The Cybersecurity and Infrastructure Security Agency (CISA) has issued a warning about a critical vulnerability in Microsoft SharePoint Server, identified as CVE-2026-45659. This flaw, recently added to CISA’s Known Exploited Vulnerabilities (KEV) Catalog, is currently being exploited in live cyberattacks.
Understanding the Vulnerability
This vulnerability involves the deserialization of untrusted data, categorized under CWE-502. It allows attackers with valid credentials to execute arbitrary code remotely through the network. This flaw primarily impacts on-premises deployments of Microsoft SharePoint Server, posing substantial risks to enterprises that utilize SharePoint for document management and collaboration.
According to CISA, attackers can create malicious serialized payloads processed by the SharePoint server, leading to remote code execution (RCE). This type of vulnerability is particularly concerning as it can circumvent conventional security measures by exploiting legitimate user contexts.
Immediate Action Required
CISA added CVE-2026-45659 to its KEV catalog on July 1, 2026, with a remediation deadline set for July 4, 2026. The agency stresses the urgency for federal agencies and organizations to address this vulnerability promptly to mitigate potential security breaches.
While there is no direct evidence linking this flaw to ransomware activities, its active exploitation significantly increases its threat level. Organizations are advised to adhere to vendor-provided mitigation strategies and comply with Binding Operational Directive (BOD) 26-04, which emphasizes the prioritization of security updates based on risk.
Mitigation and Security Measures
Organizations should evaluate the internet exposure of their affected SharePoint servers and apply necessary patches or mitigations immediately. Security experts highlight that deserialization vulnerabilities have frequently been exploited in enterprise applications, making them a common attack vector.
An attacker could exploit the vulnerability by using stolen or low-privilege credentials to gain access and execute arbitrary code on the server. This could involve submitting a malicious request that triggers a vulnerable deserialization process, allowing the deployment of web shells or persistent access.
CISA recommends implementing forensic triage procedures to identify potential compromises. Indicators of compromise may include unusual SharePoint activity, unexpected process executions, or abnormal network traffic from SharePoint servers.
The KEV catalog is a vital tool for defenders, offering a curated list of vulnerabilities actively exploited in attacks. By prioritizing the remediation of such vulnerabilities, including CVE-2026-45659, organizations can significantly lower their exposure to ongoing threat campaigns.
Given the short timeframe for remediation and the active threat status, cybersecurity teams should prioritize patching this vulnerability. Failure to act swiftly could result in the exposure of sensitive enterprise data and compromise internal systems.
