CVE-2026-30821: Flowise: Arbitrary File Upload via MIME Spoofing

Vulnerability Description

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Vulnerability Overview - The /api/v1/attachments/:chatflowId/:chatId endpoint is listed in WHITELISTURLS, allowing unauthenticated access to the file upload API. - While the server validates uploads based on the MIME types defined in chatbotConfig.fullFileUpload.allowedUploadFileTypes, it implicitly trusts the client-provided Content-Type header (file.mimetype) without verifying the file's actual content (magic bytes) or extension (file.originalname). - Consequently, an attacker can bypass this restriction by spoofing the Content-Type as a permitted type (e.g., application/pdf) while uploading malicious scripts or arbitrary files. Once uploaded via addArrayFilesToStorage, these files persist in backend storage (S3, GCS, or local disk). This vulnerability serves as a critical entry point that, when chained with other features like static hosting or file retrieval, can lead to Stored XSS, malicious file hosting, or Remote Code Execution (RCE).

Vulnerable Code

- Upload Route Definition https://github.com/FlowiseAI/Flowise/blob/d17c4394a238b49327b493c89feee45f3a20bb91/packages/server/src/routes/attachments/index.ts#L7-L10 tsx // CREATE router.post('/:chatflowId/:chatId', getMulterStorage().array('files'), attachmentsController.createAttachment) export default router - Mount /api/v1/attachments to the global router https://github.com/FlowiseAI/Flowise/blob/d17c4394a238b49327b493c89feee45f3a20bb91/packages/server/src/routes/index.ts#L72-L77 tsx const router = express.Router() router.use('/ping', pingRouter) router.use('/apikey', apikeyRouter) router.use('/assistants', assistantsRouter) router.use('/attachments', attachmentsRouter) - Include /api/v1/attachments in the WHITELISTURLS list https://github.com/FlowiseAI/Flowise/blob/d17c4394a238b49327b493c89feee45f3a20bb91/packages/server/src/utils/constants.ts#L6-L26 tsx export const WHITELISTURLS = [ '/api/v1/verify/apikey/', '/api/v1/chatflows/apikey/', '/api/v1/public-chatflows', '/api/v1/public-chatbotConfig', '/api/v1/public-executions', '/api/v1/prediction/', '/api/v1/vector/upsert/', '/api/v1/node-icon/', '/api/v1/components-credentials-icon/', '/api/v1/chatflows-streaming', '/api/v1/chatflows-uploads', '/api/v1/openai-assistants-file/download', '/api/v1/feedback', '/api/v1/leads', '/api/v1/get-upload-file', '/api/v1/ip', '/api/v1/ping', '/api/v1/version', '/api/v1/attachments', '/api/v1/metrics', - Bypass JWT validation if the URL is whitelisted https://github.com/FlowiseAI/Flowise/blob/d17c4394a238b49327b493c89feee45f3a20bb91/packages/server/src/index.ts#L213-L228 tsx const denylistURLs = process.env.DENYLISTURLS ? process.env.DENYLISTURLS.split(',') : [] const whitelistURLs = WHITELISTURLS.filter((url) => !denylistURLs.includes(url)) const URLCASEINSENSITIVEREGEX: RegExp = /\/api\/v1\//i const URLCASESENSITIVEREGEX: RegExp = /\/api\/v1\// await initializeJwtCookieMiddleware(this.app, this.identityManager) this.app.use(async (req, res, next) => { // Step 1: Check if the req path contains /api/v1 regardless of case if (URLCASEINSENSITIVEREGEX.test(req.path)) { // Step 2: Check if the req path is casesensitive if (URLCASESENSITIVEREGEX.test(req.path)) { // Step 3: Check if the req path is in the whitelist const isWhitelisted = whitelistURLs.some((url) => req.path.startsWith(url)) if (isWhitelisted) { next() - Multer Configuration: Saves files without file type validation https://github.com/FlowiseAI/Flowise/blob/d17c4394a238b49327b493c89feee45f3a20bb91/packages/server/src/utils/index.ts#L1917-L1960 tsx export const getUploadPath = (): string => { return process.env.BLOBSTORAGEPATH ? path.join(process.env.BLOBSTORAGEPATH, 'uploads') : path.join(getUserHome(), '.flowise', 'uploads') } export function generateId() { return uuidv4() } export const getMulterStorage = () => { const storageType = process.env.STORAGETYPE ? process.env.STORAGETYPE : 'local' if (storageType === 's3') { const s3Client = getS3Config().s3Client const Bucket = getS3Config().Bucket const upload = multer({ storage: multerS3({ s3: s3Client, bucket: Bucket, metadata: function (req, file, cb) { cb(null, { fieldName: file.fieldname, originalName: file.originalname }) }, key: function (req, file, cb) { cb(null, ${generateId()}) } }) }) return upload } else if (storageType === 'gcs') { return multer({ storage: new MulterGoogleCloudStorage({ projectId: process.env.GOOGLECLOUDSTORAGEPROJID, bucket: process.env.GOOGLECLOUDSTORAGEBUCKETNAME, keyFilename: process.env.GOOGLECLOUDSTORAGECREDENTIAL, uniformBucketLevelAccess: Boolean(process.env.GOOGLECLOUDUNIFORMBUCKETACCESS) ?? true, destination: uploads/${generateId()} }) }) } else { return multer({ dest: getUploadPath() }) } } - Transfers uploaded files to storage without verification https://github.com/FlowiseAI/Flowise/blob/d17c4394a238b49327b493c89feee45f3a20bb91/packages/server/src/utils/createAttachment.ts#L124-L158 tsx const files = (req.files as Express.Multer.File[]) || [] const fileAttachments = [] if (files.length) { const isBase64 = req.body.base64 for (const file of files) { if (!allowedFileTypes.length) { throw new InternalFlowiseError( StatusCodes.BADREQUEST, File type '${file.mimetype}' is not allowed. Allowed types: ${allowedFileTypes.join(', ')} ) } // Validate file type against allowed types if (allowedFileTypes.length > 0 && !allowedFileTypes.includes(file.mimetype)) { throw new InternalFlowiseError( StatusCodes.BADREQUEST, File type '${file.mimetype}' is not allowed. Allowed types: ${allowedFileTypes.join(', ')} ) } await checkStorage(orgId, subscriptionId, appServer.usageCacheManager) const fileBuffer = await getFileFromUpload(file.path ?? file.key) const fileNames: string[] = [] // Address file name with special characters: https://github.com/expressjs/multer/issues/1104 file.originalname = Buffer.from(file.originalname, 'latin1').toString('utf8') const { path: storagePath, totalSize } = await addArrayFilesToStorage( file.mimetype, fileBuffer, file.originalname, fileNames, orgId, chatflowid, chatId )

