Microsoft’s security advisory listing for CVE-2025-62204 identifies a SharePoint remote code execution (RCE) weakness tied to unsafe deserialization, and administrators should treat it as an urgent patch-and-hunt item while verifying vendor mappings and telemetry before and after remediation. Microsoft’s update page for CVE-2025-62204 is online but requires the vendor’s interactive Security Update Guide to view full details, and independent trackers report the flaw as a deserialization-based RCE affecting on‑premises SharePoint installations; at the same time, coordination notices from CISA and other responders show the broader SharePoint exploitation landscape remains complex and evolving.
Source: MSRC Security Update Guide - Microsoft Security Response Center
Background
A familiar pattern: deserialization + layout endpoints
SharePoint on‑premises servers have been at the center of several high‑impact incident clusters in 2025. The most damaging chains observed in July—commonly labeled “ToolShell” in public reporting—combine an authentication/spoofing bypass against layout endpoints with an unsafe‑deserialization primitive that lets attackers execute attacker‑supplied objects during reconstruction. That combination has repeatedly produced ASPX web‑shell drops, extraction of ASP.NET machineKey material, forged __VIEWSTATE payloads, and persistent, stealthy code execution.Why this matters now
Deserialization vulnerabilities in ASP.NET applications like SharePoint are highly valuable to attackers because they frequently run within the IIS worker process (w3wp.exe) and have access to file systems, configuration files, and network resources. When an attacker achieves RCE in that process they can often escalate laterally, harvest credentials, and deploy ransomware. Microsoft and national CERTs have repeatedly emphasized immediate patching and operational mitigations for internet‑exposed SharePoint farms.What the advisory for CVE-2025-62204 actually says
Vendor listing and current public visibility
Microsoft’s Security Update Guide hosts a page for CVE‑2025‑62204. The page is present in the update guide but the interactive view requires JavaScript to display full advisory content directly, which can obscure automated scraping and some tracker feeds. Independent tracking services and vulnerability aggregators have surfaced CVE‑2025‑62204 as a deserialization issue in SharePoint with a high severity rating (feed summaries mark it around CVSS 8.0), and they report that patches are available through Microsoft update channels for supported SharePoint Server versions. Because MSRC remains the authoritative vendor source, defenders should validate the CVE‑to‑KB mapping in the Security Update Guide before acting on numeric identifiers alone.Confirmed and unconfirmed facts
- Confirmed: Microsoft has posted an advisory page for CVE‑2025‑62204 in its update guide.
- Reported by independent aggregators: the flaw is an unsafe‑deserialization vulnerability in SharePoint that can lead to remote code execution with an elevated impact rating.
- Corroborated context: CVE‑2025‑62204 sits in a larger, active ecosystem of SharePoint deserialization and layout endpoint exploitation that has produced widely exploited CVEs earlier in 2025 (for example, the “ToolShell” cluster: CVE‑2025‑53770 / CVE‑2025‑53771 and their predecessors). Administrators should treat CVE mapping carefully because multiple related CVEs and patch bypasses have circulated.
Technical anatomy — how an exploit chain like this works
Core primitives (short, practical view)
- Authentication/spoofing bypass: crafted HTTP requests to layout endpoints (commonly /_layouts/15/ToolPane.aspx or similar) with manipulated Referer or other request metadata can bypass server-side checks to produce an effective file‑write primitive.
- Unsafe deserialization: a serialized blob (often via ViewState or object serialization paths) is deserialized without proper type whitelisting or validation, allowing execution of attacker-controlled objects.
Typical exploit flow (observed and reproduced in incidents)
- Reconnaissance: automated scanners enumerate internet‑facing SharePoint farms and fingerprint versions.
- Bypass & write: attacker crafts a POST to a layouts endpoint while spoofing headers, persuading the server to write an ASPX payload (web shell) into the TEMPLATE\LAYOUTS folder. Observed web shell filenames include variants like spinstall0.aspx.
- Key theft: the web shell reads web.config or memory to exfiltrate the ASP.NET machineKey (ValidationKey and DecryptionKey). Possession of machineKey enables signed payload forgery.
