Zero-day attacks - Attacks which exploit software vulnerabilities unknown to the software vendor. These vulnerabilities are unaddressed by any existing patches or security signatures, which means traditional defenses have no way to recognize them until after the damage has already been done.

Why Zero-Day Attacks Are Especially Dangerous for Enterprise File Upload Workflows

File upload endpoints are one of the most direct paths for zero-day threats to enter any environment, and enterprises are full of them. With these endpoints at their disposal, attackers don’t need to exploit network vulnerabilities or compromise credentials to pass their malicious payloads into enterprise environments. They just need to submit files containing distinct, custom-made malware that no signature database has ever seen before.

That last part is what makes zero-day threats categorically different from known malware. All known malware has a signature; traditional antivirus tools leverage databases with millions of these signatures, and whenever a scanned file matches one, that file gets flagged. Zero-day threats don’t have a signature yet; they haven’t been catalogued or studied, and that means they haven’t been added to any database. Almost by definition, the first time a signature-based antivirus tool encounters a zero-day threat, it passes that threat onto the backend without flagging it. For enterprises running file upload workflows at scale, that’s a severe gap, and threat actors will certainly try to exploit it.

Signature-Based Detection vs. Zero-Day Detection

Here’s what the difference between signature-based detection and zero-day detection looks like in practice:

While this comparison/breakdown might make it seem like signature detection and zero-day detection compete with one another, that isn’t really the case. In reality, they’re complementary scanning approaches rather than competing. Signature-based scanning is great at efficiently dealing a broadly known and well-document threat landscape; a thriving community of public and private cybersecurity researchers add to signature databases every day, and that takes a huge volume of threats off the table. Zero-day detection handles the deadly exceptions that fall through the cracks, preventing the scenario all enterprises (should) fear: becoming the compromised test subject for a brand-new form of malware which researchers subsequently document. Signature-based and zero-day threat scanning together constitutes the most effective antivirus posture.

What Makes a File Upload a Significant Zero-Day Attack Vector

If an endpoint accepts binary payloads, it’s a potential delivery mechanism for zero-day threats – simple as that. And the risk isn’t limited to obviously executable file types, of course. Attackers can (and do) embed zero-day threats in documents, archives, and image files, and those threats don’t always trigger behavioral flags that simpler scanning approaches would catch.

Some specific threat categories are worth mentioning:

Executables

It’s common for attackers to embed executable program code in another file type (e.g., a .DLL file disguised as a PDF). This is one scenario where content verification really matters: any file claiming to be one type but exhibiting traits of another should be blocked outright.

Scripts

PHP files, Python scripts, BAT scripts, JavaScript, and the like are excellent vehicles for malicious instructions. They execute when a file is processed or viewed, which makes them a particularly common attack vector.

Macros

Macros are a long-standing and widely understood malware delivery mechanism. They’re embedded in documents many enterprise environments see with extremely high frequency: namely Word, Excel, and PowerPoint. The word is out on macro threats, but many organizations still permit macros by default.

Unsafe archives

Zip files are notoriously difficult to scan, particularly when encrypted, and attackers can turn them into “bombs” by stuffing them with an outrageously large number of files. Once unzipped, those “zip bombs” will consume a disproportionate volume of resources on a network and/or trigger downstream failures once processed.

OLE embedded objects

Older Office documents can embed objects containing vulnerabilities or executable code (OLE = Object Linking and Embedding). These are easy to overlook in scanning configurations primarily focused on identifying virus and malware signatures.

XML external entities

Malicious XML files can reference external entities, which in turn trigger unintended server-side requests or data exposure when processed. What surprises many teams it that the risk extends directly to uploaded Office files like .DOCX, .XLSX, and .PPTX, since modern Office formats are built on XML.

Insecure Deserialization

JSON and other serialized data files can carry payloads designed to exploit specific deserialization vulnerabilities in the systems that process them.

It’s not enough to look for just one or two of these threats: blocking zero-day attacks at the file upload layer means accounting for all of these vectors.

How the Cloudmersive Virus Scan API Approaches Zero-Day Detection

The Cloudmersive Virus Scan API addresses both sides of the detection problem:

1. The known threat landscape, by referencing continuously updated signature databases
2. The novel threat landscape, through advanced content analysis and configurable threat rules

ZDDR: Zero-Day Detection Rate

Zero-Day Detection Rate (ZDDR) is the percentage of zero-day threat samples, meaning threats not yet present in any antivirus signature database, that a scanning solution successfully identifies. The Cloudmersive Virus Scan API excels in this regard, maintaining a ZDDR of 98%. This means it correctly identifies 98 out of every 100 zero-day threats it encounters before those threats have been catalogued in any signature database.

