AddFileTypeValidator (still) doesn't work correctly

[jamescrowley]

Is there an existing issue for this?

• I have searched the existing issues

Current behavior

We've just hit the same problem as #14970 (which is closed) upgrading to the latest packages, but that issue says it's fixed - it doesn't seem to be.

If we log the error being swallowed by this try, catch:

https://github.com/nestjs/nest/blob/master/packages/common/pipes/file/file-type.validator.ts#L93-L95

thrown by the loadEsm call:

https://github.com/nestjs/nest/blob/master/packages/common/pipes/file/file-type.validator.ts#L75-L76

we get:

TypeError: A dynamic import callback was invoked without --experimental-vm-modules
        at importModuleDynamicallyCallback (node:internal/modules/esm/utils:226:11)
        at loadEsm (/Users/yyy/node_modules/load-esm/index.js:2:26)
        at FileTypeValidator.isValid (/Users/yyy/node_modules/@nestjs/common/pipes/file/file-type.validator.js:46:73)
        at ParseFilePipe.validateOrThrow (/Users/yyy/node_modules/@nestjs/common/pipes/file/parse-file.pipe.js:58:41)
        at ParseFilePipe.validate (/Users/yyy/node_modules/@nestjs/common/pipes/file/parse-file.pipe.js:53:24)
        at processTicksAndRejections (node:internal/process/task_queues:95:5)
        at async ParseFilePipe.validateFilesOrFile (/Users/yyy/node_modules/@nestjs/common/pipes/file/parse-file.pipe.js:43:13)
        at async ParseFilePipe.transform (/Users/yyy/node_modules/@nestjs/common/pipes/file/parse-file.pipe.js:34:13)
        at async resolveParamValue (/Users/yyy/node_modules/@nestjs/core/router/router-execution-context.js:148:23)
        at async Promise.all (index 1)
        at async pipesFn (/Users/yyy/node_modules/@nestjs/core/router/router-execution-context.js:151:13)
        at async /Users/yyy/node_modules/@nestjs/core/router/router-execution-context.js:37:30 {
      code: 'ERR_VM_DYNAMIC_IMPORT_CALLBACK_MISSING_FLAG'
    }

It seems to be a breaking change - I'm not yet clear if this is limited to just our jest config or would require a change to our production environment too?

Minimum reproduction code

empty

Steps to reproduce

No response

Expected behavior

Tests pass as previously.

Package

• I don't know. Or some 3rd-party package
• @nestjs/common
• @nestjs/core
• @nestjs/microservices
• @nestjs/platform-express
• @nestjs/platform-fastify
• @nestjs/platform-socket.io
• @nestjs/platform-ws
• @nestjs/testing
• @nestjs/websockets
• Other (see below)

Other package

No response

NestJS version

No response

Packages versions

    "@nestjs/common": "^11.1.0",
    "@nestjs/core": "^11.1.0",
    "@nestjs/platform-express": "^11.1.0",

Node.js version

20.16.0

In which operating systems have you tested?

• macOS
• Windows
• Linux

Other

No response

[micalevisk]

are you using commonjs or ESM in your app project? (the type entry of your package.json)

[jamescrowley]

Currently type is not specified in package.json (so defaults to commonjs), and tsconfig module was set to commonjs

[micalevisk]

that has to do with Jest as well. You got no errors when running that app outside of Jest, right?

then try this:

node --experimental-vm-modules node_modules/.bin/jest

[jamescrowley]

Thanks @micalevisk - that didn't resolve the issue on its own but once setting that flag, I can now see the file type validation fails because the test files are zero bytes. This previously was allowed through though, so perhaps there was another breaking change in this area?

[jamescrowley]

@micalevisk we actually encountered this first when doing a minor upgrade on via npm audit fix ("10.3.7" -> "10.4.17" for nestjs common) and then subsequently just decided to go straight to 11.x), so it's possible the zero byte files no longer being accepted was an older change that was made on 10.x some time ago, as opposed to related to what was back ported, and was just then hidden by the new requirement for experimental-vm-modules. I'll have more of a dig.

[micalevisk]

I wonder if we can replace the

nest/packages/common/pipes/file/file-type.validator.ts

Lines 75 to 77 in f2d8543

	const { fileTypeFromBuffer } =
	await loadEsm<typeof import('file-type')>('file-type');
	const fileType = await fileTypeFromBuffer(file.buffer);

with the same approach shown at

nest/sample/34-using-esm-packages/src/import-esm-package.ts

Lines 5 to 10 in f2d8543

	export const importEsmPackage = async <ReturnType>(
	packageName: string,
	): Promise<ReturnType> =>
	new Function(`return import('${packageName}')`)().then(
	(loadedModule: unknown) => loadedModule['default'] ?? loadedModule,
	);

can you please try it out in your project? just edit your node_modules/@nestjs/common/pipes/file/file-type.validator.js file

[jamescrowley]

@micalevisk that importEsmPackage method still triggers a "A dynamic import callback was invoked without --experimental-vm-modules" error for me, at least when defined as this in the JS file:

const importEsmPackage = async (packageName) => 
    new Function(`return import('${packageName}')`)().then( 
      (loadedModule) => loadedModule['default'] ?? loadedModule, 
    );

[mag123c]

I think, the starting point of this issue was CI pipeline warnings, as exemplified by discussions in #14876. From NestJS's perspective, the original FileTypeValidator (which primarily used string/regex matching for file extensions) might not have been considered a "security vulnerability" per se, but rather a simpler validation mechanism as per its initial design. However, CI/security scanning tools began flagging this approach as insufficient for robustly preventing spoofed file types, leading to these CI warnings.

To resolve these CI warnings and generally enhance file validation robustness, NestJS adopted the file-type library. This library performs more reliable validation by checking the file's actual magic numbers, which is a significant step up.

However, file-type is an ESM-only package. This shift has now led to the current set of problems for projects still using CommonJS (CJS), primarily the requirement to use Node.js flags like --experimental-vm-modules. This impacts developer experience, especially in testing environments (like Jest) and potentially in production.

In summary, while integrating an external library like file-type as a hard dependency was a solution to meet CI recommendations, it unintentionally introduced ESM/CJS compatibility issues. We now need to find a way to resolve the original CI concerns without forcing CJS users into undesirable workarounds or Node.js flag configurations.

[Yasir-Rafique]

Hi everyone,

I'd like to take a stab at resolving the persistent issue with AddFileTypeValidator failing in CommonJS (CJS) environments due to the ESM-only file-type package.

Root Cause
So what I understand is the validator uses dynamic import() to load file-type, which is not supported in CJS contexts without the --experimental-vm-modules flag. This breaks compatibility, especially for test runners like Jest or apps still using CJS.

1. Runtime Detection of Module System (Detect whether the app is running in ESM or CJS)
2. Safe Conditional Module Loading
3. Graceful Degradation (If file-type fails to load (e.g., in Jest), fall back to a lighter check (e.g., MIME type from file extension) or skip the check with a console warning.)
4. Zero-Byte File Option (Introduce an optional allowEmptyFile: boolean flag to handle the zero-byte edge case more flexibly)

Let me know if this direction sounds good. I’ll get started with a PR accordingly.

Thanks!

[itrethan]

This is still happening in v11.1.3 that made the entire type checking not working propertly, Plese raise the priority