CVE-2026-6608: Fschat FastChat Has A Content Moderation Vulnerability

MODERATE
Advisory Identifier: GHSA-f3q6-69f3-vwch
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CVSS 5.3
CWE-670
Published
Advisory Summary

CVE-2026-6608 is a MODERATE (CVSS 5.3) fastchat has a content moderation vulnerability (CWE-670: Always-Incorrect Control Flow Implementation) affecting Fschat libraries in the PyPI ecosystem. The issue involves fastchat has a content moderation bypass via arena side-by-side views. Remediation is available in the latest vendor maintenance releases.

CVE-2026-6608 at a Glance

CVE Identifier CVE-2026-6608
GHSA Identifier GHSA-f3q6-69f3-vwch
Severity MODERATE
CVSS v3.1 Base Score 5.3
CWE Classification CWE-670: Always-Incorrect Control Flow Implementation
Affected Product Fschat
Package Ecosystem PyPI
Published Date
Last Updated

Security Impact

CVSS: 5.3

The security impact metrics below are parsed directly from the CVSS v3.1 vector string (CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N).

Attack Vector Network (AV:N)
Attack Complexity Low (AC:L)
Privileges Required None (PR:N)
User Interaction None (UI:N)
Scope Unchanged (S:U)
Confidentiality None (C:N)
Integrity Low (I:L)
Availability None (A:N)

How to Fix CVE-2026-6608

To remediate this vulnerability, upgrade all affected Fschat dependencies to a patched release. Both direct and transitive dependencies should be checked, as frameworks or security libraries may pull in vulnerable versions transitively.

fschat fixed: Latest secure release 
pip install fschat>=Latest secure release
Verify Direct and Transitive Dependencies

Run the following command in your project directory to verify that all resolved dependencies use the patched release: 

pip show fschat

How to Check if You Are Affected

To determine whether your Python environment is affected by CVE-2026-6608, inspect your installed packages using pip:

pip show fschat

Analyze the terminal output to locate any version of Fschat in your dependency tree. If any resolved version matches the vulnerable ranges listed in the table below (for example, versions earlier than the patched release), your application is affected.

Note: The pip show fschat command checks currently installed packages in your active Python environment.

Technical Details

Vulnerability Mechanism & Technical Analysis

This vulnerability is classified under CWE-670: Always-Incorrect Control Flow Implementation, where the code contains a control flow path that does not reflect the algorithm that the path is intended to implement, leading to incorrect behavior any time this path is navigated. In Fschat, software state validation or input boundaries are circumvented during processing. Review the official source advisory statement below for specific affected components, classes, and reproduction details.

Source Advisory Statement

A vulnerability was detected in lm-sys fastchat up to 0.2.36. Impacted is the function add_text of the component Arena Side-by-Side View Handler. The manipulation results in incorrect control flow. The attack can be launched remotely. The exploit is now public and may be used. The root cause was fixed in commit 34eca62 for gradio_block_arena_named.py, but three other files were missed.

CVE-2026-6608 Affected Fschat Versions

1 component(s)
Product / Package Ecosystem Affected Versions Fixed Version
fschat
pypi/fschat
PyPI
≤ 0.2.36

References

NVD — CVE-2026-6608 NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-6608
Fschat Source Repository REPOSITORY
https://github.com/lm-sys/FastChat/issues/3834
Fschat Source Repository REPOSITORY
https://gist.github.com/YLChen-007/e45039d23e698222d887ee09735d9d36
Fschat Source Repository REPOSITORY
https://github.com/lm-sys/FastChat
vuldb.com/submit/792228 WEB
https://vuldb.com/submit/792228
vuldb.com/vuln/358243 WEB
https://vuldb.com/vuln/358243
vuldb.com/vuln/358243/cti WEB
https://vuldb.com/vuln/358243/cti

Frequently Asked Questions

What is CVE-2026-6608?
CVE-2026-6608 is a security vulnerability in Fschat involving FastChat has a Content Moderation Bypass via Arena Side-by-Side Views under CWE-670: Always-Incorrect Control Flow Implementation.
Which Fschat versions are affected?
Affected versions include releases prior to current release patches across affected packages: fschat.
What version fixes CVE-2026-6608?
Upgrading to current release patches (or the latest patched release for your respective branch) resolves the vulnerability.
Is fschat affected?
Yes, fschat is affected in releases prior to Latest secure release. Upgrade your dependency to Latest secure release or later.
What is the CVSS score of CVE-2026-6608?
CVE-2026-6608 has a CVSS v3.1 base score of 5.3 (MODERATE) with vector string CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N.
What CWE is associated with CVE-2026-6608?
CVE-2026-6608 is classified under CWE-670: Always-Incorrect Control Flow Implementation (The code contains a control flow path that does not reflect the algorithm that the path is intended to implement, leading to incorrect behavior any time this path is navigated.).
How can I check PyPI dependencies for CVE-2026-6608?
Run the dependency tree command: 'pip show fschat'. This command lists direct and transitive dependencies in your build configuration so you can verify if a vulnerable version is resolved.
Machine-readable OSV JSON OSV v1.5 Schema
Normalized schema output
View Raw JSON 
{
  "schema_version" : "1.4.0",
  "id" : "GHSA-f3q6-69f3-vwch",
  "modified" : "2026-04-23T14:29:57Z",
  "published" : "2026-04-20T06:31:28Z",
  "aliases" : [ "CVE-2026-6608" ],
  "summary" : "FastChat has a Content Moderation Bypass via Arena Side-by-Side Views",
  "details" : "A vulnerability was detected in lm-sys fastchat up to 0.2.36. Impacted is the function add_text of the component Arena Side-by-Side View Handler. The manipulation results in incorrect control flow. The attack can be launched remotely. The exploit is now public and may be used. The root cause was fixed in commit 34eca62 for gradio_block_arena_named.py, but three other files were missed.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N"
  }, {
    "type" : "CVSS_V4",
    "score" : "CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:L/VA:N/SC:N/SI:N/SA:N/E:P"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "PyPI",
      "name" : "fschat"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "last_affected" : "0.2.36"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-6608"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/lm-sys/FastChat/issues/3834"
  }, {
    "type" : "WEB",
    "url" : "https://gist.github.com/YLChen-007/e45039d23e698222d887ee09735d9d36"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/lm-sys/FastChat"
  }, {
    "type" : "WEB",
    "url" : "https://vuldb.com/submit/792228"
  }, {
    "type" : "WEB",
    "url" : "https://vuldb.com/vuln/358243"
  }, {
    "type" : "WEB",
    "url" : "https://vuldb.com/vuln/358243/cti"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-670" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-04-23T14:29:57Z",
    "nvd_published_at" : "2026-04-20T06:16:21Z"
  }
}