That figure matters: it represents the gap that signature-based scanning simply cannot close. An antivirus solution with strong signature coverage and weak zero-day detection leaves enterprises exposed to the threats likely to cause them the most damage.

Signature Coverage

The Virus Scan API leverages over 17 million virus and malware signatures with continuous cloud-based updates. This provides comprehensive baseline coverage across the known threat landscape alongside the zero-day detection capability.

Configurable Threat Rules

The Virus Scan API addresses common zero-day attack vectors head on with a set of configurable Boolean parameters. These allow security teams to define exactly what content is and isn’t permitted through their upload endpoints. They can be tuned independently to match the specific risk profile/risk appetite of any given workflow:

• allowExecutables: block or permit executable file content
• allowScripts: block or permit script files including PHP, Python, BAT, and JS
• allowMacros: block or permit macros embedded in Office documents
• allowPasswordProtectedFiles: block or permit encrypted files that circumvent scanning
• allowUnsafeArchives: block or permit zip bombs and unsafe archive formats
• allowOleEmbeddedObject — block or permit OLE embedded objects in Office files
• allowXmlExternalEntities: block or permit XML external entity references
• allowInsecureDeserialization: block or permit insecure deserialization payloads
• allowHtml: block or permit HTML files at the top level
• allowInvalidFiles: block or permit files that fail content verification against their declared format
• restrictFileTypes: limit accepted file formats to a specific allowlist (e.g. .pdf,.docx,.png only)

The last parameter on this list, restrictFileTypes, is particularly useful for workflows where only a narrow set of file formats is ever expected. Rather than blacklisting specific threats individually, it inverts the model by whitelisting only the file types we need.

The Response Object

In our API response, we get a flag for each detection signal independently. The full object contains the following:

• CleanResult: a top-level Boolean indicating if a file passed all configured security checks
• FoundViruses: an array of viruses identified in the scan (if any)
• VerifiedFileFormat: the actual detected file format, if one could be detected
• Individual Boolean flags for each threat category (ContainsExecutable, ContainsMacros, ContainsScript, etc.)
• Content Information - an array of additional metadata for each scanned file, including SHA1 hash, relevant subfile details (each of which are scanned independently), and Authenticode signature status

This combination of top-level and granular result allows scanning workflows to support both pass/fail routing and more nuanced/tiered review logic.

Deploying Zero-Day Detection in an Enterprise File Upload Workflow

The most effective deployment point for any antivirus will always be the file intake step, immediately after a file is received and before it gets processed or stored anywhere downstream. Introducing a scan at this point in the pipeline ensures no harmful interactions occur if a file hasn’t been evaluated.

The configuration approach afforded by the Virus Scan API depends entirely on the sensitivity of the workflow in question. For a general-purpose document intake endpoint, enabling the recommended default flags provides strong baseline threat coverage without requiring detailed knowledge of every possible threat vector. For higher-sensitivity workflows, like financial document processing or claims intake, for example, adding restrictFileTypes to limit accepted formats gives an additional layer of defense that doesn’t depend on threat detection at all.

The API’s subsecond typical response time means scanning can be added inline without introducing meaningful latency into the request pipeline, which removes the common objection to synchronous scanning at high request volumes.

Takeaways

In this article, we learned that:

• Zero-day attacks are defined as those which exploit unknown vulnerabilities with no existing signature. This makes them invisible to signature-only detection policies.
• Signature-based and zero-day detection are complementary, not competing. Both are a requirement for effective threat coverage.
• The Cloudmersive Virus Scan API maintains a 98% ZDDR, catching the vast majority of zero-day threats before they’ve been catalogued by a signature database.
• Configurable API threat rules mean security teams can take control of exactly what content is permitted through each upload endpoint.
• Restricting file types via restrictFileTypes inverts the detection model by only permitting a defined allowlist of file formats.
• The Virus Scan API can be deployed anywhere you want, but it’s always best to incorporate an antivirus layer at the point of file intake.

For expert advice on deploying the Cloudmersive Virus Scan API in your enterprise environment, reach out to the Cloudmersive team directly.