- Forged persistence: attacker generates legitimately signed __VIEWSTATE blobs that SharePoint accepts, creating a persistence channel that can survive some partial remediations.
Who’s at risk and which products are affected
Affected hosting models
- On‑premises SharePoint Server (Subscription Edition, SharePoint Server 2019, SharePoint Server 2016 where vendor KBs apply) are the primary risk surface for deserialization chains of this class. SharePoint Online (Microsoft 365) is not affected by on‑premises deserialization vectors in the same way.
Internet exposure multiplies risk
Publicly reachable SharePoint farms are the highest priority because the exploit chain is triggered by unauthenticated or low‑privilege HTTP(S) requests. Scanners have found thousands of potential targets, and even a modest scanning-to-exploit success rate creates a large operational incident scope.Legacy and unsupported versions
Older, unsupported SharePoint versions (2010/2013 and custom older builds) often lack vendor updates and therefore remain exposed; the only practical options for those environments are isolation, migration, or compensating controls such as VPN‑only access or a hardened WAF.How to verify CVE‑to‑KB mappings and avoid tracking mistakes
- Use Microsoft’s Security Update Guide (MSRC) as the authoritative mapping for KB numbers and product builds. The vendor’s page for the CVE is the single source of truth for which update packages apply to each SKU.
- Cross‑check the MSRC mapping in at least one independent tracker (CISA advisory, a major vendor blog, or a reputable security news outlet) to confirm real‑world exploitation status and additional operational guidance.
- If a CVE string in a third‑party ticket cannot be found in the MSRC Security Update Guide or the NVD/MITRE entries, treat that label as unverified and confirm the KB numbers and builds directly with Microsoft before rolling changes into production tracking. Several prior SharePoint advisories produced spurious or transposed CVE labels early in the disclosure cycle.
Immediate prioritized response for administrators (operational checklist)
Apply the following steps in order; each reduces attack surface or invalidates attacker persistence.- Patch first: identify every SharePoint host, map its product SKU (Subscription Edition, 2019, 2016, language packs), and apply the exact Microsoft security update(s) published for the CVE—don’t rely on a generic “Windows Update fixed it” assumption. Validate the KB or build number post-install.
- Rotate ASP.NET machineKey farm‑wide: after installing updates, rotate machineKey values (ValidationKey and DecryptionKey) across the farm and restart IIS on each node to invalidate any stolen keys. Use the SharePoint Central Administration rotation job or Set‑SPMachineKey / Update‑SPMachineKey in PowerShell. This step is essential to prevent attackers from re‑using forged __VIEWSTATE payloads.
- Enable AMSI and enterprise antimalware: configure the Antimalware Scan Interface (AMSI) in SharePoint (Full Mode if available) and deploy Microsoft Defender Antivirus or equivalent with AMSI hooks to scan HTTP request bodies for payloads.
- Deploy/validate EDR and hunting queries: enable EDR telemetry (Microsoft Defender for Endpoint or your EDR platform), and run detections for suspicious w3wp.exe child processes (PowerShell, cmd), Base64‑encoded POST bodies to ToolPane.aspx, and unexpected files in the Layouts directory (for example, spinstall0.aspx). Microsoft and vendors have published hunting queries and YARA/SIGMA patterns.
- Isolate internet‑exposed hosts if AMSI cannot be enabled quickly: if you cannot rapidly harden and update a public SharePoint instance, take it offline or place it behind a VPN or authenticated proxy until full mitigations are in place.
- Search for POST requests to /_layouts/15/ToolPane.aspx and for Referer headers that match SignOut.aspx spoofing patterns; log and block high‑confidence anomalies.
- Inspect Layouts directories and file modification times across servers in the farm; unknown ASPX files are high‑confidence indicators.
Incident response: if compromise is confirmed
- Immediately isolate the compromised host and preserve forensic artifacts (IIS logs, web.config snapshots, process memory if feasible). Assume machineKey and credentials may be exfiltrated.
- Replace credentials and secrets: rotate service accounts, certificates, and any secrets that may have been exposed. Plan for a coordinated, farm‑wide machineKey rotation and controlled IIS restarts.
