CVE-2026-66304 in Skype for Business Server
Summary
by MITRE • 09/08/2026
Server-side request forgery (ssrf) in Skype for Business allows an unauthorized attacker to disclose information over a network.
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Analysis
by VulDB Data Team • 09/08/2026
The vulnerability identified as Server-Side Request Forgery, or SSRF, within Microsoft Skype for Business represents a critical class of web application security flaws that allow attackers to manipulate the server into making requests to unintended destinations. In this specific context, an unauthorized attacker can exploit weaknesses in how the application processes user-supplied input to force the backend system to initiate HTTP or other network connections to internal resources that are not directly accessible from the external internet. This type of attack leverages the trust relationship between the web server and its underlying infrastructure, effectively bypassing perimeter security controls such as firewalls because the requests originate from a trusted source within the protected network zone. The core technical flaw typically involves insufficient validation or sanitization of input parameters that are used to construct outbound URLs or API calls, allowing an attacker to inject malicious payloads that redirect these server-side operations toward internal services like metadata endpoints, database interfaces, or administrative consoles.
From an operational perspective, the impact of this vulnerability is severe as it facilitates unauthorized information disclosure across the network. By directing the Skype for Business server to communicate with internal systems, attackers can extract sensitive data such as configuration files, authentication tokens, private keys, or proprietary business logic stored on adjacent servers that were never intended to be exposed publicly. This capability effectively neutralizes many traditional defense-in-depth strategies because the attacker does not need direct network access to these internal assets; they only require a valid account or entry point into the Skype for Business application itself. The ability to probe and interact with internal services can serve as a stepping stone for further lateral movement, potentially leading to full compromise of the corporate intranet if combined with other vulnerabilities in downstream systems that respond differently based on their identity or configuration when accessed from different sources.
This vulnerability aligns closely with Common Weakness Enumeration identifier CWE-918, which specifically defines Server-Side Request Forgery as a weakness where a web server receives a URL or similar request and makes it to retrieve content without validating the user's intent. Furthermore, in terms of offensive security tactics mapped by MITRE ATT&CK, this behavior corresponds to techniques involving Taint Analysis and potentially Data from Local System if the SSRF allows reading local files via file:// protocols, though primarily it falls under reconnaissance phases where an attacker gathers information about internal network topology and service configurations. The exploitation often involves crafting specific HTTP requests with manipulated Host headers or URL parameters that point to localhost addresses such as 127.0.0.1 or internal IP ranges like 192.168.x.x, exploiting the server's ability to resolve these names internally while remaining invisible to external network monitoring tools designed to block direct inbound connections from untrusted sources.
Mitigation strategies for this vulnerability require a multi-layered approach focusing on input validation and network segmentation. Developers must implement strict allow-listing of permitted domains or IP addresses that the application is authorized to contact, rejecting any request targeting internal ranges such as RFC 1918 private address spaces unless explicitly required by business logic. Additionally, implementing egress filtering at the firewall level can restrict outbound connections from the Skype for Business servers to only known and necessary external endpoints, thereby limiting the blast radius even if an SSRF is successfully exploited. It is also critical to ensure that internal services do not rely solely on IP-based access controls since requests originating from within the network will appear legitimate; instead, these services should enforce strong authentication mechanisms independent of source IP address. Regular security testing including dynamic application security scanning and penetration tests focused on SSRF scenarios are essential to identify such flaws before they can be leveraged by malicious actors in production environments.