CVE-2025-54895: Local Privilege Escalation in Windows NEGOEX/SPNEGO

Microsoft’s advisory for CVE-2025-54895 warns that an integer overflow or wraparound in the SPNEGO Extended Negotiation (NEGOEX) security mechanism can be triggered by an authorized local actor to elevate privileges, turning a legitimate local account into a pathway to SYSTEM-level control if left unpatched. (msrc.microsoft.com)

Background​

Why NEGOEX matters to Windows authentication​

SPNEGO (Simple and Protected GSS‑API Negotiation Mechanism) and its extension NEGOEX are central pieces of Windows authentication plumbing. They act as the negotiation layer that lets clients and servers agree whether to use Kerberos, NTLM, or other mechanisms for authenticating a session. Because NEGOEX sits at the boundary between network protocols and credential handling, defects in its parsing or arithmetic logic can have outsized impact on identity and privilege security across endpoints and servers.
This is not theoretical: Microsoft and multiple security vendors have tracked several high‑severity NEGOEX‑related flaws across 2022–2025, including both remote code execution and privilege escalation variants — a pattern that demonstrates the real operational risk when the negotiation layer miscomputes lengths, counts, or buffer sizes. (socprime.com, nvd.nist.gov)

How CVE numbering maps to real world threats​

There are multiple NEGOEX CVEs circulating in 2025. Some of the best‑known are RCE bugs (remote) that allow unauthenticated attackers to run code by sending crafted NEGOEX packets; others are local elevation‑of‑privilege (EoP) issues that require some local access but let an attacker amplify privileges to SYSTEM. CVE‑2025‑54895, per Microsoft’s entry, sits in the latter class: an integer overflow / wraparound resulting in local privilege escalation. (msrc.microsoft.com, crowdstrike.com)

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What Microsoft’s advisory says (short, exact summary)​

• The underlying cause: integer overflow or wraparound in the NEGOEX implementation inside Windows authentication components.
• The impact: Elevation of privilege — a locally authorized attacker can elevate privileges on a vulnerable system.
• The attack vector: Local (authorized user or process context required).
• The recommended action: Apply Microsoft’s security update(s) addressing CVE‑2025‑54895 as soon as practical; consult the MSRC vulnerability page for exact KBs and affected builds. (msrc.microsoft.com)

Note: the Microsoft Security Response Center (MSRC) advisory page for this CVE is the authoritative source for affected builds, KB numbers, and mitigations — the MSRC UI requires JavaScript to render fully, so administrators should open the MSRC entry directly from a browser to confirm the exact remediation artifacts for their environment.

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Technical analysis: what an integer overflow / wraparound in NEGOEX can mean​

The bug class and typical exploitation path​

An integer overflow or wraparound happens when code uses a fixed‑width integer to compute the size of a buffer, or the length of data to copy, and the computed size exceeds the integer’s representable range. When values wrap around (for example, a 32‑bit length that becomes small after overflow), subsequent allocations or copy loops allocate or assume the wrong size and allow adjacent memory to be overwritten.
Within NEGOEX — which must parse nested negotiation tokens, lengths, extents and capability sets — a single miscalculated length can lead to:

• corrupted internal state,
• out‑of‑bounds writes or reads,
• corrupted pointers or vtable entries,
• and ultimately a path to write controlled data into a privileged process context.

CVE‑2025‑54895’s classification as an EoP indicates the exploit scenario is likely to be invoked by a local subject that can trigger NEGOEX parsing (for example, a local service, a user process that calls APIs that use NEGOEX, or a helper utility), then use the overflow to escalate. Because the attacker is authorized locally, the vulnerability reduces the precondition barrier — an account that already has some access can use this flaw to pivot to complete control. (msrc.microsoft.com)

How this differs from the RCE NEGOEX bugs we saw earlier​

Earlier NEGOEX flaws in 2025 included heap buffer overflows and use‑after‑free conditions that allowed remote unauthenticated attackers to execute arbitrary code simply by sending crafted packets across the network. Those RCE bugs (for example, high‑profile July disclosures) were rated critical and wormable because no local account was needed and no user interaction was required. CVE‑2025‑54895 is different in scope: it requires local authorization to reach the vulnerable code path but still delivers very high impact through privilege elevation. Administrators must treat both classes seriously but prioritize response differently: RCE patches for exposed services are immediate emergency fixes; local EoP fixes are essential for reducing lateral‑movement and post‑compromise escalation risk. (socprime.com, rapid7.com)

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Affected components and likely affected builds (how to confirm for your estate)​

