OS Command Injection in chainlit (CVE-2026-45018)
Summary
OS command injection in chainlit (CVE-2026-45018). Successful exploitation can lead to full system takeover. Exploitable via POST /mcp. Mitigation: upgrade to 2.12.0 or later.
Original description
The description as published by the source.
Am I affected?
Only if your deployment sets features.mcp.enabled = true in .chainlit/config.toml. MCP has been disabled by default since v2.7.0, so most Chainlit deployments are not affected. No authentication is required: /mcp is reachable by any client that can open a session.
Summary
When MCP is enabled (features.mcp.enabled = true), the POST /mcp endpoint for stdio transport accepts a user-controlled fullCommand string. The validate_mcp_command() function checks the executable name against a configurable allowlist but does not inspect or restrict the arguments. An attacker can pass npx -y -c 'ARBITRARY COMMAND' to execute arbitrary shell commands on the server with the privileges of the Chainlit process.
Affected / patched versions
| CVE | CVE-2026-45018 |
| Affected | >=2.4.0rc0, <2.12.0 with features.mcp.enabled = true (introduced in PR #1977, the change that added MCP support) |
| Patched | 2.12.0 (releasing 2026-08-25) |
Details
validate_mcp_command() in backend/chainlit/mcp.py uses shlex.split() to parse the command string and validates only the executable name (e.g., npx, uvx) against config.features.mcp.stdio.allowed_executables. Arguments are returned unchecked and passed directly to StdioServerParameters, which spawns a subprocess.
Since npx supports -c for arbitrary shell execution, npx -y -c 'PAYLOAD' passes the allowlist check while running whatever the attacker specifies…
Source: GitHub Security Advisories (Global) NVD GitHub Advisory
Auto-generated summary
Assembled by SecPulse from the source data (CVSS, CWE, affected versions) using templates — no AI involved. Check the original description and references below for exact details.
A vulnerability tracked as CVE-2026-45018 has been found in chainlit.
Attackers can target a specific entry point like POST /mcp over the network to misuse the product.
Successful exploitation can lead to full system takeover. CVSS score: 9.8/10.
What to do: upgrade chainlit to 2.12.0 or later.
If unsure, ask your IT team or search "chainlit CVE-2026-45018" on the vendor's site.
CVE-2026-45018 (chainlit) — CWE-78 / CVSS v3 9.8
Attack vector: remote (network-reachable) / unauthenticated / no user interaction
Attack surface: POST /mcp / stdio / fullCommand / npx
Patched: 2.12.0 — apply immediately
Workaround: set features.mcp.: enabled
Plan: 1) Audit SBOM/dependencies, 2) Stage→prod upgrade, 3) Add WAF/proxy monitoring on affected endpoints, 4) Hunt IOCs in logs.
Refs: see the GHSA / vendor advisory / patched release linked on this page.
OS command injection (CWE-78) exists in chainlit. Attackers reach the vulnerable code path via POST /mcp without authentication.
Check the vendor advisory for the affected versions of chainlit. Attack surface: POST /mcp / stdio / fullCommand / npx.
Severity: Critical (CVSS 9.8/10). Successful exploitation can lead to full system takeover.
Update to 2.12.0.
Workaround: set features.mcp.: enabled
Search webserver/proxy logs for unusual POST /mcp requests with malformed payloads or SQL meta-characters. Run grep -r 'chainlit' . against your dependency files (package-lock.json, requirements.txt, go.sum) to find affected services.
Response actions
Concrete steps and command examples for SOC/SRE teams to execute in order
-
1Identify exposure identify
grep -r 'chainlit' . | grep -v node_modulesリポジトリと本番環境の依存ファイル (package-lock.json / requirements.txt / go.sum / Gemfile.lock 等) で
chainlitを grep し、稼働しているサービス・バージョンを把握する。 -
2Hunt for indicators of compromise detect
grep 'POST /mcp' /var/log/nginx/access.log | grep -E '(unusual_payload|sqli_pattern)'アクセスログで
POST /mcpへの異常なリクエスト (不正な認証ヘッダ・SQLメタ文字)を過去 30〜90日分捜索。WAF/SIEM があれば該当パスのアラート発火履歴を確認。 -
3Apply temporary workaround mitigate
set features.mcp.: enabledパッチが適用されるまでの応急処置として、set features.mcp.: enabled を実施。回避策の副作用 (機能低下) を確認した上で。
-
4Apply patch patch
Upgrade chainlit to 2.12.0ステージング環境で 2.12.0 に上げて回帰テスト → 本番反映。回帰テストはアプリの主要ハッピーパスと、Step 2 で見つけた異常検知の続報チェックを含めること。
-
5Post-deployment verification verify
Replay attack against POST /mcp on staging to confirm patch closes the vectorパッチ適用後、ステージングで PoC または同等の悪用パターンを再現して脆弱性が閉じたことを確認。本番では Step 2 と同じログクエリでアラート再発が無いか継続監視。