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Merged
jba merged 1 commit into from
Dec 8, 2025
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internal/docs: document UserID for session hijacking prevention #697

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15 changes: 12 additions & 3 deletions docs/protocol.md
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Original file line number Diff line number Diff line change
Expand Up @@ -316,9 +316,18 @@ If you create your own with
If you are using Go 1.24 or above,
we recommend using [`crypto/rand.Text`](https://pkg.go.dev/crypto/rand#Text)

- _Binding session IDs to user information_. This is an application requirement, out of scope
for the SDK. You can create your own session IDs by setting
[`ServerOptions.GetSessionID`](https://pkg.go.dev/github.com/modelcontextprotocol/go-sdk/mcp#ServerOptions.GetSessionID).
- _Binding session IDs to user information_. The SDK supports this mitigation through
[`TokenInfo.UserID`](https://pkg.go.dev/github.com/modelcontextprotocol/go-sdk/auth#TokenInfo.UserID).
When a [`TokenVerifier`](https://pkg.go.dev/github.com/modelcontextprotocol/go-sdk/auth#TokenVerifier)
sets `UserID` on the returned `TokenInfo`, the streamable transport will:
1. Store the user ID when a new session is created.
2. Verify that subsequent requests to that session include a token with the same `UserID`.
3. Reject requests with a 403 Forbidden if the user ID doesn't match.

**Recommendation**: If your `TokenVerifier` can extract a user identifier from the token
(such as a `sub` claim in a JWT, or a user ID associated with an API key), set
`TokenInfo.UserID` to enable this protection. This prevents an attacker with a valid
token from hijacking another user's session by guessing or obtaining their session ID.

## Utilities

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15 changes: 12 additions & 3 deletions internal/docs/protocol.src.md
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Original file line number Diff line number Diff line change
Expand Up @@ -242,9 +242,18 @@ If you create your own with
If you are using Go 1.24 or above,
we recommend using [`crypto/rand.Text`](https://pkg.go.dev/crypto/rand#Text)

- _Binding session IDs to user information_. This is an application requirement, out of scope
for the SDK. You can create your own session IDs by setting
[`ServerOptions.GetSessionID`](https://pkg.go.dev/github.com/modelcontextprotocol/go-sdk/mcp#ServerOptions.GetSessionID).
- _Binding session IDs to user information_. The SDK supports this mitigation through
[`TokenInfo.UserID`](https://pkg.go.dev/github.com/modelcontextprotocol/go-sdk/auth#TokenInfo.UserID).
When a [`TokenVerifier`](https://pkg.go.dev/github.com/modelcontextprotocol/go-sdk/auth#TokenVerifier)
sets `UserID` on the returned `TokenInfo`, the streamable transport will:
1. Store the user ID when a new session is created.
2. Verify that subsequent requests to that session include a token with the same `UserID`.
3. Reject requests with a 403 Forbidden if the user ID doesn't match.

**Recommendation**: If your `TokenVerifier` can extract a user identifier from the token
(such as a `sub` claim in a JWT, or a user ID associated with an API key), set
`TokenInfo.UserID` to enable this protection. This prevents an attacker with a valid
token from hijacking another user's session by guessing or obtaining their session ID.

## Utilities

Expand Down
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