Microsoft’s MSRC vulnerability entry is the canonical inventory of affected SKUs and build numbers; administrators should use it as the single source of truth when mapping CVE‑2025‑54895 to their assets. Because MSRC content is rendered by a dynamic web app, open the MSRC page directly to view the full affected platform matrix and KB identifiers before scheduling deployments. (msrc.microsoft.com)
In practice, NEGOEX code appears in multiple Windows authentication consumers (LSASS, user‑mode libraries used by network services, and protocol stacks). Historically, NEGOEX CVEs have touched both client and server SKUs, and many supported Windows 10/11 and Windows Server builds were patched in prior NEGOEX advisories. Expect a similar broad set of affected builds for an NEGOEX integer overflow unless MSRC explicitly narrows it. (crowdstrike.com, sentiguard.eu)

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Immediate mitigation and response checklist (operational playbook)​

If your environment includes Windows endpoints or servers (especially domain‑joined hosts, RDP/SMB/IIS servers, or systems that run authentication stacks for services), treat CVE‑2025‑54895 as a high‑priority patching item. Follow this step sequence to reduce risk quickly and methodically:

• Inventory and prioritize
• Identify all Windows endpoints and servers that host authentication services or run agent software that may invoke NEGOEX (domain controllers, AD‑joined servers, remote desktop hosts, IIS with Windows Authentication).
• Flag internet‑facing or VPN‑reachable assets for top priority.
• Retrieve official patch artifacts
• Open the MSRC CVE‑2025‑54895 advisory and capture the KB numbers and affected build list for your product versions. (msrc.microsoft.com)
• Test in staging
• Validate the patch in a representative staging environment, focusing on authentication flows (Kerberos/NTLM handshakes), single sign‑on integrations, and third‑party software that interacts with Windows authentication.
• Deploy with urgency
• For high‑risk hosts (Internet‑facing, DCs, RDP/SMB servers): push the patch as soon as staging validation completes.
• For broad desktop fleets: use phased deployment with accelerated timelines; postpone non‑essential feature updates only if compatibility testing is incomplete.
• Short‑term compensating controls (if patching will be delayed)
• Limit local access to sensitive hosts and reduce the number of authorized local accounts.
• Enforce least privilege and remove unnecessary local administrative rights.
• Harden local LSA/credential protection: enable Windows Defender Credential Guard where supported and practical.
• Increase monitoring and EDR sensitivity for LSASS anomalies, process token changes, and suspicious use of impersonation APIs.
• Post‑deployment validation
• Confirm KB installation across the estate via WSUS, SCCM/MDM, or the Microsoft Update Catalog.
• Re‑run authentication and SSO smoke tests; validate no regressions in login flows for smartcards, PKINIT, or certificate‑based auth.
• Hunt and detect
• Create SIEM detections for:
• Unexpected local process crashes of lsass.exe or immediate restarts (event source: LsaSrv).
• Sudden privilege escalations, token duplication events, or creation of services with SYSTEM privileges.
• Calls to unusual NEGOEX interfaces from local accounts that don’t normally perform authentication negotiation.

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Detection and threat‑hunting guidance (concrete signals to monitor)​

• Event IDs and process faults
• Monitor Windows Event logs for LSASS or LsaSrv errors, particularly unexpected crashes or faulting module entries; these have historically been associated with NEGOEX memory‑corruption attempts. (research.kudelskisecurity.com)
• EDR telemetry and token elevation signals
• Hunt for sequences of:
• Local process spawning that leads to token manipulation (e.g., OpenProcessToken, SetTokenInformation)
• Unusual use of SeImpersonatePrivilege or impersonation token duplication
• Creation of scheduled tasks or services by non‑privileged users immediately following authentication events
• Network and protocol telemetry (supporting signals)
• While CVE‑2025‑54895 is local, cross‑referencing network logs for unusual SPNEGO negotiation attempts from local services that subsequently show anomalous user activity can help correlate suspected exploitation chains.
• Endpoint configuration audits
• Identify hosts with large numbers of local admin accounts, excessive service accounts, or developer/test tooling that grants local elevated rights — these amplify the practical risk of a local EoP.

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Why this vulnerability matters to enterprise defenders (risk analysis)​

• Elevated blast radius from low‑privilege footholds
• Attackers frequently chain lower‑privilege access (phished user, broken web app leading to user shell, or malicious macro) into local privilege escalations to persist and move laterally. A NEGOEX integer overflow that permits EoP shortens that chain significantly.
• Reputation and trust in authentication stacks
• Authentication mechanisms are in the trust path for multiple controls (file shares, service bindings, SSO). Flaws here are particularly valuable to attackers because they can create clean, high‑privilege tokens that appear legitimate to downstream systems.
• Patch prioritization nuance
• Because this CVE requires local authorization, some organizations might deprioritize it relative to remote RCE bugs. That is a mistake: an attacker who already has foothold inside — which is the most common real‑world scenario for targeted intrusions — can use a local EoP to seize domain admin or host compromise in minutes.
• Operational complexity
• Hotfixes to core authentication libraries can trigger side effects in SSO, smartcard, or PKI‑based flows. Careful staging, testing, and rollback planning are essential; at the same time, defenders must not delay fixes for long. Use a controlled and rapid rollout model to balance stability and security. (crowdstrike.com)