PoC

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PoC Description - Create a local file named shell.js containing arbitrary JavaScript code (or a malicious payload). - Send a multipart/form-data request to the /api/v1/attachments/891f64a2-a26f-4169-b333-905dc96c200a/:chatId endpoint without any authentication (login, session, or API keys). - During the upload, retain the filename as shell.js but spoof the Content-Type header as application/pdf. - This exploits the server's reliance solely on the client-provided file.mimetype, forcing it to process the malicious JS file as an allowed PDF, thereby confirming unauthenticated arbitrary file upload.

PoC

bash curl -X POST \ "http://localhost:3000/api/v1/attachments/891f64a2-a26f-4169-b333-905dc96c200a/$(uuidgen)" \ -F "files=@shell.js;type=application/pdf"

<img width="1916" height="1011" alt="image" src="https://github.com/user-attachments/assets/45679d95-00b9-4bee-9c94-7bd9403554d5" />

Impact

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1. Root Cause The vulnerability stems from relying solely on the MIME type without cross-validating the file extension or actual content. This allows attackers to upload executable files (e.g., .js, .php) or malicious scripts (.html) by masquerading them as benign images or documents.

2. Key Attack Scenarios

- Server Compromise (RCE): An attacker uploads a Web Shell and triggers its execution on the server. Successful exploitation grants system privileges, allowing unauthorized access to internal data and full control over the server. - Client-Side Attack (Stored XSS): An attacker uploads files containing malicious scripts (e.g., HTML, SVG). When a victim views the file, the script executes within their browser, leading to session cookie theft and account takeover.

3. Impact This vulnerability is rated as High severity. The risk is particularly critical if the system utilizes shared storage (e.g., S3, GCS) or static hosting features, as the compromise could spread to the entire infrastructure and affect other tenants.