Cyberdudebivash

GitHub MCP Server Vulnerabilities — Decoded (CyberDudeBivash Deep-Dive) By CyberDudeBivash

Powered by: cyberdudebivash.com | cyberbivash.blogspot.com

TL;DR (Exec Brief)

“MCP servers” (Model Context Protocol servers) expose tools—files, shell commands, HTTP fetchers, databases, vector search, cloud APIs—to AI assistants and agents. Thousands of experimental MCP servers live on GitHub, and many are deployed by teams without a hardened threat model. The result: data exfiltration, command execution, SSRF, path traversal, supply-chain compromise, and privacy leaks—often via zero-click prompt injection or insecure defaults.

This guide explains how MCP servers break, shows attacker playbooks, and gives a copy-paste hardening blueprint (code & configs) you can apply today—without killing developer velocity.

1) Threat Model for MCP Servers

Surfaces

  • Tool Endpoints: file system, shell, network, cloud, databases, search.
  • Transport: JSON-RPC/WebSocket/HTTP between client (agent) and server.
  • Caller Trust: local desktop agents, browser extensions, chatbots, or remote services.
  • Content Sources: URLs, documents, repos—often under attacker control.

Attacker Objectives

  • Read secrets (SSH keys, cloud creds, tokens).
  • Run code on the MCP host (RCE → lateral movement).
  • Pivot to internal services (SSRF, cloud metadata).
  • Poison results (supply-chain, model override, response manipulation).
  • Exfiltrate sensitive data silently through tool outputs.

Common Wrong Assumptions

  • “It’s local, so it’s safe.” (Local ≠ safe; any untrusted prompt/content can trigger tools.)
  • “We require a token.” (But do you scope it? Rotate it? Bind to loopback?)
  • “Read-only file access can’t harm us.” (It can leak keys, configs, customer data.)

2) The Top Vulnerability Classes (with fixes)

2.1 Path Traversal & Workspace Escape

Smell: readFile("../../.ssh/id_rsa") works.
Fixes:

  • Define a workspace root; resolve & verify all paths are inside it.
  • Disallow symlinks or resolve them to real paths before checks.
  • Maintain an allowlist per tool (/workspace/data, not /).

Node.js pattern

import { promises as fs } from "fs";
import path from "path";

const ROOT = "/srv/workspace";

function safeJoin(p: string) {
  const real = path.resolve(ROOT, p);
  if (!real.startsWith(path.resolve(ROOT) + path.sep)) {
    throw new Error("Path escape blocked");
  }
  return real;
}

export async function readSafe(relPath: string) {
  const abs = safeJoin(relPath);
  const st = await fs.lstat(abs);
  if (st.isSymbolicLink()) throw new Error("Symlink blocked");
  return fs.readFile(abs, { encoding: "utf8", flag: "r" });
}

2.2 Command Injection & Over-Privileged Shell Tools

Smell: exec("grep " + userArg + " file"), runners with --privileged.
Fixes:

  • Use execFile with explicit args; never exec string concatenation.
  • Whitelist commands; deny shell meta-characters; set time & memory limits.
  • Run tools in rootless containers with no new privileges.

Node.js pattern

import { execFile } from "child_process";
execFile("/usr/bin/grep", ["-n", "needle", "file.txt"], { timeout: 3000 }, cb);

Docker run (rootless)

docker run --read-only --pids-limit=256 --network=none \
  --security-opt no-new-privileges --cap-drop=ALL \
  -v /srv/workspace:/workspace:ro --user 1000:1000 mcp-server:stable

2.3 SSRF & Internal Pivoting

Smell: A “fetch” tool can hit http://169.254.169.254/ (cloud metadata) or http://db.internal:5432.
Fixes:

  • Allowlist schemes/hosts/ports; block link-local, loopback, RFC1918 by default.
  • Require DNS-over-HTTPS to trusted resolvers; enforce TLS with SNI/ALPN.
  • Strip sensitive headers; block redirects to private IPs.

