In affected versions, the SDK's server-side bearer authentication had no way to check that a token was issued for your server (its audience). Where one authorization server issues tokens for several services, a valid token for another service was also accepted by your MCP server.
Yes, if your HTTP server uses requireBearerAuth() or verifyBearerToken(), its authorization server also issues tokens for other services, and your verifyAccessToken() does not check the token's audience (aud) itself.
Affected versions:
@modelcontextprotocol/sdk 1.6.0 through 1.31.0@modelcontextprotocol/server 2.0.0 through 2.2.0@modelcontextprotocol/express 2.0.0 through 2.0.1@modelcontextprotocol/server-legacy 2.0.0 through 2.3.0Upgrade to:
@modelcontextprotocol/sdk 1.32.0 or later@modelcontextprotocol/server 2.3.0 or later, together with @modelcontextprotocol/express 2.0.2 or later if you use it@modelcontextprotocol/server-legacy 2.3.1 or laterUpgrading alone changes nothing. The patched versions add an expectedResource setting that is off by default. To turn the check on, make both of these changes together:
requireBearerAuth({ verifier, expectedResource: new URL('https://mcp.example.com/mcp') }).verifyAccessToken() as AuthInfo.resource, also a URL.A token for another service then gets 401. With only the first change, every request gets 401.
If you cannot upgrade yet, reject any token whose aud does not include your server inside verifyAccessToken(). That closes the issue on every version.
| Software | Affected versions |
|---|---|
@modelcontextprotocol / sdk
|
>= 1.6.0, < 1.32.0 |
@modelcontextprotocol / server
|
>= 2.0.0, < 2.3.0 |
@modelcontextprotocol / express
|
>= 2.0.0, < 2.0.2 |
@modelcontextprotocol / server-legacy
|
>= 2.0.0, < 2.3.1 |
A vulnerability database tells you what is broken. Attack surface management tells you where it is running: it discovers your internet-facing assets and flags the ones affected by issues like this one.
What is attack surface management? →A security vulnerability is a weakness in software, hardware, or configuration that can be exploited to compromise confidentiality, integrity, or availability. Many vulnerabilities are tracked as CVEs (Common Vulnerabilities and Exposures), which provide a standardized identifier so teams can coordinate patching, mitigation, and risk assessment across tools and vendors.
CVSS (Common Vulnerability Scoring System) estimates technical severity, but it doesn't automatically equal business risk. Prioritize using context like internet exposure, affected asset criticality, known exploitation (proof-of-concept or in-the-wild), and whether compensating controls exist. A "Medium" CVSS on an exposed, production system can be more urgent than a "Critical" on an isolated, non-production host.
A vulnerability is the underlying weakness. An exploit is the method or code used to take advantage of it. A zero-day is a vulnerability that is unknown to the vendor or has no publicly available fix when attackers begin using it. In practice, risk increases sharply when exploitation becomes reliable or widespread.
Recurring findings usually come from incomplete Asset Discovery, inconsistent patch management, inherited images, and configuration drift. In modern environments, you also need to watch the software supply chain: dependencies, containers, build pipelines, and third-party services can reintroduce the same weakness even after you patch a single host. Unknown or unmanaged assets (often called Shadow IT) are a common reason the same issues resurface.
Use a simple, repeatable triage model: focus first on externally exposed assets, high-value systems (identity, VPN, email, production), vulnerabilities with known exploits, and issues that enable remote code execution or privilege escalation. Then enforce patch SLAs and track progress using consistent metrics so remediation is steady, not reactive.
SynScan combines attack surface monitoring and continuous security auditing to keep your inventory current, flag high-impact vulnerabilities early, and help you turn raw findings into a practical remediation plan.