CVE-2025-62557: High Risk Office UAF Memory Bug and Immediate Patch Guide

Microsoft’s advisory for CVE-2025-62557 confirms a memory‑corruption flaw in Microsoft Office that can be weaponized for local remote‑code‑execution (RCE) scenarios — a use‑after‑free (UAF) in Office’s document parsing that, if chained successfully, allows attacker code to run with the privileges of the affected Office process.

Background / Overview​

Microsoft’s Security Response Center (MSRC) lists CVE‑2025‑62557 in its December 2025 update rollup for Microsoft Office, characterizing the issue as a Use‑After‑Free (CWE‑416) leading to Remote Code Execution when a specially crafted Office file is parsed. That advisory entry is the authoritative record for the vulnerability, and it is the starting point for any enterprise triage and remediation activity. Because MSRC’s update guide is the canonical mapping of CVEs to KBs and product builds, operations teams should always confirm the exact per‑SKU KB numbers for their estate before declaring systems remediated.
Independent vulnerability aggregators and patch summaries corroborate the presence and impact of CVE‑2025‑62557. Public feeds report a CVSS v3.1 base score in the high range (8.4) for this CVE and classify the bug as a high‑impact, local‑execution memory corruption that does not require prior privileges but typically requires the vulnerable component to parse a crafted file. There is some variation in public reporting about whether the vendor classifies the issue as critical in headline language — a discrepancy that often arises for Office parsing bugs because the vendor’s impact label (“Remote Code Execution”) focuses on attacker outcome while CVSS emphasizes the exploit mechanics (local parsing vs. network‑facing).

Why this matters now​

Microsoft Office remains a top initial‑access vector for attackers because adversaries have effective delivery channels (phishing, cloud share links, collaboration attachments) and many modern document exploits do not rely on macros or scripting. When parsing code contains memory vulnerabilities — use‑after‑free, out‑of‑bounds writes/reads, or type confusions — skilled exploit authors can convert those primitives into reliable execution chains even in the presence of modern mitigations like ASLR, DEP/NX and CFG. The practical consequence is that a successful exploit can lead to code running in the context of the logged‑on user and then be leveraged for credential theft, lateral movement, ransomware, or persistent backdoors.

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What we know about CVE‑2025‑62557​

Technical summary (authoritative points)​

• Vulnerability class: Use‑After‑Free (UAF) in Microsoft Office’s document parsing code.
• Impact: Remote Code Execution — attacker‑controlled data can result in arbitrary code executing under the Office process identity.
• CVSS (public aggregator): CVSS v3.1 base score ~8.4 (high). Public feeds list vector components consistent with a local parsing requirement (AV:L) but with high confidentiality/integrity/availability impact.
• Exploitability: Public trackers flag the flaw as a local parsing bug (an attacker must cause a vulnerable Office process to parse the malformed file), but server‑side document processors or preview services that automatically parse uploads can convert the vector into a network‑accessible attack path.
• Vendor status: Microsoft published the CVE in its December 2025 Patch Tuesday set and mapped affected Office packages to per‑SKU KB updates; the MSRC update guide is the authoritative KB mapping resource. Because the MSRC UI is JavaScript driven, administrators should use the Microsoft Update Catalog or their patch management tools to retrieve exact package identifiers for their environments.

Known vs. unknown​

Known:

• The defect exists and Microsoft published a fix mapping for affected Office packages — treat the issue as confirmed and actionable.

Unknown or not publicly disclosed (as of publication):

• Microsoft has not published low‑level exploit code or a public proof‑of‑concept (PoC) in raw advisory text; public PoCs often follow independent research reports or when attackers weaponize patches. Until a vendor technical write‑up or trusted researcher blog emerges, details of the exact heap/stack manipulation and exploit reliability remain internal. Flag any granular exploitation claims that lack vendor or multi‑source technical corroboration.

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Attack scenarios and real‑world risk​

Typical exploitation chain​

• Attacker crafts a malicious Office document or embeds a malformed object (image, metafile, OLE blob) that triggers the UAF code path.
• Delivery occurs via phishing email, shared cloud link, a compromised website, or an uploaded file to a web service.
• The victim’s Office application parses the file (either via an explicit open or an automatic preview/thumbnail generation), triggering memory corruption.
• The attacker’s code executes in the context of the Office process. From there, the attacker can drop additional payloads or attempt privilege escalation.

