CVE-2025-55317: Local Privilege Escalation in MAU via Link Following

Microsoft has published an advisory identifying CVE-2025-55317, a local elevation-of-privilege flaw in Microsoft AutoUpdate (MAU) caused by improper link resolution before file access — commonly described as a link-following or symlink/junction weakness — that can allow an authorized local attacker to escalate to higher privileges on an affected host. The issue affects MAU’s file-access logic and, according to the vendor advisory, occurs when the updater follows attacker-controlled reparse points (or otherwise resolves links) before validating or restricting the privileged file operation, enabling the attacker to coerce MAU into performing writes or replacements in protected locations. The advisory for CVE-2025-55317 is hosted on the Microsoft Security Response Center. (msrc.microsoft.com)

Background​

What is Microsoft AutoUpdate (MAU)?​

Microsoft AutoUpdate (MAU) is Microsoft’s updater for non-Windows platforms (notably macOS) and for certain Microsoft apps. It runs background checks, downloads update payloads and executes update actions — tasks that inherently require elevated file operations and, in some configurations, root-level access. Because MAU runs with trusted privileges to modify application files and system resources, any bug in its file-handling or privilege checks is a high-value target for local attackers and malware authors. This class of issue has appeared multiple times across MAU advisories in 2024–2025, showing a recurring pattern around unsafe file operations and privilege management. (nvd.nist.gov, app.opencve.io)

What “link following” means and why it matters​

A link-following vulnerability arises when privileged code performs file operations (create, write, move, delete, or load) and mistakenly follows filesystem reparse points — symbolic links, junctions, or other reparse types — that were created by a less-privileged user. If MAU resolves a reparse point that points to a protected system path or another user’s resource, the privileged operation can be redirected to an attacker-controlled target. This can result in overwriting system binaries, replacing configuration files, or planting malicious executables that will later be executed with higher privileges.
This pattern is well understood: trusted maintenance utilities, installer services, and auto-update agents routinely combine elevated privileges with operations touching user-writable directories. When they do not canonicalize and validate targets (ownership, existence, and expected path), they become an elevation vector. Recent MAU advisories show Microsoft patching variants of this class repeatedly, underscoring the recurring nature of the risk. (app.opencve.io)

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The CVE-2025-55317 advisory — summary and verification​

• Advisory headline (vendor): Improper link resolution before file access ('link following') in Microsoft AutoUpdate (MAU) allows an authorized attacker to elevate privileges locally. (msrc.microsoft.com)
• Affected component: Microsoft AutoUpdate (MAU).
• Impact: Local elevation of privilege (EoP) — attacker with some account-level access on the host can trick MAU into performing privileged file operations.
• Exploit vector: Local (requires the attacker to be able to run code or place files with their user permissions on the machine).
• Public exploit status: As of this advisory publication, no public proof-of-concept exploit was distributed by Microsoft; public exploit availability is often tracked in public CVE/NVD feeds (see note below). (msrc.microsoft.com, nvd.nist.gov)

Verification and cross-check:

• Microsoft’s MSRC page for CVE-2025-55317 is the authoritative advisory location. That page is served as a client-side application and requires JavaScript to render the full content; an automated fetch returned the page shell. For definitive remediation steps and exact versions, administrators should use the MSRC advisory referenced by Microsoft. (msrc.microsoft.com)
• Similar MAU CVEs published earlier in 2025 (for example CVE-2025-47968 and other MAU-related CVEs) document the same family of risks — improper input validation, improper privilege management, and link-following behaviors — and have been tracked in the NVD and independent vendor reports. Cross-referencing these prior entries helps validate the technical pattern described in Microsoft’s advisory for CVE-2025-55317. (nvd.nist.gov, app.opencve.io)

Cautionary note: public aggregators and vulnerability databases sometimes lag vendor pages or index different CVE identifiers in bundled advisories. For this reason, defenders must treat the vendor advisory as authoritative for exact affected versions, CVSS scores, and patch identifiers; the advisory is the primary source for remediation instructions. (msrc.microsoft.com)

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Technical analysis — how an attacker would abuse this flaw​

Attack prerequisites​

• Local access to the target host as an authorized (non-privileged) user, or the ability to run code under such an account.
• The ability to create files and, crucially, create or manipulate reparse points (junctions/symlinks) in locations the attacker can write to (typically under the user profile, temp directories, or application-controlled paths).
• Absence of host hardening that prevents reparse-point creation or that restricts the updater from loading resources from user-writable directories.

