Spreadsheets behave differently than other documents. Where a Word DOCX file is typically authored once and read, an Excel workbook is more often built to actively compute; it pulls from external sources, recalculates when it’s opened, and often maintains live connections to data that lives somewhere else. That computational design is part of what gives XLSX its immense business value, but it’s also what makes it worth examining very carefully from a security standpoint.

As we’ve defined in previous entries to our format vulnerability series, a vulnerability is a weakness in software that a crafted file can exploit. The XLSX threat picture covers both the parsing vulnerabilities common to XML-based Office formats and a unique set of risks specific to how spreadsheets work. Those include external data connections, formula-based network requests, and a widespread assumption that a file without macros is a file without meaningful risk.

What’s inside an XSLX file?

We’ll start with the big picture. XLSX is Microsoft Excel’s modern document format, introduced (along with many others) with Office 2007 as part of the Open XML standard. Like DOCX, it’s structured as a ZIP container holding XML parts and supporting resources. The surface-level similarity XLSX has to its Open XML cousins matters because it creates a familiar problem: neither a ZIP signature nor a simple file extension string - .xlsx - reveals anything meaningful about what the file actually contains.

The XML inside an XLSX is organized around the workbook and its sheets. The key parts include the workbook structure and sheet references, individual XML file carrying cell data and formulas, a shared strings file for text values reused across cells, style definitions, chart drawing data, and – occasionally – a connections file that defines external data sources the workbook is configured to contact.

The above identifiers serve the purpose of confirming what kind of container software is looking at. What identifiers can’t tell software is what each workbook’s cells contain, which external sources its connections are referencing, and what formulas the file will evaluate on open.

H4: A quick note on macros

As we’ve done in all our articles on Office format vulnerabilities, we’ll make a quick note about macros here.

Excel supports macros, but XLSX cannot store a VBA macro project. The specific lettering in the file extension here matters. The macro-enabled Excel format is XLSM (.xlsm), and it’s a deliberately separate extension designed to give content policies a clear signal when Excel files contain executable code. That boundary is worth knowing (it’s one of the more useful format-level distinctions in the Office ecosystem), but it doesn’t account for the rest of XLSX’s threat surface.

A macro-free workbook can still carry things we should be very worried about, including external connections, crafted formula structures, embedded objects, and other content designed to exploit the software that reads it.

How XLSX content can expose software vulnerabilities

The answer to this question starts with an understanding of how XLSX files are read by software programs.

Opening an XLSX file starts a parsing sequence that touches multiple XML parts. That sequence covers workbook structure, cell content and formulas, chart definitions, style data, and drawing objects (i.e., charts and graphs). Any defect in the code handling those parts is a potential target for threat actors to exploit.

Malformed XML attributes, unexpected numeric values in cell records, and specially (maliciously) crafted chart data can all reach vulnerable code paths in Excel or similar third-party libraries capable of reading XLSX files. The Excel formula engine, which is responsible for evaluating cell expressions across a workbook, is itself a complex processing system that has long been a target for vulnerability research separate from the XML parsing layer.

Excel exploits can be really niche and tricky; CVE-2021-42292 is a good example of this. The documented vulnerability was an Excel security feature bypass that allowed a malicious crafted file to circumvent Protected View (the sandboxed processing environment intended to screen files from untrusted sources) and land directly in Excel’s full parsing pipeline. In a spiritually similar way the formula engine, Protected View bypasses are a category of their own: they don’t exploit the spreadsheet content directly, but they remove the processing constraint that’s supposed to limit what the content can do. The practical result from this particular vulnerability was that any crafted content in the file, including formular structures, XML, or embedded objects, could get a clear path to Excel’s full processing environment, where the assumptions of the Protected View sandbox would no longer apply.

If that sounds extremely subtle and mundane, we’re on the right track. Vulnerability exploitation hardly ever needs to be flashy to create opportunities for disastrous outcomes.

External data connections and formula-based risks

We’ve talked a bit about Open XML files having a shared threat surface. This particular threat angle distinguishes XLSX most clearly from that group, and from Word DOCX in particular.