2.4 Prompt Injection → Tool Abuse

Smell: Retrieved content tells the agent “read /etc/passwd” and it obeys.
Fixes:

  • Separate retrieval from tool execution; require user or policy approval for privileged tools.
  • Add content origin labels (trusted vs untrusted) and capability mapping.
  • Use policy-as-code to deny high-risk tool calls that originate from untrusted content.

2.5 Authentication, Authorization & Binding

Smell: MCP server listens on 0.0.0.0:3001 with a static token in .env.
Fixes:

  • Bind to 127.0.0.1 by default; if remote, use mTLS or short-lived OAuth tokens.
  • Scope tokens to capabilities; rotate frequently; rate-limit calls.

2.6 Logging-Driven Data Exfiltration

Smell: Tool logs include full file contents, cookies, or API keys.
Fixes:

  • Redact with patterns (AWS keys, JWTs).
  • Log arguments metadata, not raw payloads.
  • Provide a privacy mode toggle for regulated data.

2.7 Dependency & Supply-Chain Risks

Smell: latest tags, no lockfiles, unsigned releases.
Fixes:

  • Pin with lockfiles; review maintainers; use provenance and artifact signing (e.g., cosign).
  • CI gates for SCA, lint, license, and security scorecards.

2.8 Deserialization & Message Validation

Smell: Server trusts any JSON struct from client.
Fixes:

  • Validate every RPC with schemas (zod/pydantic); cap sizes; reject unknown fields.
  • Enforce max concurrency and time-outs to prevent DoS loops.

Type-safe schema

import { z } from "zod";

const ReadInput = z.object({ path: z.string().max(256) });
type ReadInput = z.infer<typeof ReadInput>;

function handleRead(input: unknown) {
  const args = ReadInput.parse(input); // throws on bad input
  return readSafe(args.path);
}

3) Attacker Playbooks 

  1. Content-to-Tool Hijack: Malicious web page in retrieval contains instructions; agent forwards to MCP; file tool reads ~/.aws/credentials.
  2. Dependency Backdoor: Popular MCP server on GitHub updates a transitive package; build pipeline pulls a poisoned version; server exfiltrates tokens on start.
  3. Local Runner Escape: Self-hosted runner launches MCP with --privileged; PR modifies tool config; attacker gets RCE on the CI host.
  4. SSRF Pivot: “Fetch URL” tool used to probe internal IP ranges and cloud metadata endpoints; attacker harvests credentials and pivots to cloud.

4) Defense-in-Depth Blueprint 

4.1 Policy Guardrails

  • Default-deny: tools require explicit allowlists (commands, paths, hosts).
  • Trust labels: Untrusted content cannot trigger privileged tools without human approval or policy exception.
  • Rate limits and budgets per session (time, CPU, outbound requests).

4.2 Process Isolation

  • Run MCP in rootless, read-only containers; mount a narrow workspace.
  • Add seccomp/AppArmor/SELinux; drop all Linux capabilities.
  • Separate build and runtime images (distroless, non-root).

4.3 Network Controls

  • Egress allowlist; block 169.254.169.254, 127.0.0.0/8, 10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16 unless explicitly needed.
  • Disable redirects across trust boundaries; enforce TLS.

4.4 Identity & Secrets

  • Use short-lived credentials (OIDC/WIF) instead of static keys.
  • Store secrets outside the repo; provide a redaction layer in logs.

4.5 Software Supply Chain

  • Require signed artifacts (cosign) and verify on start.
  • Generate SBOM and track vulnerabilities; set CI gates for critical issues.
  • Pin to trusted registries; no latest.

4.6 Observability & Detection

  • Emit security-aware audit logs: tool, args schema, decision policy, result size.
  • Alert on policy bypass, path escape attempts, blocked SSRF.
  • Periodic self-tests that try forbidden operations; alert if they pass.

