Use github.com/dgrijalva/jwt-go instead of custom logic to decode OAuth token

Signed-off-by: Ed Snible <snible@us.ibm.com>

Author: esnible

mcp/github_tool/README.md

@@ -178,4 +178,3 @@ oc --context kind-agent-platform apply -f /tmp/expose-github-tool.yaml
 
 At this point, you can do the MCP curls above if you define `MCP=http://github-tool.127-0-0-1.sslip.io:8888/mcp`
 
-

mcp/github_tool/go.mod

@@ -2,7 +2,10 @@ module github.com/kagenti/mcp-mitm
 
 go 1.24.9
 
-require github.com/mark3labs/mcp-go v0.41.1
+require (
+	github.com/dgrijalva/jwt-go v3.2.0+incompatible
+	github.com/mark3labs/mcp-go v0.41.1
+)
 
 require (
 	github.com/bahlo/generic-list-go v0.2.0 // indirect

mcp/github_tool/go.sum

@@ -4,6 +4,8 @@ github.com/buger/jsonparser v1.1.1 h1:2PnMjfWD7wBILjqQbt530v576A/cAbQvEW9gGIpYMU
 github.com/buger/jsonparser v1.1.1/go.mod h1:6RYKKt7H4d4+iWqouImQ9R2FZql3VbhNgx27UK13J/0=
 github.com/davecgh/go-spew v1.1.1 h1:vj9j/u1bqnvCEfJOwUhtlOARqs3+rkHYY13jYWTU97c=
 github.com/davecgh/go-spew v1.1.1/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
+github.com/dgrijalva/jwt-go v3.2.0+incompatible h1:7qlOGliEKZXTDg6OTjfoBKDXWrumCAMpl/TFQ4/5kLM=
+github.com/dgrijalva/jwt-go v3.2.0+incompatible/go.mod h1:E3ru+11k8xSBh+hMPgOLZmtrrCbhqsmaPHjLKYnJCaQ=
 github.com/frankban/quicktest v1.14.6 h1:7Xjx+VpznH+oBnejlPUj8oUpdxnVs4f8XU8WnHkI4W8=
 github.com/frankban/quicktest v1.14.6/go.mod h1:4ptaffx2x8+WTWXmUCuVU6aPUX1/Mz7zb5vbUoiM6w0=
 github.com/google/go-cmp v0.5.9 h1:O2Tfq5qg4qc4AmwVlvv0oLiVAGB7enBSJ2x2DqQFi38=

mcp/github_tool/gtlib/upstream.go

@@ -2,8 +2,6 @@ package lib
 
 import (
 	"context"
-	"encoding/base64"
-	"encoding/json"
 	"fmt"
 	"log/slog"
 	"net/http"
@@ -12,6 +10,7 @@ import (
 	"strings"
 	"time"
 
+	"github.com/dgrijalva/jwt-go"
 	"github.com/mark3labs/mcp-go/client"
 	"github.com/mark3labs/mcp-go/client/transport"
 	"github.com/mark3labs/mcp-go/mcp"
@@ -343,27 +342,25 @@ func getAuthorizationHeaderFromBearer(auth string) string {
 	}
 
 	token := strings.TrimPrefix(auth, "Bearer ")
-	var decodedToken map[string]interface{}
-
-	// The token will be several base64 pieces separated by '.'.  Split in '.', use base64.StdEncoding.DecodeString(eachPart)
-	// This method is insecure, because it trusts the bearer token, rather than verifying it cryptographically or by
-	// passing it to an OIDC endpoint
-	tokenParts := strings.Split(token, ".")
-	if len(tokenParts) == 3 {
-		// Only look at the middle part
-		tokenPart := tokenParts[1]
-		decodedPart, err := base64.StdEncoding.DecodeString(tokenPart)
-		if err != nil {
-			fmt.Printf("@@@ couldn't decode part: %v of %q\n", err, tokenPart)
-		} else {
-			// fmt.Printf("part is %q\n", decodedPart)
-			err = json.Unmarshal(decodedPart, &decodedToken)
-			if err != nil {
-				fmt.Printf("@@@ couldn't unmarshal part: %v\n", err)
-			}
-		}
+
+	// For explanation see https://stackoverflow.com/questions/55698770/decode-jwt-without-validation-and-find-scope
+	jwtToken, _, err := new(jwt.Parser).ParseUnverified(token, jwt.MapClaims{})
+	if err != nil {
+		fmt.Printf("Bearer token couldn't be parsed: %v\n", err)
+		return ""
 	}
 
+	// TODO check that jwtToken hasn't expired, is valid, etc.
+
+	claims, ok := jwtToken.Claims.(jwt.MapClaims)
+	if !ok {
+		fmt.Printf("Parsed token not MapClaims: %v\n", err)
+		return ""
+	}
+
+	// fmt.Printf("Got claims %#v\n", claims)
+	decodedToken := claims
+
 	/*
 		x := map[string]interface {}{
 			"acr":"1",