As we’ve described, spreadsheets are routinely built to pull data from outside themselves – be it from databases, other files, or web sources – and XLSX encodes those connections directly in the file’s XML. Basically, the same feature that supports live dashboards and automated reporting workflows also means a workbook can initiate network requests the moment it opens.

External data connections defined in the workbook’s connections XML can point to anything from database servers to web services or file paths. In some configurations, Excel attempts to refresh those connections automatically. A connection targeting an attacker-controlled server can expose the client’s IP, harvest credentials that Excel transmits with the request, or server crafted data designed to exploit the receiving application once it arrives.

External workbook references in formulas work similarly. A formula referencing a path on a remote server (including UNC paths) can cause Excel to contact that server during formula evaluation. That contact has been used to capture NTLM authentication hashes from the connection system without any further user interaction.

Power Query, Excel’s built-in data transformation engine, can fetch content from web URLs, local files, and other sources using M language queries stored within the workbook. Power Query queries are yet another mechanism by which opening a spreadsheet can result in insecure outbound network activity.

Embedded content and external references create different risks

Workbooks are data-dense by nature, and that includes a range of content beyond cell values: think embedded objects, linked images, charts pulling from external series data, and hyperlinks embedded in cells or shapes. Each category introduces a different kind of external dependency, or a different application, into the processing chain.

What XLSX content validation can reveal

Validating content means determining that its structure matches formatting standards. For an XLSX file, that means verifying the ZIP container, confirming that the XML parts conform to the Open XML schema, and checking the workbook’s content against a defined acceptance policy. That examination can identify malformed structure, flag workbooks carrying external data connections, and surface embedded objects that have no place in a given context (e.g., an upload portal or cross-network file share).

The limits to XLSX validation are the same ones that apply across Open XML documents in this series. A structurally valid XLSX can still carry connections configured to call out to attacker-controlled infrastructure, and it can hold formula references that will trigger network activity on evaluation. It can have specially crafted XMLS that reaches a parser vulnerability in the application processing it. Schema conformance establishes that a file is well-formed, but it doesn’t speak to what the file will do when Excel (or any other application) opens it.

The connection and formula risks add a wrinkle that other format types don’t share: some XLSX threats activate before a human reader makes any decision at all. A connection refresh that fires on open, or a formula that evaluates a result automatically, can initiate network activity at the moment the file loads. Inspection needs to occur before that moment, or the threat has already been missed.

The malware an XLSX file can help deliver

While we’re not primarily focused on traditional malware delivery in this article, it’s hard to paint a full picture of Excel security concerns without that piece.

Without native macro support for the .xlsx extension, XLSX isn’t the primary delivery vehicle for macro-based malware; that role belongs to XLSM and the legacy XLS format (both of which we’ll cover in this series). But XLSX files have been used, and continue to be used, in multi-stage malware delivery attacks that exploit the format’s connectivity features to deliver, rather than directly execute, code.

Phishing campaigns have used XLSX files with external connections or formula references to harvest credentials during what looks like a routine file open event. The workbook itself may contain no overtly malicious content; rather, the threat is in the network request that the file is responsible for triggering, and what the server on the other end ultimately collects and/or returns.

Applying Cloudmersive 360-Degree Content Protection to XLSX files

Cloudmersive’s Advanced Virus Scan API provides 360-Degree Content protection, combining virus and malware detection with configurable content verification in a single step. XLSX, along with the entire Open XML family, is supported for both malware scanning and internal content verification.

For XLSX files, the API’s controls can verify permitted file types, flag workbooks carrying external data connections, identify embedded objects, and surface content that conflicts with a policy. That includes the connection and external reference features that make XLSX threats distinctive; content-level inspection can identify a workbook configured to reach out to external infrastructure before it ever opens in Excel.

For file upload and file share portals and other services that accept spreadsheet content, separating a detected threat from a policy violation (i.e., an unexpected external connection) allows for appropriate handling of each.

To discuss the right controls for your XLSX files, please feel free to contact a Cloudmersive security expert.