CVE-2025-62556: Excel Remote Code Execution Explained (AV L vs AV N)

Microsoft’s advisory for CVE-2025-62556 labels the issue as a Microsoft Excel Remote Code Execution vulnerability, yet the published CVSS vector shows an Attack Vector of Local (AV:L) — a seemingly contradictory pairing that, on closer inspection, reflects two different ways of answering two separate questions about the same flaw.

Background / Overview​

Microsoft’s Security Response Center and many vendor advisories commonly use the short phrase Remote Code Execution (RCE) in CVE titles to signal impact and attacker reach: a remote adversary can ultimately cause arbitrary code to run on a victim system. CVSS, by contrast, is a mechanistic scoring system that decomposes exploitability into discrete metrics. One of those metrics — Attack Vector (AV) — describes where the vulnerable code must execute at the moment the exploit triggers. When that moment occurs inside a local application process (for example, Excel parsing a spreadsheet on an endpoint), CVSS records AV:L, even though the malicious file may have been delivered remotely by email, cloud link, or web download. This intentional separation explains the headline-versus-vector mismatch.
The distinction matters: the CVE title is an operational alarm bell — it tells defenders what an attacker can achieve and from where they might initiate the attack — while the CVSS AV field clarifies the technical mechanics required for the vulnerability to be exploited. Treating these as complementary signals gives a fuller, more actionable risk picture.

Why “Remote” appears in the CVE title while CVSS shows AV:L​

Two different questions, two different answers​

• CVE/advisory title: What can the attacker achieve and from where can they attempt it?
  The title “Remote Code Execution” signals that an external actor — not physically present — can, through some delivery channel, cause arbitrary code to run on target machines if the exploitation chain completes.
• CVSS Attack Vector: Where and how must code run at the moment the vulnerability is triggered?
  CVSS’s AV metric captures the locality of the triggering operation. If the vulnerable code executes as part of a local process parsing a file on an endpoint (for instance, Excel opening a workbook), the correct classification is AV:L. CVSS deliberately separates the delivery phase from the trigger phase to avoid “double-counting” the network delivery step and to preserve comparability across thousands of vulnerabilities.

Practical phrasing: remote delivery, local execution​

A typical Excel parsing exploit chain looks like this:

• Attacker crafts a malicious workbook designed to trigger a memory-safety defect (use-after-free, heap overflow, OOB write/read, etc..
• The attacker delivers the file remotely (spear-phishing email, shared cloud link, public download, or file share).
• A victim opens (or a client-side preview component renders) the workbook in local Excel.
• Excel’s parser processes the malformed content, triggers memory corruption, and attacker-supplied code executes in the context of the Excel process.

Delivery occurs remotely; the exploit trigger happens locally — hence RCE in the CVE title and AV:L in CVSS. This is canonical for document-parsing vulnerabilities.

The technical anatomy: how a file-only exploit becomes RCE​

Why Excel parsing is a high-risk surface​

Excel supports complex file formats, legacy binary records, Open XML parts, embedded OLE objects, ActiveX controls, formula engines, and various native renderers. Many of these components are implemented in native C/C++ code running inside the Excel process. Native parsers historically present fertile ground for memory-safety bugs — use-after-free, heap overflow, type confusion, and out-of-bounds read/write — which attackers can weaponize by controlling file contents. When an attacker controls enough of a file’s structure, they can manipulate parser state, corrupt memory, and ultimately hijack control flow to execute arbitrary payloads without using macros or scripts. These are data-only exploits and are particularly dangerous because they bypass macro-blocking mitigations.

How the exploit chain works in practice​

• Initial foothold: Malicious spreadsheet arrives via a low-friction channel — email attachment, cloud share, or a compromised website.
• Trigger: The victim opens the file in Excel (or a preview pane uses the same vulnerable parser), which executes native code paths that interpret the file structure.
• Memory corruption: The malformed document causes the parser to corrupt memory in a controlled way (for example, overwriting a function pointer or vtable).
• Payload execution: The attacker’s crafted payload is invoked, running with the privileges of the logged-in user (often limited but sufficient for data theft, lateral movement, or follow-on payload installation).