Typical exploitation chain (conceptual)​

• The attacker creates a specially crafted reparse point in a directory MAU can access (for example, a temp folder or an app cache).
• The attacker places or prepares a malicious payload under the reparse point target or arranges for MAU to copy or move a file through that location.
• MAU, running with elevated privileges, resolves the reparse point and performs the privileged file action (write/replace/move) into a protected target — for example, replacing a library, service binary, or configuration file that will be executed by a privileged process.
• The attacker triggers or waits for a privileged process to run the attacker-owned artifact, resulting in local privilege escalation to root (macOS) or SYSTEM/administrator (Windows, where MAU equivalents exist).

This is a canonical TOCTOU/link-following class of bug; prior MAU advisories and independent writeups show similar sequences in practice. (cybersecurity-help.cz)

Why the requirement “authorized attacker” matters​

Microsoft’s advisory phrasing typically indicates whether exploitation is possible from an unauthenticated remote context, an unauthenticated local user, or an authorized local user. CVE-2025-55317 is described as requiring an authorized actor — meaning the attacker needs local credentials on the machine. That makes the bug less likely to be used for remote, pre-authentication compromise, but extremely useful for post-compromise privilege escalation, lateral movement, ransomware escalation, or malicious insiders. Many real-world incidents chain a local EoP like this with an initial foothold delivered by phishing or malicious documents.

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Real-world impact scenarios​

• Workstation escalation: A non-admin user on a corporate workstation exploits the MAU bug to obtain SYSTEM/root and then disables security controls, exfiltrates credentials, or plants ransomware.
• Developer/build servers: Developer machines and CI/build agents commonly run tools that interact with system locations. A single compromised developer workstation using MAU’s updater path can result in supply-chain-like persistence or build-time tampering.
• Multi-user or shared hosts: Kiosks, lab PCs, or shared macOS workstations are higher-risk because multiple users have local access and can create or place reparse points.
• Targeted insider threat: An authorized user with minimal privileges abuses MAU to create persistent elevated access without triggering obvious network signatures.

The overarching theme: even if exploitation is local, the blast radius is significant because MAU is a trusted updater that touches critical files; the vulnerability becomes a force multiplier for attackers once they obtain any local foothold.

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Detection and telemetry — what to hunt for​

Successful detection is difficult because MAU operations can appear normal in process and file logs. Effective detection hinges on correlating reparse-point creation or unexpected file operations in user-writable directories with subsequent privileged file writes into system locations.
Suggested telemetry sources:

• Sysmon (if deployed): EventID 11 (FileCreate), EventID 15 (FileCreateStreamHash), EventID 1 (ProcessCreate), EventID 7 (ImageLoad).
• Windows/macOS file-system auditing: monitor for reparse point (symlink/junction) creation in user profile and temp directories.
• EDR alerts: watch for processes normally associated with MAU performing file modifications in system directories.
• File integrity monitoring (FIM): unexpected changes to binaries in /usr/local/bin, /Library, /Applications, C:\Program Files, or C:\Windows.

Example hunts (copy/adapt to your platform):

• PowerShell (Windows) pseudo-hunt to find reparse points in user temp/AppData:
• Get-ChildItem -Path C:\Users\AppData\Local\Temp,C:\Users\AppData\Roaming -Recurse -Force | Where-Object { $_.Attributes -band [IO.FileAttributes]::ReparsePoint } | Select FullName, LastWriteTime
• Sysmon/ELK pseudo-correlation:
• Alert when a reparse point is created (FileCreate or FileCreateStreamHash) AND within N minutes, MAU process opens/writes to a path under C:\Windows or /usr/bin.

Community hunting guidance for MAU-style link-following EoP has coalesced around the above signals; Microsoft and independent analysts recommend correlating reparse-point creation with immediate privileged writes by the updater process.

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Immediate mitigations and long-term mitigations​

Immediate operational actions (for admins and users)​

• Apply the vendor patch as the primary mitigation — follow Microsoft’s advisory for the exact update and version numbers. The vendor advisory is the authoritative source for the precise patch package. (msrc.microsoft.com)
• If an immediate patch is not possible for operational reasons:
• Restrict local account capabilities and reduce the number of users with installation or update privileges.
• Harden directories MAU touches: reduce write permissions to temp and application cache directories where appropriate.
• Temporarily restrict MAU execution via policy controls or local configuration to prevent automatic runs while you plan patch deployment.
• Increase monitoring and hunt for indicators described in the Detection section; collect forensic evidence if suspicious activity is detected.

Recommended long-term controls​

• Remove privileged code paths that load resources from user-writable directories. Design update agents to use dedicated, sandboxed temporary directories with appropriate ownership checks.
• Enforce stricter path canonicalization: verify ownership and that the final resolved path is within an expected safe directory and not a reparse target.
• Implement signed update artifacts and integrity checks to reduce the value of arbitrary file writes even if an attacker can manipulate a path.
• Apply least privilege to system management utilities: minimize the time and scope that an updater runs with elevated tokens.