5) Secure Defaults Checklist 

  •  Server binds to 127.0.0.1 unless ALLOW_REMOTE=1.
  •  Path allowlist & symlink blocking.
  •  Command allowlist via execFile only; timeouts set.
  •  Network allowlist; metadata & RFC1918 blocked.
  •  Schema validation (size limits, unknown fields rejected).
  •  Token scoping & rotation; rate-limits.
  •  Read-only containers, no new privileges, capabilities dropped.
  •  Signed releases & SBOM published; lockfiles committed.
  •  Redacted logs; privacy mode toggle.
  •  Security.md with coordinated disclosure process.

6) Pentest Playbook for MCP Servers 

  • Traversal tests: ../../ paths, symlink loops, alternate encodings.
  • Command tests: meta-chars, argument explosions, timeouts.
  • SSRF tests: DNS rebinding, redirects to private IPs, IPv6 edge cases.
  • Prompt injection: make untrusted content request high-risk tools.
  • DoS: massive payloads, zip bombs, recursive calls.
  • Supply chain: tamper with dependencies, unsigned image refusal.

7) Compliance & Best-Practice Mapping

  • NIST SSDF: secure build & release, secrets handling, code review, incident response.
  • OWASP ASVS/Top-10: input validation, authz, logging, SSRF, file handling.
  • SLSA + Sigstore: provenance and signature verification for MCP server artifacts.
  • Zero Trust: verify identity, device, and workload before granting tool capabilities.

8) Quick-Start Templates

Kyverno policy (Kubernetes) — require signed images & non-root

apiVersion: kyverno.io/v1
kind: ClusterPolicy
metadata:
  name: mcp-secure-run
spec:
  validationFailureAction: Enforce
  rules:
  - name: require-nonroot
    match: { resources: { kinds: ["Pod"] } }
    validate:
      message: "Run as non-root with read-only root FS"
      pattern:
        spec:
          securityContext:
            runAsNonRoot: true
          containers:
          - securityContext:
              readOnlyRootFilesystem: true
              allowPrivilegeEscalation: false
  - name: verify-signature
    match: { resources: { kinds: ["Pod"] } }
    verifyImages:
    - imageReferences: ["registry.example.com/mcp-server:*"]
      attestors:
      - entries:
        - keyless:
            issuer: "https://token.actions.githubusercontent.com"
            subject: "repo:org/repo:ref:refs/heads/main"

.env.example (never commit secrets)

MCP_BIND_ADDR=127.0.0.1
MCP_PORT=3030
ALLOW_REMOTE=0
TOKEN_ISSUER_URL=
TOKEN_AUDIENCE=
WORKSPACE_ROOT=/srv/workspace
NETWORK_ALLOWLIST=api.github.com,example.com

9) Publishing Block (CyberBivash Blogspot)

Title: GitHub MCP Server Vulnerabilities Decoded — Real Risks, Real Fixes
Meta Description (≤160 chars): A practical guide to securing GitHub MCP servers: stop path traversal, SSRF, RCE, and prompt-injection with copy-paste hardening blueprints.
Slug: /github-mcp-server-vulnerabilities-decoded-secure-hardening-guide
Internal Links: OIDC short-lived creds, artifact signing with cosign, SBOM/provenance primer.
Affiliate-ready CTAs: 1Password Business (secrets rotation), Malwarebytes/Bitdefender (EDR/XDR), Cloudflare Zero Trust (ZTNA), Proton/NordVPN (secure admin plane).


#MCP #AIAgents #SupplyChainSecurity #PromptInjection #SSRF #RCE #DevSecOps #SBOM #Sigstore #SLSA #ZeroTrust #Kubernetes #APIsecurity #DataProtection #CyberDudeBivash

10) CyberDudeBivash — Your Secure MCP Launchpad

We help teams ship secure-by-default MCP servers: workspace confinement, policy guardrails, signature verification, and zero-trust identity—in one sprint.

Leave a comment

Design a site like this with WordPress.com
Get started