Vulnerability in mcp (CVE-2026-63119)
Summary
Vulnerability in mcp (CVE-2026-63119). Risk of unauthorized operations or information disclosure. Exploitable via limit. Mitigation: upgrade to 0.23.0 or later.
Original description
The description as published by the source.
Summary
The stdio transports in MCP::Server::Transports::StdioTransport and MCP::Client::Stdio read newline-delimited JSON-RPC frames using IO#gets with no limit argument. CRuby's IO#gets with no limit reads from the current position until the next separator (\n) with no upper bound on the returned string length. A peer that streams bytes without ever emitting a newline causes gets to accumulate the entire stream in a single Ruby String until the process is killed by the operating-system OOM killer.
This is the same vulnerability class tracked in sibling MCP SDKs as GHSA-74gp-qhv5-v493 (Kotlin), GHSA-wqgc-pwpr-pq7r (TypeScript), GHSA-655q-2283-6jgj (Python), and others. It was identified during a cross-SDK audit; ruby-sdk had no prior report.
Affected code
Verified at main @ cf44475c.
Server transport — lib/mcp/server/transports/stdio_transport.rb
# line 23
while @open && (line = $stdin.gets) # <-- no limit argument
response = @session.handle_json(line.strip)
...
# line 76
while @open && (line = $stdin.gets) # <-- no limit argument
begin
parsed = JSON.parse(line.strip, symbolize_names: true)
$stdin is the raw process global (only set_encoding is applied at line 16); there is no wrapper imposing a length limit.
Client transport — [lib/mcp/client/stdio.rb](https://github.com/modelcon…
Source: NIST National Vulnerability Database 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-2026-63119 has been found in mcp.
Attackers can target a specific entry point like limit over the network to misuse the product.
Risk of unauthorized operations or information disclosure. CVSS score: 6.2/10.
What to do: upgrade mcp to 0.23.0 or later.
If unsure, ask your IT team or search "mcp CVE-2026-63119" on the vendor's site.
CVE-2026-63119 (mcp) — CWE-400 / CVSS v3 6.2
Attack vector: local / unauthenticated / no user interaction
Attack surface: limit / gets / String / main
Patched: 0.23.0 — apply immediately
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.
A vulnerability (CWE-400) exists in mcp. Attackers reach the vulnerable code path via limit without authentication.
Check the vendor advisory for the affected versions of mcp. Attack surface: limit / gets / String / main.
Severity: Medium (CVSS 6.2/10). Risk of unauthorized operations or information disclosure.
Update to 0.23.0.
Until the patch is applied: disable the affected feature, apply WAF rules, or restrict access via network ACLs.
Search webserver/proxy logs for unusual request patterns matching this CVE's known IOCs. Run grep -r 'mcp' . 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 'mcp' . | 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
mcpを grep し、稼働しているサービス・バージョンを把握する。 -
2Apply patch patch
Upgrade mcp to 0.23.0ステージング環境で 0.23.0 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を含めること。
-
3Post-deployment verification verify
Confirm patched version is live in production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。本番ではアクセスログ・WAF でアラート再発が無いか継続監視。
Affected packages
References
- advisory nvd.nist.gov
- package github.com
- web github.com
- web github.com
- web github.com
- web github.com