Microsoft’s prior fixes for MAU and similar updater bugs have focused on not following reparse points, validating path ownership, and hardening search and load paths — these are canonical developer-side remediations for link-following weaknesses.

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Verification, cross-references, and what we could not confirm​

• Microsoft’s MSRC advisory page for CVE-2025-55317 is the primary, authoritative source of information; it is the document administrators should follow for exact patch identifiers, affected versions, and CVSS scoring. The advisory page is client-side rendered and needs a browser to view the full content. (msrc.microsoft.com)
• Public vulnerability trackers frequently index MAU issues: multiple MAU CVEs in 2025 (for example CVE-2025-47968, CVE-2025-29800, CVE-2025-24036, and others) document similar local EoP patterns and show how Microsoft grouped and fixed them across updates. These public CVE/NVD/OpenCVE records provide independent confirmation of the technical class, though exact mappings between MSRC identifiers and aggregator entries should be cross-checked when applying enterprise patches. (nvd.nist.gov, app.opencve.io)
• At the time of writing, no public exploit or in-the-wild mass exploitation was recorded for CVE-2025-55317 in public feeds that index active exploit telemetry. That said, historically, link-following EoP bugs are valuable post-compromise primitives and warrant urgent remediation in enterprise environments. This statement is based on the absence of publicly posted proof-of-concept exploit code in mainstream feeds; defenders should assume exploitation may be possible and follow Microsoft’s remedial guidance. (cybersecurity-help.cz)

Unverified or potentially ambiguous claims:

• If incident responders find third-party posts or forum threads that refer to a different CVE identifier for the same MAU behavior, treat the vendor advisory as the source of truth. Public aggregators sometimes list related CVEs under separate identifiers or consolidate multiple fixes in a single advisory; cross-check version numbers and KB/patch IDs before acting.

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Practical checklist — step-by-step for IT teams​

• Identify endpoints running MAU: inventory Microsoft Office, MAU agent versions on macOS endpoints and any other MAU-managed apps.
• Confirm whether CVE-2025-55317 patch is published for your builds via MSRC; obtain KB numbers or package identifiers from Microsoft’s advisory page. (msrc.microsoft.com)
• Prioritize high-risk hosts: shared machines, developer workstations, remote employees’ machines, and servers used for building or packaging software.
• Deploy the Microsoft update using your existing patch management tooling (WSUS, Intune, Jamf Pro, Munki, or vendor-specific update workflows).
• After patching, re-run hunts for reparse points and check for suspicious correlations; look for unexpected file modifications that occurred before the patch.
• If signs of compromise are found, escalate to full incident response: collect volatile memory, preserve system and EDR logs, and isolate affected hosts.

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Why this class of bug keeps recurring (analysis and recommendations for vendors)​

The link-following EoP family is a recurring trouble spot for maintenance utilities because development and QA often focus on happy-path updates, not adversarial filesystem states. Reparse points are a legitimate filesystem feature, and unless developers actively defend against reparse-target attacks (canonicalization, ownership checks, avoiding user-writable search paths), privileged operations are at risk.
Vendors shipping update agents should:

• Treat any resource loaded from user-writable directories as untrusted.
• Avoid elevated operations that implicitly trust user-controlled path components.
• Use dedicated, strictly permissioned temporary directories for update staging.
• Add automated tests that simulate attacker-controlled reparse points and verify safe handling.

These are engineering fixes that prevent whole classes of future EoP bugs. Multiple prior MAU fixes adopted this pattern.

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Closing assessment​

CVE-2025-55317 is a classic and dangerous example of a local elevation-of-privilege bug rooted in link following by a privileged updater. While it requires an attacker with local access, its potential for post-compromise escalation — especially on shared, developer, or poorly hardened endpoints — makes it a high-priority patch for administrators. The authoritative guidance and patch instructions live on Microsoft’s MSRC advisory for CVE-2025-55317; organizations should treat the vendor advisory as the definitive source, apply updates promptly, and deploy the detection and hardening steps outlined above to reduce residual risk. (msrc.microsoft.com, nvd.nist.gov)
Additional context and community analysis on the same class of MAU issues (link-following, improper input validation, improper privilege management) are available through public CVE/NVD entries and independent vulnerability trackers; defenders should cross-check the MSRC advisory against those references when planning patch rollouts and hunts. (app.opencve.io, cybersecurity-help.cz)

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The immediate action remains straightforward and urgent: locate MAU instances in your environment, confirm Microsoft’s published fixed versions for CVE-2025-55317 on the MSRC advisory page, and deploy the vendor-supplied updates without delay. (msrc.microsoft.com)

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Source: MSRC Security Update Guide - Microsoft Security Response Center