Implementation guide
OWASP AISVS v1.01
Model Context Protocol (MCP) Security · Secure Transport
No fallback to older MCP protocol versions
Verify that MCP clients enforce a minimum acceptable protocol version and do not retry with, or fall back to, a protocol version or legacy handshake below that minimum.
OWASP AISVS v1.01, requirement 10.3.4
In plain English
An MCP client should set a minimum protocol version and refuse to go below it. If a client falls back automatically when asked, an attacker or a malicious server can push the connection down to an older version without the newer security features.
What good looks like
- A minimum acceptable protocol version is configured in the client.
- Servers offering only older versions, or legacy handshakes, are refused.
- The client does not retry with a lower version after a failure.
- Downgrade attempts are logged.
How teams usually do it
- A version check in the client's initialisation code, with the minimum set by policy.
- An inventory of servers still on old versions, with a plan to upgrade or retire them.
Suggested evidence
- Client configuration showing the minimum version.
- A test with a server that offers only an older version, showing the connection is refused.
Common mistakes
- Falling back automatically when a server claims not to support the current version.
- A permanent exception for one legacy server that becomes the route everyone uses.
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Requirement text from the OWASP AI Security Verification Standard, licensed under CC BY-SA 4.0. This guidance is shared under the same licence.