CVE-2026-50522: Microsoft SharePoint Deserialization of Untrusted Data Vulnerability

CVE-2026-50522 is a deserialization of untrusted data vulnerability in Microsoft SharePoint. An unauthorized attacker could use it to execute code over a network. For organizations that run SharePoint, that means a path to full compromise of the application host if the flaw is reachable and unmitigated. Confirm exact product editions, builds, and fixed releases against the vendor advisory before acting.

CISA has directed stakeholders to apply mitigations in line with vendor instructions and BOD 26-04 prioritization and forensics guidance, evaluate internet exposure, and discontinue use if mitigations are unavailable. Ransomware use of this CVE is not documented in the provided facts.

How it works

This issue is classed as CWE-502: deserialization of untrusted data. SharePoint (and similar collaboration platforms) often accept serialized objects in web requests, workflow or service payloads, or other network-facing inputs. When an application deserializes data without adequately restricting type, source, or content, an attacker who can supply or influence that data may cause the runtime to instantiate unexpected types and run attacker-controlled logic.

In practical terms, a remote, unauthorized attacker who can reach the vulnerable SharePoint endpoint may trigger code execution in the context of the SharePoint process. Exact request shape, authentication requirements, and gadget chains are not specified in the provided facts; treat any public proof-of-concept material with caution and validate behavior only in isolated lab conditions against vendor documentation.

Am I affected? How to find it in your systems

Microsoft SharePoint is commonly deployed as on-premises server farms, hybrid configurations, or related Microsoft collaboration stacks used for document management, intranet portals, and business workflows. Inventory every host and service that runs SharePoint Server components, including web front ends, application servers, and any internet-facing or partner-facing sites.

• Build a software inventory from configuration management, SCCM/Intune, vulnerability scanners, and manual review of SharePoint Central Administration and installed Microsoft products; record build/version strings and compare them to the fixed versions listed in the vendor advisory.
• Map network exposure: identify which SharePoint URLs are reachable from the internet, VPN, or untrusted segments; note load balancers, reverse proxies, and authentication front ends in the path.
• Review configuration for features that accept complex or serialized input (custom web parts, workflows, service applications, API endpoints). Disable or restrict unused services where policy allows.
• For exploitation signs, inspect IIS and SharePoint ULS logs, Windows Security and System events, and EDR telemetry for anomalous process creation from w3wp.exe or SharePoint-related services, unexpected serialized payloads in request bodies, new persistence (scheduled tasks, services, web shells under SharePoint directories), and outbound connections inconsistent with normal farm behavior. Specific IOCs are not provided here; align detection with vendor and CISA guidance as it is published.

How to remediate

Patch first. Apply the Microsoft updates named for CVE-2026-50522 in the official vendor advisory, following your standard change and test process for SharePoint farms (including compatibility checks for custom solutions). Confirm successful installation via build numbers and Microsoft’s verification steps.

• After patching, re-validate that previously exposed endpoints no longer exhibit the vulnerable behavior and that farm health (search, user profiles, workflows) remains intact.
• Harden the stack for this weakness class: run SharePoint under least privilege, keep the underlying Windows and .NET components current, remove unused deserialization formatters or legacy custom code paths that accept untrusted serialized input, and enforce strong authentication and authorization on all administrative and API interfaces.
• Ensure compliance with CISA BOD 26-04 prioritization for internet-facing and high-value assets, and retain forensic artifacts per CISA forensics triage expectations if compromise is suspected.

If you can't patch immediately

Reduce reachability and monitor aggressively until the vendor fix is installed.

• Segment SharePoint hosts so only required clients and management jump hosts can reach administrative and application ports; remove direct internet exposure where business allows, or place the farm behind a reverse proxy with strict allow-lists.
• Apply virtual patching at a WAF or reverse proxy only if rules are specifically validated for this issue class and do not break legitimate SharePoint traffic; generic rules are incomplete substitutes for the vendor patch.
• Disable or restrict nonessential SharePoint services, custom endpoints, and features that accept complex client input until patched.
• Increase logging and EDR sensitivity on SharePoint servers; alert on anomalous child processes, file writes under web roots, and unusual authentication patterns. Prepare an incident response path that includes isolation of the farm and preservation of IIS/ULS/EDR evidence.
• If mitigations cannot be applied, follow CISA direction to evaluate discontinuing use of the product for exposed assets until a fix is available.

If your data may have been exposed

Actively exploited remote code execution flaws in collaboration platforms frequently lead to web shells, credential theft, and lateral movement into file stores and identity systems. If you have indicators of exploitation or uncertain patch status on an exposed farm, treat the environment as potentially compromised: isolate affected hosts, preserve logs, rotate credentials and secrets that SharePoint could access, and follow your incident response and CISA forensics triage procedures. You can also run a free exposure scan of your email addresses against known breach datasets to see whether associated accounts appear in prior public dumps while you complete internal investigation.