Why “local” doesn’t mean “safe”​

CVSS Attack Vector AV:L (Local) can be misleading for defenders. Office document parsing vulnerabilities are often scored AV:L because the vulnerable code runs on the host, but the delivery of the malicious document is trivial at scale via email or cloud shares. The bigger operational risk is server‑side services that automatically parse or render user uploads (webmail preview engines, CMS attachment processors, thumbnailing services), which transform local parsing bugs into network‑accessible attacks. In those configurations the exposure is functionally AV:N (Network). Triage must therefore consider both endpoints and any server that processes user‑supplied documents.

Evidence of exploitation​

• At time of initial public advisory, there was no authoritative public PoC tied to CVE‑2025‑62557. Aggregators report the CVE and classify it as high‑impact; however, the absence of a confirmed PoC or observed in‑the‑wild exploitation at publication does not imply low priority — Office RCEs historically see rapid weaponization. Treat the advisory as urgent and assume attackers will attempt to reverse‑engineer patches or develop exploits.

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Immediate actions for administrators (prioritized checklist)​

• Patch: Identify the exact Office SKUs and build numbers in your environment and apply the Microsoft updates that map to CVE‑2025‑62557 via Microsoft Update, WSUS, MECM/Intune, or the Update Catalog. Patching is the definitive remediation step.
• Inventory and prioritize:
• Map all servers and services that accept or parse user uploads (webmail, SharePoint/Collab, CMS, OCR services, thumbnailing pipelines). These are top priority for immediate patching or isolation.
• Identify critical admin endpoints (jump hosts, finance PCs) and push updates there early in the rollout.
• Compensating controls (if immediate patching is impossible):
• Disable automatic preview panes in Exchange/Outlook and file‑explorer preview handlers for at‑risk groups.
• Enforce Office Protected View for files from the internet and untrusted sources.
• Route attachments through a mail gateway sandbox/detonation chamber; quarantine suspicious documents.
• Apply Attack Surface Reduction (ASR) rules and application allow‑listing (AppLocker/WDAC) to stop Office apps from spawning child shells.
• Hardening and detection:
• Ensure EDR/AV telemetry collection is enabled and hunt for Office processes spawning unexpected children (cmd.exe, powershell.exe, wscript.exe).
• Alert on unusual outbound connections immediately following Office document opens and on new persistence artifacts (scheduled tasks, services, run keys).
• Communication and rollback planning:
• Stage updates (pilot → phased → full) when possible, but prioritize internet‑facing servers and devices with upload/preview capabilities for immediate remediation.
• Take backups and have rollback plans for critical systems. Validate update installs by verifying KB numbers in your patch reporting.

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Detection & response: hunting recipes​

Behavioral detection outperforms simple signature matching for memory‑safety exploits that don’t rely on macros. Focus on these signals:

• Office app (WINWORD.EXE, EXCEL.EXE, POWERPNT.EXE) spawning command interpreters or unusual child processes.
• Immediate network egress from Office processes to suspicious destinations after document opens.
• New scheduled tasks, services or startup registry keys following an Office process execution.
• Mail gateway sandbox detonations that flag document parsing anomalies or abnormal system calls.
• EDR memory artifacts showing abnormal heap/free chains or crashes within Office parsers — preserve these for vendor coordination and forensic analysis.

Containment steps on suspected compromise:

• Isolate the endpoint from the network and preserve forensic evidence (EDR snapshot, the malicious file, email headers).
• Hunt for lateral movement artifacts and credential theft indicators. Revoke exposed sessions and rotate credentials if compromise is confirmed.

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Technical analysis and exploitation potential​

Exploitation primitives​

Use‑after‑free bugs are a common root cause for Office RCEs. The attack typically requires precise manipulation of memory layout (heap grooming) to place attacker‑controlled data where a freed pointer will be dereferenced. Common exploitation techniques include:

• Grooming the heap with predictable allocations to control the content at freed memory addresses.
• Overwriting vtable entries or function pointers to redirect execution flow to attacker code.
• Chaining information‑disclosure primitives (out‑of‑bounds reads) to defeat ASLR and other mitigations before triggering code‑execution.

Difficulty and realism​

Modern mitigations increase exploitation complexity, but they do not make exploitation impractical. Skilled exploit developers can convert memory primitives into reliable RCEs — particularly in targeted attacks where conditions can be tested and tuned. The real‑world ease of exploitation depends on many variables (platform build, defender telemetry, presence of mitigations like CFG/ASLR, and whether server‑side parsing is involved). Always assume motivated actors will attempt weaponization.

