MCP Python SDK's OAuth Flaw: How a Malicious Server Could Steal Client Secrets
A security advisory published September 28, 2026, alongside a technical writeup from Cycode, disclosed a flaw in the official Model Context Protocol Python SDK that let a malicious MCP server redirect a connecting client to an attacker controlled token endpoint. The SDK failed to verify that the authorization server endpoint returned during login actually belonged to the legitimate service, which meant a client could end up handing its client secret, authorization code, and PKCE proof key to an attacker. The fix landed in versions 1.30.0 and 2.2.0 on September 7, with the public advisory following three weeks later. No known active exploitation has been reported, but any team running an MCP client against untrusted or semi trusted servers should treat this as a today problem, not a changelog line to skim.
MCP Python SDK's OAuth Flaw: How a Malicious Server Could Steal Client Secrets
A security advisory published September 28, 2026, alongside a same day technical writeup from Cycode, disclosed a flaw in the official Model Context Protocol Python SDK's OAuth implementation. The SDK did not verify that the authorization server endpoint details returned during a login flow actually belonged to the legitimate service the client thought it was talking to, which meant a malicious MCP server could redirect the client to an attacker controlled token endpoint and collect whatever credentials the client sent there.
What the flaw actually does
| Detail | What the advisory says |
|---|---|
| Root cause | SDK did not validate that returned authorization endpoint details belonged to the legitimate login service |
| Attack path | Malicious server redirects client to an attacker controlled token endpoint during login |
| What gets exposed | Client secret, authorization code, and PKCE proof key |
| Affected versions | 1.x: 1.9.1 through 1.29.1; 2.x: 2.0.0 through 2.1.1 |
| Fixed versions | 1.30.0 and 2.2.0, released September 7, 2026 |
| Extra step for some providers | ClientCredentialsOAuthProvider and PrivateKeyJWTOAuthProvider require configuring an explicit issuer parameter |
| Disclosure date | September 28, 2026, three weeks after the fix shipped |
| Known active exploitation | None reported at time of disclosure |
Why OAuth endpoint validation matters more for MCP than for a typical API client
A conventional OAuth client usually talks to a small, known set of identity providers configured ahead of time by whoever builds the integration. An MCP client is built around a different assumption: it connects to whatever server a user or an agent configuration points it at, potentially a server it has never talked to before, sometimes one discovered dynamically. That model is precisely why authorization server endpoint validation cannot be an afterthought in an MCP client. If the client trusts whatever endpoint details a server hands back during the OAuth handshake, a malicious or compromised server can simply supply its own token endpoint and the client will dutifully send its client secret, authorization code, and PKCE proof key there, no different in effect from a classic OAuth mix up attack, just delivered through a protocol designed specifically to make connecting to new, less vetted servers routine.
PKCE, the Proof Key for Code Exchange extension, exists specifically to prevent an intercepted authorization code from being redeemed by an attacker who does not also have the original code verifier. This flaw defeats that protection by a different route: if the token endpoint itself is attacker controlled, PKCE's verifier check happens on the attacker's own infrastructure, which the attacker of course accepts, so the protection PKCE is meant to provide never actually engages against the party who needs to be stopped.
Who is actually exposed
The exposure scales with how permissively an environment connects to MCP servers. A team running MCP clients exclusively against a small, hand vetted list of internal or well known servers has a narrower window, though "narrower" is not "zero," since a previously trustworthy server could itself be compromised and start returning malicious endpoint details without the client having any way to notice under the vulnerable SDK versions. A team or an individual developer connecting MCP clients to servers discovered dynamically, pulled from a public registry, or configured by end users rather than centrally vetted has meaningfully more exposure, because the entire value proposition of that setup depends on connecting to servers nobody has manually reviewed. The broader identity and authorization model MCP deployments should be building toward covers this in more depth, and this vulnerability is a concrete illustration of why endpoint trust cannot be assumed at the protocol layer.
This is not the same issue as the Excel MCP path traversal CVE
It is worth being explicit that this OAuth flaw is unrelated to the CVE-2026-40576 Excel MCP server path traversal vulnerability covered here previously. That issue was a file system path traversal bug in a specific third party MCP server implementation, allowing arbitrary file access through a poorly sanitized path parameter. This issue is an OAuth endpoint validation gap in the official Python SDK itself, affecting any client built on the vulnerable SDK versions regardless of which server it connects to. Both point at the same underlying pattern, MCP's server ecosystem growing faster than its security tooling, but they are different bugs in different layers of the stack, and a team that patched one should not assume the other is covered.
What to do about it today
Confirm your MCP Python SDK version and upgrade to 1.30.0 or 2.2.0 if you are anywhere in the affected ranges, 1.9.1 through 1.29.1 or 2.0.0 through 2.1.1. If your integration uses ClientCredentialsOAuthProvider or PrivateKeyJWTOAuthProvider specifically, confirm the new issuer parameter is configured, since the patched SDK version alone is not sufficient for those provider types without that additional configuration. Audit which MCP servers your clients currently connect to and separate the list into centrally vetted servers versus dynamically discovered or user configured ones, since that distinction determines how urgently the rest of this applies to your specific deployment.
When this applies to your stack
If your team has any MCP client in production, whether built in house or through a framework that wraps the official Python SDK, this is worth a version check today rather than at the next dependency update cycle. The three week gap between the fix shipping and the public advisory means any team not actively tracking SDK release notes has likely been running vulnerable code without knowing it.
Contra Collective builds and hardens production AI agent infrastructure for enterprise clients, including the MCP client and server configurations that need this kind of endpoint trust model reviewed before deployment rather than after an incident. If your team cannot currently produce a clean answer to which MCP SDK versions are running in production, that answer is worth having before the next dependency scan.
FAQ
Has this vulnerability been assigned a CVE number? Not as of the September 28, 2026 advisory. The issue was disclosed and patched without a formal CVE identifier attached at time of publication.
Do I need to rotate credentials if I was running an affected version? Treat any client secret used with an affected SDK version against an untrusted or semi trusted MCP server as potentially exposed, and rotate it after upgrading. If you only ever connected to a small, fully trusted set of servers that you control, the practical exposure is lower but not provably zero.
Does upgrading the SDK version alone fully fix this for every provider type? No. For ClientCredentialsOAuthProvider and PrivateKeyJWTOAuthProvider specifically, the advisory notes an additional issuer parameter must be configured on top of the version upgrade.
Is this related to the broader MCP registry and exposed server issues covered elsewhere? It is adjacent but distinct. Exposed server issues generally concern MCP servers themselves being misconfigured or publicly reachable without authentication. This is a flaw in how MCP clients validate OAuth endpoint authenticity during login, a different point in the connection lifecycle.
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