# CVE-2026-6608: lm-sys fastchat Arena Side-by-Side View add_text control flow

> Attackers can exploit incorrect control flow in the Arena Side-by-Side View handler to manipulate application logic.

- **Severity:** Medium 6.9 (`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`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 20 Apr 2026
- **Updated:** 20 Apr 2026
- **Also known as:** GHSA-f3q6-69f3-vwch, PYSEC-2026-2485

## What is CVE-2026-6608?

CVE-2026-6608 is a medium-severity vulnerability in lm-sys fastchat, published on 20 Apr 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official 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.

Weakness: CWE-670 (Always-Incorrect Control Flow Implementation)

## Which versions are affected by CVE-2026-6608?

3 open source packages ship an affected version, including fschat.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `fschat` | PyPI | All versions to 0.2.36 | None |
| `fschat` | PyPI | All versions to 0.2.36 | None |
| `fschat` | PyPI | All versions to 0.2.36 | None |

## How do I fix CVE-2026-6608?

No fix has been published for CVE-2026-6608 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-6608 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-6608 exists.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-6608 and fix it if it is affected.

What it is: CVE-2026-6608 is a medium-severity vulnerability in lm-sys fastchat, published on 20 Apr 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-6608 exists.

Affected packages and fixed versions:
- fschat (PyPI): affected all versions up to 0.2.36; no fix listed
- fschat (PyPI): affected all versions up to 0.2.36; no fix listed
- fschat (PyPI): affected all versions up to 0.2.36; no fix listed

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-6608/ (Markdown: https://insights.sbomify.com/CVE-2026-6608/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-6608
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-6608
- OSV: https://osv.dev/vulnerability/CVE-2026-6608

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-6608/
Summaries are generated and may contain mistakes; the official description above is the record.