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How to verify a system is patched (practical steps)​

• Use the Microsoft Update Catalog or your enterprise patch tool to match CVE‑2025‑62557 to the exact KB(s) for the Office installation type (Click‑to‑Run, MSI, LTSC, macOS/Android variants). MSRC lists the CVE but maps to multiple per‑SKU KBs that must be matched to your builds.
• Confirm patch install:
• For Windows desktops running Office MSI: check Control Panel → Programs & Features → View installed updates, or query WMI for the KB.
• For Click‑to‑Run Office (Microsoft 365 Apps): check the build number and apply the appropriate Channel update; Microsoft’s update history pages list the fixed builds.
• For macOS/Android Office variants: install the vendor updates via the App Store/Play Store or managed app deployment platform.
• Validate endpoint telemetry after patching: ensure there are no post‑patch regressions, and monitor EDR for unusual Office process behavior during and immediately after the rollout.

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Report Confidence: what it is and why it matters for CVE triage​

MSRC and many vendors include a report confidence signal that describes how certain the public record is about the vulnerability’s technical root cause. This is operationally important:

• Confirmed: vendor acknowledgement and a validated patch exist — treat remediations as definitive and patch immediately.
• Reasonable: independent researcher analysis supports a likely root cause — treat as high priority because public details can enable rapid exploitation.
• Unconfirmed: only the existence is publicized with no technical detail — triage by impact class and exposure.

For CVE‑2025‑62557, Microsoft’s advisory presence and inclusion in the December 2025 update set indicate a confirmed remediation mapping; organizations should treat the issue as actionable and prioritize patching accordingly. Still, flag any low‑level exploit claims that lack multi‑source corroboration and handle them cautiously.

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Risks, strengths and operational pitfalls​

Strengths of Microsoft’s advisory model​

• The MSRC update guide provides a single, vendor‑authoritative mapping of CVEs to KBs and builds, which is essential for accurate patching.
• Using an impact label like “Remote Code Execution” signals the operational severity clearly to both technical and non‑technical stakeholders.

Common operational pitfalls to avoid​

• Relying solely on CVE headline language without reading the CVSS vector and advisory body can misprioritize responses; “Remote” in the title describes attacker origin and impact, while the CVSS Attack Vector documents which local or network steps are required.
• Using third‑party mirrors for KB→CVE mapping without validating against MSRC or the Microsoft Update Catalog can lead to deploying the wrong package for a build. Always verify KB numbers per SKU.
• Underestimating server‑side risks: forgetting that webmail and preview services may parse documents and convert a local parsing vulnerability into an unauthenticated network attack surface. Prioritize patching of any document‑processing server.

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Long‑term hardening and risk reduction​

• Enforce least privilege: standard user accounts reduce the impact of successful document exploitation.
• Apply application allow‑listing (AppLocker/WDAC) and ASR rules that limit Office from launching interpreters or executables.
• Use mail‑gateway sandboxing and detonation for unknown senders and attachments.
• Maintain robust EDR telemetry and retention to allow post‑incident hunts and memory analysis for modern memory corruption exploits.

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Conclusion​

CVE‑2025‑62557 is a confirmed use‑after‑free vulnerability in Microsoft Office that presents a high operational risk because it enables arbitrary code execution in the context of Office processes. Aggregated public scoring places the CVSS base around 8.4 while vendor advisories emphasize the RCE outcome; the difference in emphasis reflects the tension between impact‑first messaging and CVSS exploitation mechanics. Administrators should treat the advisory as urgent: map per‑SKU KBs from Microsoft’s update guide, prioritize patching for endpoints and any server that parses uploaded documents, enforce Protected View and mail‑gateway sandboxing, and hunt for the behavioral indicators described above. Where immediate patching is not possible, use the compensating controls listed to reduce exposure while you deploy vendor updates. Note: MSRC remains the authoritative source for KB mappings; always validate your chosen update packages against Microsoft’s update catalog for the specific Office channel and build you manage. If low‑level exploit details surface from multiple trusted researchers, adjust detection and mitigation plans accordingly — and flag any unverified technical claims until they are corroborated by both vendor and independent analyses.

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Source: MSRC Security Update Guide - Microsoft Security Response Center