Because user interaction is usually required (opening or previewing the file), CVSS commonly records UI:R (User Interaction required) alongside AV:L in such scenarios. But the operational risk remains high because the attacker’s delivery path is remote and delivery channels are ubiquitous.

The critical exception: when AV becomes Network (AV:N)​

There is one operationally important exception to the “delivery remote, trigger local” model: server-side parsing.
If a network-exposed service or gateway (e.g., email attachment previewer, Office Online Server, collaborative platform, or document-conversion service) parses the same vulnerable file using the affected parser on behalf of remote clients, then the vulnerable code runs inside a network-bound process. In that case the CVSS Attack Vector should be AV:N, because an attacker can trigger the vulnerable code remotely without requiring the target endpoint user to open the file.
This distinction materially changes exposure: a client-only AV:L bug generally requires social engineering and per-victim interaction, whereas a server-side AV:N bug can be exploited at scale against any user of the service without explicit user action. Advisories highlight this server-side risk when applicable; defenders must inventory where document parsing occurs in their environment and treat server-side hosts as critical patch targets.

Triage and prioritization: what defenders must understand​

Why AV:L is not “low risk”​

Interpreting AV:L as low priority is a dangerous mistake. Several practical facts make Excel RCEs attractive to attackers:

• Ubiquity of Excel: Excel is widely deployed across enterprises and consumer devices, increasing the probability that malicious documents will reach a user.
• Low-friction delivery: Email, cloud collaboration, and shared drives enable massive distribution at minimal cost.
• Effective social engineering: Phishing, invoice lures, and contextual document baits reliably persuade recipients to open attachments.
• Preview and server-rendering: Many environments render attachments or previews automatically, which can remove explicit user actions and increase exposure.

Therefore, even when CVSS lists AV:L, RCE-class advisories should be treated as urgent and prioritized for patching and mitigation.

Practical triage checklist (operations-friendly)​

• Confirm affected versions: Map Excel/Office builds across your estate against Microsoft’s advisory and deploy the matching security updates promptly. Patching is the definitive remediation.
• Inventory parsing endpoints: Identify servers and services that parse or preview Office documents (email gateways, webmail, Office Online Server, DLP engines, CMS converters). Treat those hosts as high-priority if they use the affected parsing stack.
• Harden ingestion paths:
• Quarantine or block suspicious attachments.
• Enforce attachment type policies and strip high-risk content where possible.
• Route attachments through sandbox/detonation services.
• Enforce Protected View & Isolation:
• Ensure Office opens files from the Internet in Protected View.
• Deploy Application Guard or similar isolation for Office where available.
• Limit macro risk: Disable automatic macros, allow only signed macros from trusted publishers, and enforce application control policies.
• Least privilege and app control: Ensure users operate as standard (non-elevated) accounts and apply AppLocker, WDAC, or other policies to restrict post-exploitation activity.
• EDR & detection: Tune Endpoint Detection and Response to detect unusual child processes, persistence attempts, and telemetry consistent with exploit chains.
• User awareness & phishing-resistant MFA: Train users and apply phishing-resistant multi-factor authentication to reduce the success rate of follow-on account takeover attempts.

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Mitigations and technical hardening (detailed)​

• Patch management: Prioritize Microsoft’s Excel/Office updates through your existing patch channels (Microsoft Update, WSUS, Intune, MECM). This is non-negotiable — timely patch deployment is the strongest protection.
• Mail gateway controls: Enable attachment sandboxing, disable automatic content previews for external mail, and quarantine potentially malicious attachments for analysis.
• Disable automatic preview panes: On clients where feasible, disable preview panes in Outlook or other mail clients that render Office formats automatically.
• Application isolation: Use Office Protected View for files from the Internet and supplement with Application Guard where available to contain parsing processes.
• Application control: Use AppLocker or Windows Defender Application Control policies to restrict which binaries may execute and which processes Office can spawn.
• Network segmentation: Isolate servers that perform document parsing or conversion from general user traffic and apply strict egress/ingress filtering.
• Monitoring & threat hunting: Look for anomalous Excel child processes, suspicious command-line activity, and unusual network connections originating from Office processes.

