Microsoft’s advisory classifies CVE-2025-54901 as a buffer over-read (out‑of‑bounds read) in Microsoft Office Excel that can disclose process memory contents when a crafted spreadsheet is opened.
In Office/Excel, these out‑of‑bounds reads often occur in code paths that deserialize nested binary records or reinterpret buffers across object types (for example, shape metadata, embedded objects, OLE items, or nested workbook records). Attackers craft malformed values that cause mis‑sized allocations or invalid casts; when code later reads those regions incorrectly it can leak memory. Historical vendor advisories for Excel in 2025 describe this same pattern across multiple CVEs.
Common delivery vectors that attackers use:
If you’d like, I can:
Source: MSRC Security Update Guide - Microsoft Security Response Center
Executive summary
- What it is: CVE-2025-54901 is an information‑disclosure vulnerability in Microsoft Excel caused by a buffer over‑read in Excel’s file‑parsing code.
- Impact: A maliciously crafted workbook (XLS/XLSX/XLSB or embedded object) may cause Excel to read memory beyond a buffer boundary and expose contents of process memory (heap/stack), potentially leaking secrets such as pointers, heap layout, or other sensitive data. That information can be useful to attackers to bypass memory‑safety mitigations or as an initial primitive in more complex exploit chains.
- Exposure: The issue is triggered by opening (or in some configurations, previewing or server‑side rendering of) a crafted file in a vulnerable Excel client. User interaction (opening or previewing a file) is typically required.
- Remediation: Microsoft’s Security Update Guide / MSRC advisory lists the affected Excel/Office builds and the security update(s) that mitigate the issue; install the supplied updates for your Office servicing channel as the definitive fix.
Technical background — what “buffer over‑read” means here
A buffer over‑read (out‑of‑bounds read) occurs when code reads memory beyond the bounds of an allocated buffer. In complex document parsers like Excel’s, structured binary records are deserialized into in‑memory objects; when length fields, type checks, or bounds checks are missing or incorrect, the parser can read from adjacent memory. The immediate effect is information disclosure — memory contents that should be private may be revealed to the attacking data flow — but such a read can also be the stepping stone to more powerful exploits (heap layout disclosure helps defeat ASLR, and disclosed pointers may assist type confusion or heap grooming).In Office/Excel, these out‑of‑bounds reads often occur in code paths that deserialize nested binary records or reinterpret buffers across object types (for example, shape metadata, embedded objects, OLE items, or nested workbook records). Attackers craft malformed values that cause mis‑sized allocations or invalid casts; when code later reads those regions incorrectly it can leak memory. Historical vendor advisories for Excel in 2025 describe this same pattern across multiple CVEs.
What Microsoft says (and why you should treat MSRC as authoritative)
Microsoft publishes each advisory and the per‑product, per‑channel update KBs in the Security Update Guide (MSRC). That advisory is the canonical source for:- Which Excel/Office builds and platforms are affected.
- The exact KB/article identifiers or packages that remediate the issue.
- Microsoft’s official severity assessment and mitigation guidance.
Affected products and typical exposure model
Microsoft’s advisory page enumerates affected Excel/Office builds (consult the MSRC entry for the definitive list). Practically speaking, any user who can open Excel files is in the risk set: desktop Excel users who open a crafted workbook, mail clients or mail servers that preview attachments using Excel parsers, or any server‑side service that renders Excel files could be exposed. Because preview panes or server‑side rendering sometimes invoke the same parsing logic as desktop Excel, vulnerability surface can extend beyond direct user action in some environments.Common delivery vectors that attackers use:
- Spear‑phishing emails with malicious attachments.
- Files placed on shared drives or collaboration platforms.
- Supply‑chain or public downloads (malicious attachments on website or archive).
- Malicious embedded objects inside otherwise benign documents.
Exploitation scenarios and attacker goals
- Information disclosure only: An attacker might solely aim to read process memory to exfiltrate sensitive in‑process data (rare but possible).
- Recon/primitive for further exploitation: More commonly, information disclosure is used to leak heap layout or pointers that allow an attacker to bypass mitigations (e.g., ASLR) and develop a reliable exploit that escalates to code execution when combined with other primitives (type confusion, use‑after‑free, or write primitives). Historical Office bugs show disclosure → heap grooming → remote/local code execution chains.
- Post‑exploit impacts: if used in a chain to produce code execution, consequences range from credential theft and lateral movement to persistent implant deployment and ransomware. Even as an information‑leak alone, the revealed layout data materially aids attackers crafting follow‑on exploits.
Likelihood of exploitation in the wild
- The immediate risk is driven by how quickly proof‑of‑concepts (PoCs) appear and whether attackers can weaponize the read primitive in isolation or require additional bugs. Historically, once an Office CVE is public, skilled actors and researchers quickly attempt to produce PoCs or make exploit code available. Treat published Office CVEs as urgent until telemetry shows otherwise.
- Because exploitation typically requires file open (or preview), mass automated exploitation is less trivial than a remote network service exploit — but targeted phishing or commodity phishing campaigns can still yield many successful compromises.
Immediate remediation and mitigation (prioritized)
- Patch (definitive) — Highest priority
- Retrieve the exact KB(s) for CVE‑2025‑54901 from Microsoft’s Security Update Guide and deploy them across each Office servicing channel you use. Use centralized tools (WSUS, SCCM/ConfigMgr, Intune) to roll out and verify installation. Microsoft’s advisory is the authoritative patch mapping.
- Short‑term compensating controls (if you cannot patch immediately)
- Enforce Protected View for files from the internet and untrusted locations.
- Disable or restrict macros (even though this CVE does not require macros, macro lockdown reduces overall attack surface).
