logging, then baseline normal error rates to surface anomalies.

Monitoring Recommendations

Forward Skype for Business Server and IIS logs to a centralized SIEM for retention and query.
Alert on repeated 4xx and 5xx responses containing sensitive keywords such as Exception, StackTrace, or internal domain names.
Monitor egress traffic from Skype for Business servers for anomalous data volumes following error spikes.

How to Mitigate CVE-2026-66306

Immediate Actions Required

Apply the Microsoft security update referenced in the Microsoft Security Update CVE-2026-66306 advisory.
Restrict network exposure of Skype for Business endpoints to trusted networks and authenticated users where feasible.
Educate users to avoid clicking untrusted links or opening unsolicited meeting invites that could trigger error conditions.

Patch Information

Microsoft has published the fix through the Microsoft Security Response Center. Administrators should deploy the update to all Skype for Business Server front-end and edge roles. Verify build numbers after patching against the vendor advisory.

Workarounds

Configure Skype for Business Server to suppress detailed error responses and return generic error pages to remote clients.
Place Skype for Business front-end services behind a reverse proxy that strips sensitive headers and rewrites verbose error bodies.
Limit inbound federation and anonymous access until patching is complete.

bash

# Configuration example: enforce generic error handling in IIS for Skype for Business web services
%windir%\system32\inetsrv\appcmd.exe set config "Default Web Site" -section:system.webServer/httpErrors /errorMode:"Custom" /commit:apphost
%windir%\system32\inetsrv\appcmd.exe set config "Default Web Site" -section:system.webServer/asp /scriptErrorSentToBrowser:"False" /commit:apphost