EUVD-2026-71144
GHSA-46fr-7j3v-277v
Severity by source
CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:L/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X
Network-reachable, unauthenticated registration (AV:N/AC:L/PR:N/UI:N) yields full prompt/response interception (C:H) and forged responses/spoofing (I:H); SSRF/probing gives limited availability impact (A:L).
Primary rating from Vendor (VulnCheck).
CVSS VectorVendor: VulnCheck
Lifecycle Timeline
2DescriptionCVE.org
FastChat contains an authentication bypass vulnerability in the /register_worker endpoint that allows unauthenticated attackers to register arbitrary worker addresses and perform server-side request forgery. Attackers can register malicious workers under victim model names to intercept user prompts, images, and responses, or probe internal network ports across the worker mesh.
AnalysisAI
Server-side request forgery and model spoofing in lm-sys FastChat let unauthenticated remote attackers register arbitrary workers via the controller's /register_worker endpoint, which performs no authentication. By registering a malicious worker address under a legitimate model's name, an attacker can silently intercept user prompts, uploaded images, and model responses, or coerce the controller into probing internal network ports across the worker mesh. …
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Attack ChainAIDerived
Hypothetical attack flow derived from CVE metadata
Vulnerability AssessmentAI
| Exploitation | Requires network reachability to the FastChat controller service and its /register_worker endpoint, which by design performs no authentication (CWE-306) - so exploitation succeeds against default configurations wherever the controller is reachable by the attacker. … Additional conditions and limiting factors are described in the full assessment. |
| Risk Assessment | The CVSS 4.0 vector (AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:L) describes a straightforward network-reachable, low-complexity, unauthenticated attack with high confidentiality and integrity impact - consistent with the described unauthenticated registration abuse - yielding a 9.3 base score. … Full risk analysis with EPSS, KEV, and SSVC signal comparison available after sign-in. |
| Exploit Scenario | Full exploit scenario with step-by-step reproduction available after sign-in. |
| Remediation | No vendor-released patch identified at time of analysis; the references are an open GitHub issue (#3886) and the VulnCheck advisory rather than a tagged fixed release, so monitor https://github.com/lm-sys/FastChat/issues/3886 and the VulnCheck advisory for a fixed version. … Detailed patch versions, workarounds, and compensating controls in full report. |
Recommended ActionAI
Within 24 hours: Inventory all FastChat deployments in production and document worker mesh topology; restrict network access to the /register_worker endpoint using firewall rules or reverse-proxy authentication to block unauthenticated registration. …
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A Server-Side Request Forgery (SSRF) vulnerability exists in the POST /worker_generate_stream API endpoint of the Contro
A Denial of Service (DoS) vulnerability exists in the file upload feature of lm-sys/fastchat version 0.2.36. Rated high
A Server-Side Request Forgery (SSRF) vulnerability exists in lm-sys/fastchat version 0.2.36. Rated high severity (CVSS 7
A Server-Side Request Forgery (SSRF) vulnerability was identified in the lm-sys/fastchat web server, specifically in the
In lm-sys/fastchat Release v0.2.36, the server fails to handle excessive characters appended to the end of multipart bou
An open redirect vulnerability in lm-sys/fastchat Release v0.2.36 allows a remote unauthenticated attacker to redirect u
Resource exhaustion in lm-sys FastChat up to 0.2.36 allows remote attackers to trigger denial of service by sending mani
Information disclosure in lm-sys FastChat up to version 0.2.36 allows remote unauthenticated attackers to manipulate the
Same technique Authentication Bypass
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External POC / Exploit Code
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