The .doc extension is a legacy of the Office era that predated XML and ZIP containers. It’s familiar enough to feel unremarkable, but DOC’s internal structure is precisely what sets it apart from every format in Word’s modern lineup. Those structural differences carry real security consequences.

DOC files aren’t ZIP archives, and they don’t use XML. Instead, content is stored in a binary structure called the Compound File Binary Format, which is read by completely different parsing code than the kind used for DOCX, DOTX, and DOCM. In order to understand what DOC security actually requires, we need to understand what those structural differences mean for parser vulnerabilities, macros, and the techniques specific to this format.

What’s inside a DOC file?

The Compound File Binary Format (also known as OLE2 or CFB) organizes content into named streams and storages within a binary container. The container resembles a miniature file system; text formatting data, embedded objects, and macro projects each occupy separate streams within that system.

This differs fundamentally from DOCX, which packages XML parts inside a ZIP archive. A DOC file has no ZIP structure to enumerate and no XML to parse. That means any application reading a DOC file needs to navigate binary streams directly, which puts binary parsing code – with its own history and its own vulnerabilities – at the center of this format’s security picture.

The identifiers shown above describe the container type. The actual streams a DOC file carries, and what those streams contain or do, can’t be determined from the surface.

DOC macros: no separate format required this time

In our previous articles on DOCX and DOTX, we’ve discussed how those formats handle macros by keeping them in distinct format variants: DOCM for macro-enabled documents, and DOTM for macro-enabled templates. That clear separation was a deliberate OOXML design choice. The goal was to explicitly indicate the presence of executable code within the file – and one could argue DOC is the reason for that decision.

DOC offers no equivalent boundary. A standard .doc file can natively contain a full VBA macro project, stored in its own stream within the Compound File Binary container. There’s no extension distinction between a DOC file that runs code and one that doesn’t; both are just .doc.

As you might be aware, macros can be configured to execute automatically on document events. AutoOpen fires when a document is opened; AutoClose fires when a document is closed; AutoExec runs when Word itself starts with the document loaded. None of these require any user action between the document loading.

In a malicious DOC file, the distance between “dormant macro” and “executed macro” is filled by social engineering. A large share of DOC-based macro attacks rely on the attacker convincing a reader to click “Enable Content”. That’s typically done using in-document text that makes enabling macros look like a necessary step to view the file.

How DOC content can expose software vulnerabilities

Even before a macro runs – even in a DOC file with no macro project at all – binary stream parsing creates its own attack surface.

The code that reads DOC binary structure has accumulated a rather notable CVE record over decades of Office versions. Anything from malformed record fields to out-of-bounds stream references to unexpected binary arrangements can trigger defects in the parsing application. Depending on the specific vulnerability, the result may be a crash, an information disclosure, or – in the most severe worst-case scenario – attacker-controlled code execution without any user action beyond opening the file.

One of the most frequently cited examples of this is CVE-2017-118882. This vulnerability existed in the legacy Equation Editor component embedded in Office documents, including DOC files. A specially crafted equation object could cause memory corruption in the Equation Editor process, enabling code execution with no macro involved. The flaw had been in the codebase for approximately seventeen years before it was finally patched. Exploitation continued long after the path was released, reflecting the extended lag between patch availability and deployment. That last point is a pattern that compounds the risk of any widely used legacy format.

Legacy features that expand the attack surface

Macros aren’t the only DOC mechanism that’s been exploited outside the macro security model.

Dynamic Data Exchange (DDE) is a legacy inter-application communication mechanism that can be found embedded in DOC files. A DDE field can direct Word to contact an external application and pass it a command to execute. Since DDE processing is a completely separate mechanism from VBA macros, it isn’t governed by macro security settings in older configurations, and that’s made it an independently useful tool for attackers. In later Office versions, controls were eventually introduced to disable DDE resolution, but those controls only apply where they’ve been enforced.

It’s also worth noting that, despite the differences in format and structure, embedded OLE objects function in DOC the same way they do in DOCX: the containing document and the embedded object are processed as separate artifacts. Each has its own application dependencies and vulnerability exposure.

Embedded content and features create different risks

Some of what a DOC file can contain introduces another application, network endpoint, or executable logic into the chain. The legitimate uses for these features are, of course, real (OLE embedding, linked data sources, and external references all have completely valid applications), but the security implications depend on what they actually contain and how they’re configured.

What DOC content validation can reveal for us

From what we know about DOC file structure, it follows naturally that validating the format means verifying the Compound File Binary container. This means checking that its streams are structurally intact, that the file conforms to the DOC format specification, and that its contents don’t validate an enterprise content acceptance policy. Rigorous validation can flag broken container structures, identify the presence of VBA storage streams where macros aren’t expected, and surface embedded objects that a particular service has no need to accept.

There are two limits to this worth being explicit about, however. First, structural validity alone doesn’t establish that VBA content is safe, that a DDE field is inactive, or that the binary structures a parser encounters are free of exploitable flaws. A well-formed DOC file can still carry a macro that runs on open or a DDE reference pointing at a command. It can also still hold crafted binary content that targets a specific parser defect. Format validation won’t surface any of those exact problems.

Second, the DOC format has a documented history of parser vulnerabilities that spans multiple decades at this point. Keeping the software that reads and processes DOC files current is an independent requirement, and content inspection can’t substitute for it.

The malware a DOC file can help deliver

DOC’s macro capabilities have solidified it as one of the most durable malware delivery formats in circulation. VBA-based DOC malware predates most modern detection controls, and it has persisted because the core mechanism (code that runs when the document opens) remains effective.

AutoOpen macros that retrieve and execute secondary payloads are a foundational technique. Contemporary implementations of that technique tend to add layers on top (thing encoded strings, multi-stage loaders, built-in Windows utilities, etc.). In these attacks, the DOC file is a delivery vehicle rather than the final payload; its job is to get something else running.

What keeps this relevant is the compatibility argument. Whether it’s because of legacy workflows, external partners, or older systems, many organizations can’t afford to refuse .doc files outright. A file type that can’t be categorically blocked at the network perimeter can only be managed carefully through inspection.

Applying Cloudmersive 360-Degree Content Protection to DOC files

Cloudmersive’s Advanced Scan API provides 360-Degree Content Protection, combining virus and malware detection with configurable content inspection in a single step. The DOC file format is supported for both malware scanning and internal content verification.

The API’s controls can reject invalid container structures, flag DOC files carrying VBA storage streams, and identify embedded OLE objects in contexts where they shouldn’t appear. Those checks work alongside malware detection rather than replacing it: a file that matches a known malware signature and a file whose content violates a policy are two different findings, and the API’s response allows us to handle each appropriately.

For services that still need to accept DOC files (particularly for compatibility reasons), combining content inspection with regular patching of DOC-capable readers addresses the format’s content risks and the parser vulnerability exposure that no inspection step alone can fully close.

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