CVE-2026-102866 in SharePoint Server
Summary
by MITRE • 10/13/2026
Improper verification of cryptographic signature in Microsoft Office SharePoint allows an unauthorized attacker to elevate privileges over a network.
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Analysis
by VulDB Data Team • 10/13/2026
The vulnerability described involves a critical failure in the validation mechanism for cryptographic signatures within Microsoft Office SharePoint, specifically affecting components responsible for processing remote requests and configuration updates. This flaw stems from improper verification logic that fails to rigorously validate the authenticity or integrity of digital signatures attached to incoming data packets or serialized objects before they are processed by the application server. In a typical secure implementation, any modification to signed content should result in signature validation failure; however, due to this defect, an attacker can manipulate payload contents while preserving valid cryptographic markers, thereby bypassing security controls designed to prevent unauthorized changes. This class of issue is formally categorized under CWE-347, which denotes Improper Verification of Cryptographic Signature, and represents a significant deviation from secure coding practices required for handling sensitive administrative operations in enterprise environments.
From an operational perspective, the ability to forge or alter signed requests allows an unauthenticated attacker to execute arbitrary commands with elevated privileges on the SharePoint server. By exploiting this weakness, an adversary can trick the system into accepting maliciously crafted inputs as legitimate administrative actions. This effectively grants the attacker control over critical services and data repositories hosted by the platform. The impact extends beyond simple information disclosure; it enables full compromise of the underlying infrastructure, including potential lateral movement within the network domain. Attackers may leverage this privilege escalation to install backdoors, exfiltrate sensitive corporate documents, or disrupt business continuity by altering site configurations and user permissions without detection through standard audit logs that rely on signature validity as a trust indicator.
The exploitation vector for this vulnerability is primarily remote over a network, aligning with the ATT&CK technique T1078, Valid Accounts, when combined with initial access methods such as phishing or credential theft to reach vulnerable endpoints, and more directly relates to MITM attacks if session tokens are intercepted and replayed due to signature validation failures. It also intersects with techniques involving command injection (T1059) and privilege escalation (T1068), as the core issue is the system's blind trust in manipulated signed payloads. The severity of this flaw is heightened by its potential for automated exploitation across large enterprise deployments where SharePoint serves as a central hub for collaboration and document management, making it an attractive target for state-sponsored actors or cybercriminal syndicates seeking persistent access to high-value targets.
Mitigation strategies must focus on immediate patching and architectural hardening. Organizations should apply the latest security updates provided by Microsoft that address this specific signature verification logic error in their SharePoint environments. In addition to software remediation, network segmentation plays a crucial role in limiting exposure; ensuring that SharePoint servers are not directly accessible from untrusted networks reduces the attack surface significantly. Implementing strict input validation and enforcing mutual TLS (mTLS) for internal service-to-service communication can further mitigate risks associated with signature manipulation. Security teams should also monitor logs for anomalous patterns indicative of signature bypass attempts, such as unexpected changes to configuration files or unusual administrative actions originating from non-standard sources, ensuring rapid detection and response capabilities are in place alongside the technical fixes.