- Consider full rebuilds for highly‑trusted hosts: if web shells or lateral movement are observed, rebuilding from known‑good images and restoring from verified backups is typically the safest route. Continue monitoring historical logs for prior compromise windows.
Strengths and weaknesses of the current response posture
Notable strengths
- Microsoft, CISA, and major security vendors have publicly coordinated guidance and detection content, and vendor hotpatches and KBs for affected SharePoint builds have been published quickly for supported products. This reduces confusion for administrators who follow the MSRC mapping.
- The attacker chain is well‑understood operationally: researchers and incident responders have enumerated practical indicators (ToolPane posts, Layouts folder artifacts, machineKey theft), enabling focused hunting and WAF rule development.
Persistent risks and limitations
- CVE labeling and mapping confusion: multiple related CVEs, patch bypasses, and re‑weaponized fixes earlier in 2025 produced inconsistent CVE labels across trackers. That means operations teams can mis‑classify or delay patching if they rely only on a CVE string without validating the MSRC KB mapping.
- Legacy installations and patch lag: many organizations run older SharePoint builds that are unsupported or difficult to patch in short maintenance windows; those farms remain prime targets.
- Instrumentation gaps: not all SharePoint farms have AMSI enabled, EDR deployed, or centralized logging configured; these telemetry gaps make both detection and confident remediation much harder.
Verification status and cautionary notes
- MSRC lists CVE‑2025‑62204 in the Security Update Guide; however, the Security Update Guide can be opaque to automated scrapers and some feeds, so defenders must use the MSRC UI or official KB pages to confirm which update package(s) apply to each SharePoint SKU. Treat MSRC as the canonical source.
- Some third‑party trackers show CVE‑2025‑62204 with a high severity and characterize it as a deserialization RCE with high impact, but at the time of writing central repositories such as the NVD/MITRE public CVE pages may not yet show a full entry for this specific numeric label. If a CVE cannot be found in NVD, consult the vendor KB and alerting agencies for the authoritative remediation steps.
- Where individual feeds or reports quote exploit counts, actor attributions, or precise exploitation timelines for CVE‑2025‑62204 specifically, treat those numbers as provisional until corroborated by independent vendor telemetry or coordinated government advisories. Earlier SharePoint incidents did include confirmed active exploitation (ToolShell variants) and real ransomware deployments; similar behavior is plausible—but each CVE’s exploitation footprint should be verified independently.
Long‑term defensive lessons
- Harden and reduce internet exposure: place SharePoint behind authenticated gateways, restrict traffic via WAFs and network ACLs, and minimize direct public access to management endpoints.
- Enforce defense in depth: patch management, AMSI-enabled scanning, EDR coverage, WAF detection rules, and centralized logging together make exploitation harder and detection faster.
- Inventory and update discipline: maintain accurate SKU and language‑pack inventories so that KB mappings are applied correctly; automated OS and app inventory tools (SCCM/Intune/WSUS) are essential in large estates.
Conclusion
CVE‑2025‑62204 is published in Microsoft’s update guide as a SharePoint deserialization remote code execution issue and should be treated as a high‑priority remediation and hunting item for on‑premises SharePoint servers. Administrators must verify the correct KBs in Microsoft’s Security Update Guide before mapping fixes in ticketing systems, apply updates immediately, rotate ASP.NET machineKey values across the farm, enable AMSI and EDR telemetry, and hunt for web shells and anomalous ToolPane activity. The broader pattern of SharePoint exploitation in 2025 demonstrates that chained application‑layer weaknesses (bypass + deserialization) have real operational impact—remediation requires both timely patching and careful post‑patch verification. Administrators should prioritize: 1) identify all on‑prem SharePoint hosts and the exact product SKUs, 2) apply Microsoft’s security updates for the CVE and associated KBs, 3) rotate machine keys and restart IIS farm‑wide, 4) enable AMSI + enterprise antimalware and EDR, and 5) hunt and remediate any web shells or signs of prior compromise. If evidence of compromise exists, assume keys and credentials were exposed and conduct a full incident response with credential and secret rotation and controlled rebuilt of affected hosts.Source: MSRC Security Update Guide - Microsoft Security Response Center