- Apply Attack Surface Reduction (ASR) rules in Microsoft Defender for Endpoint to block Office apps from spawning child processes; this prevents many post‑exploit actions.
- Disable automatic previewing of attachments in mail clients or route attachments through a sandbox/detonation service to prevent server‑side or client preview triggers.
- Use application allow‑listing (AppLocker/WDAC) to reduce impact of any exploit that attempts to write or execute payloads.
- Network & content controls
- Harden mail gateways and content ingestion: detonate/sandbox attachments and block known malicious sources.
- Limit direct access to shared drives for users in high‑risk roles; scan stored attachments for known bad indicators and quarantine suspicious Excel files.
Detection and hunting guidance
Hunt for the most indicative behaviors rather than file patterns (parsers can be weaponized in many ways):- Monitor Excel (excel.exe) processes for:
- Unexpected child process creation (cmd.exe, PowerShell, wscript/cscript).
- Rapid or unusual network connections initiated immediately after Excel activity.
- Attempts to write executable or script files into system locations (ProgramData, System32, user profile bin directories).
- EDR/telemetry indicators:
- Alerts when Office processes spawn non‑Office processes.
- Memory leaks or crash dumps tied to Excel opens immediately preceding other suspicious activity.
- Newly created persistence artifacts (scheduled tasks, services) shortly after document open.
- Collection artifacts to assist post‑incident analysis:
- Memory snapshot (forensics) from compromised host, Excel crash dumps, relevant EDR logs, and the original malicious file if available. These help determine if the out‑of‑bounds read was observed, and whether it was chained into exploitation.
Operational playbook for enterprise remediation (concise checklist)
- Inventory: enumerate all Office/Excel builds and map to the MSRC advisory’s affected list. Use managed update tooling to identify which endpoints are vulnerable.
- Patch deployment: schedule an emergency/accelerated rollout using your normal change control but with an expedited pilot to catch regressions. Validate KB presence and Office build numbers post‑install.
- Interim controls: enable Protected View, apply ASR rules, disable previews, restrict macros, and sandbox attachments until patches are confirmed across the estate.
- Hunting & monitoring: run EDR hunts for Office processes spawning child processes and other IOCs described above; prioritize triage of any matches.
- User communications: issue a short advisory instructing users not to open unexpected spreadsheets and to forward suspicious messages to IT, with examples of common phishing signs.
For home users and small business operators
- Update Office now: Excel → File → Account → Update Options → Update Now (or run Microsoft Update). Patching is the single most effective action.
- Avoid opening Excel files from unknown or unexpected sources. If you must view an untrusted file, use Office for the web (browser‑based preview) as a safer first step — many parsing bugs trigger only in desktop Office.
- Keep anti‑malware and OS updates current; layered defenses reduce post‑exploit impacts.
Timeline and context (why this matters now)
2025 continued to see a stream of memory‑safety bugs in Office components — use‑after‑free, heap overflows, and out‑of‑bounds reads — with repeated patterns: a crafted workbook triggers parsing logic, memory corruption or disclosure occurs, and attackers may chain primitives for code execution. Microsoft’s monthly security updates and advisories have repeatedly covered similar Excel issues, which is why each new CVE in the same family is treated with urgency. Administrators must not treat this as an isolated incident but as part of a persistent, targeted threat against document parsers.Threat modelling and prioritization advice
- Prioritize patching for endpoints that: handle email attachments, run with elevated privileges, are in high‑value business units (finance, HR, IT), or provide server‑side rendering (mail servers, SharePoint/Office Web Apps, document scanners).
- If assets are unmanaged, prioritize user training and policy controls (disable previews, require sandboxed inspection for external attachments).
- Assume skilled attackers will attempt to combine information‑leak primitives with other bugs; therefore, defense‑in‑depth (patching + ASR + EDR + network controls + user training) is essential.
If you detect a suspected exploit attempt
- Isolate the affected endpoint(s) immediately (network and user access).
- Preserve volatile evidence: memory dump, Excel process dump, EDR logs showing the file open and subsequent activity, and keep the suspicious file for analysis.
- Triage for post‑exploit indicators: unusual child process creation, persistence artifacts, credential theft tools, lateral movement attempts. If confirmed, follow incident response procedures (contain, eradicate, remediate, restore).
Indicators, artifacts and telemetry useful to defenders
- MD5/SHA256 of any suspicious Excel files (archive them).
- Excel crash dumps/time of crash correlated with email delivery times.
- EDR events showing excel.exe spawning PowerShell or cmd.exe.
- Network flows from the host immediately after document open, especially to suspicious IPs or C2‑like destinations.
Final recommendations (actionable, prioritized)
- Obtain the CVE‑2025‑54901 MSRC advisory and the per‑channel KB(s); deploy the update to all affected Office installations as soon as possible. Microsoft’s Security Update Guide is the canonical source for these KB mappings.
- Until you’ve patched: enforce Protected View, disable preview handlers for email, tighten macro settings, and apply ASR rules that prevent Office apps from launching other processes.
- Use EDR to hunt for Office processes spawning non‑Office child processes and prioritize triage of those alerts. Collect memory and crash dumps when you suspect an exploit attempt.
- Communicate clearly with users: do not open unexpected spreadsheets and report suspicious attachments to IT for sandbox analysis.
If you’d like, I can:
- Pull the exact affected product list and KB identifiers from Microsoft’s Security Update Guide and format them into a deployment checklist for your environment (I’ll need the servicing channels you use: e.g., Microsoft 365 Apps Current Channel, Monthly Enterprise Channel, Office MSI, Office for Mac, etc.).
- Generate EDR/SIEM query examples (YARA/ELM/EDR rule pseudocode) to hunt for the most likely post‑exploit behaviors described above.
Source: MSRC Security Update Guide - Microsoft Security Response Center