These layered mitigations reduce the window of opportunity for attackers and make successful exploitation, follow-on activity, and lateral movement more difficult.

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Attack surface nuance: what to audit in your environment​

• Which endpoints have Excel installed, and which build numbers do they run?
• Do any central services (mail gateways, DLP, webmail, document previewers, Office Online) perform server-side parsing or rendering of uploaded Office files?
• Are there business workflows that rely on automated preview or conversion of Excel files?
• What percentage of your user base runs with administrative privileges or has local admin rights?
• How quickly can your organization deploy critical patches to endpoints and servers?

If server-side parsing is present, treat exposure as effectively network-accessible and move those assets to the top of the patch queue. If only client-side parsing exists, prioritize patching but also reduce user exposure by hardening ingestion and enforcing Protected View and application isolation.

Risk assessment and likelihood of exploitation​

Even when a vulnerability requires local triggering (AV:L), the likelihood of exploitation remains significant because:

• Delivery mechanisms are trivial and inexpensive for attackers.
• Social engineering success rates are non-trivial in enterprise contexts.
• Document-based RCEs are often data-only and can bypass macro restrictions.
• Historical patterns show that once reliable exploit techniques exist for native parsers, weaponization follows rapidly.

Many recent Excel CVEs have carried high CVSS base scores and have been rapidly exploited or weaponized in targeted phishing campaigns. Organizations should treat CVE-2025-62556 and similar advisories as urgent, even if early public reporting shows no confirmed active exploitation.

Strengths and risks of vendor messaging — critical analysis​

Notable strengths​

• The “Remote Code Execution” headline is an effective triage signal: it communicates maximum potential impact rapidly to defenders, prompting prompt attention.
• CVSS metrics provide the granular mechanics needed for automated risk-scoring systems and triage logic.
• When read together, the title plus CVSS encode both impact and technical exploitability in a way that enables more nuanced response decisions.

Potential risks and sources of confusion​

• Organizations that ingest only CVE titles in automated tooling may misinterpret a Remote Code Execution label as implying AV:N and prioritize incorrectly.
• Conversely, teams that focus only on CVSS AV may under-prioritize an RCE-class advisory because they infer “local = low risk.”
• Overlooking server-side parsing services is a common blind spot; those services can convert AV:L client exploits into AV:N server-side catastrophes if left unpatched.
• Incomplete understanding of the difference between delivery origin and execution locality can lead to misallocated mitigation efforts.

The correct operational posture is to read both the title and the CVSS metrics, verify the presence (or absence) of server-side parsing paths in your environment, and prioritize remediation accordingly.

How to communicate this to stakeholders (concise messaging)​

• For IT leadership: “CVE-2025-62556 is an Excel vulnerability that can result in arbitrary code execution if a user opens a crafted spreadsheet. Microsoft’s advisory lists the impact as Remote Code Execution, but CVSS lists Attack Vector as Local (AV:L) because exploitation requires a local parser to run. Patch endpoints and any servers that parse Office documents immediately.”
• For security operations: “Audit email gateways, preview services, Office Online, and any server-side document processors for exposure. Apply mitigation controls (quarantine attachments, enable Protected View, and enforce application isolation) while deploying patches.”
• For end users: “Do not open untrusted Excel attachments. If a document is unexpected, verify with the sender by a separate channel before opening. Report suspicious attachments to security.”

Conclusion​

The apparent contradiction between the CVE title “Remote Code Execution” and CVSS Attack Vector AV:L for CVE-2025-62556 is not an error but a reflection of two complementary communication goals: the vendor headline warns defenders that a remote adversary can cause arbitrary code to run on endpoints, while CVSS documents the technical context of the exploit trigger, which in many document-parsing cases happens inside a local process. Both messages are valuable and must be read together to form a correct risk picture.
Operationally, treat Excel RCE advisories as high priority regardless of AV:L. Patch quickly, inventory parsing points in your environment (clients and servers), harden ingestion paths, enforce isolation and least privilege, and prioritize servers that render documents for external users. Doing so reduces the risk of successful exploitation and limits the blast radius if an exploit emerges.

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Source: MSRC Security Update Guide - Microsoft Security Response Center