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Strengths of Microsoft’s fix and residual risks​

• Strengths
• Vendor patch closes the root arithmetic/logic issue in NEGOEX code paths. Provided KBs should be the authoritative remediation; MSRC is the canonical place to verify affected builds. (msrc.microsoft.com)
• Previous NEGOEX remediations give defenders detection playbooks and community guidance to accelerate triage.
• Residual risks and caveats
• Third‑party or embedded products that statically link Windows authentication libraries may not inherit fixes automatically—check vendor advisories for appliances, appliances firmware, or software appliances that bundle Windows auth stacks.
• Incomplete patch rollouts or misapplied updates keep the attack surface open; attackers commonly harvest partially patched estates.
• If defenders rely solely on patching and ignore least‑privilege and credential protection, the window for post‑compromise escalation remains.

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Practical Q&A: common admin concerns​

Q: Can this be exploited remotely?​

A: CVE‑2025‑54895, as described in Microsoft’s advisory, is a local elevation‑of‑privilege bug requiring an authorized local actor or process to reach the vulnerable code path. This differs from earlier remote NEGOEX RCEs that were exploitable over the network. Administrators should nonetheless check for chains that start with remote user compromise (phishing, web RCE) that lead to local exploitation. (msrc.microsoft.com, socprime.com)

Q: Should we block NEGOEX/Negotiate or disable PKU2U?​

A: Blanket disabling of negotiation mechanisms can break legitimate authentication flows. Temporary mitigations such as disabling PKU2U authentication (if not required) or limiting which hosts accept particular negotiation types may be appropriate for high‑risk systems. Always test changes in a lab because Kerberos/NTLM/PKI interactions are sensitive to policy changes. Where available, follow Microsoft’s guidance on settings that reduce exposure for specific NEGOEX issues.

Q: What is the priority ranking for this CVE?​

A: High for hosts that accept local logins from many users, for servers with many service accounts or delegated tasks, and for systems where local account compromise is plausible. For internet‑facing systems, prioritize remote RCE fixes; for domain controllers and servers with authentication duties, prioritize both remote and local NEGOEX patches. (crowdstrike.com)

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Verification and cross‑checks (transparency about what we can and cannot confirm)​

Microsoft’s MSRC advisory is the definitive reference for CVE‑2025‑54895 and must be used to confirm build‑level impact and KBs. The MSRC entry is rendered by a JavaScript app and should be consulted directly in a browser for the full advisory content. (msrc.microsoft.com)
As of publication, public analysis that mentions NEGOEX vulnerabilities more broadly (including RCE and other EoP CVEs) is abundant; many security vendors have published high‑level analysis and detection guidance for NEGOEX CVEs earlier in 2025. Those analyses support the technical context above and the operational recommendations for patching, monitoring, and compensating controls. However, if you require third‑party write‑ups specifically enumerating CVE‑2025‑54895 details (external CVE trackers, vendor blogs, or advisories), those may lag the MSRC advisory by hours or days — always confirm against MSRC for KB/patch artifacts before deploying. (socprime.com, rapid7.com)
If any public claims about exploit code, in‑the‑wild exploitation, or active weaponization are made, treat them with caution until multiple independent telemetry sources confirm exploitation patterns. Where possible, use EDR or network telemetry to corroborate suspicious activity before assuming active exploitation.

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Recommended next steps (concise action list for IT managers and security teams)​

• Open the MSRC advisory for CVE‑2025‑54895 and copy the KB and affected build matrix to your patch plan. (msrc.microsoft.com)
• Prioritize patching for domain controllers, authentication servers, and internet‑exposed hosts.
• Harden accounts by removing unnecessary local admin privileges and enabling credential protection features where available.
• Tune EDR/SIEM to detect LSASS faults, token duplication, impersonation calls, and unusual privilege escalations.
• Communicate patch windows and rollback plans to application owners to avoid unexpected SSO/service disruptions.
• Validate post‑patch authentication flows and finalize the rollout once compatibility is confirmed.

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Conclusion​

CVE‑2025‑54895 is another reminder that the negotiation layer of authentication — the code that decides which credentials to use and how to exchange them — is as mission‑critical as the credential store itself. An integer overflow in NEGOEX that allows local privilege elevation is a powerful tool for attackers who already have a foothold, and it demands rapid, coordinated action: confirm the MSRC advisory for your exact builds, test the supplied patches, and deploy them with urgency while shoring up credential protections and monitoring.
Treat NEGOEX and SPNEGO vulnerabilities as part of your identity‑centric risk model: patch quickly, harden account and token protections, and hunt proactively for evidence of local exploit attempts. The vendor advisory is the authoritative source for the fix; consult it directly and plan patching accordingly. (msrc.microsoft.com)

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Source: MSRC Security Update Guide - Microsoft Security Response Center