ensure jwt signature verification happens when decoding claim (#22)

nats-io · Nov 1, 2024 · fc68be5 · fc68be5
1 parent b1eefff
commit fc68be5
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Showing 2 changed files with 29 additions and 8 deletions.
diff --git a/NATS.Jwt.Tests/NatsJwtTests.cs b/NATS.Jwt.Tests/NatsJwtTests.cs
@@ -336,4 +336,11 @@ public void TestMultipleExports()
         string jsonStr = JsonSerializer.Serialize(json, new JsonSerializerOptions { WriteIndented = true });
         output.WriteLine(jsonStr);
     }
+
+    [Fact]
+    public void TestDecodeUserClaimWithTamperedJWTThrowsError()
+    {
+        var jwt = "eyJ0eXAiOiJKV1QiLCJhbGciOiJlZDI1NTE5LW5rZXkifQ.eyJqdGkiOiJPSk9CUkZDQ0NGNEMzU1JWQzRLNEhTVFNYRlBTSVZBSzJPRUxOMlZXNE9GQ0IzQTVMMkNBIiwiaWF0IjoxNzMwMzk3NTU0LCJpc3MiOiJVQklUR0VSQk9JVEZDWkJHNTNUSkk3M1BHTjdBMzdPTVkyWE5YUU82VUZUSlA1TE5VWVFORUpXSSIsIm5hbWUiOiJVWFgiLCJzdWIiOiJVQklUR0VSQk9JVEZDWkJHNTNUSkk3M1BHTjdBMzdPTVkyWE5YUU82VUZUSlA1TE5VWVFORUpXSSIsIm5hdHMiOnsicHViIjp7ImFsbG93IjpbImFsbG93Llx1MDAzRSJdfSwic3ViIjp7ImFsbG93IjpbInN1YnNjcmliZS5cdTAwM0UiXX0sInN1YnMiOi0xLCJkYXRhIjotMSwicGF5bG9hZCI6LTEsInR5cGUiOiJ1c2VyIiwidmVyc2lvbiI6Mn19.SjIBpWWLNCZmgYZwrFHEJSTkm5M9bik0kgQyG-3V9Nn5sTrfO1Llj3hs7z9R7b1rCyGsFm1RkpZAVAnS5ay2BA";
+        Assert.Throws<NatsJwtException>(() => _natsJwt.DecodeUserClaims(jwt));
+    }
 }
diff --git a/NATS.Jwt/NatsJwt.cs b/NATS.Jwt/NatsJwt.cs
@@ -303,14 +303,15 @@ public T DecodeClaims<T>(string jwt, JsonTypeInfo<T> jsonTypeInfo)
         }
 
         // Verify the signature
-        // var signingInput = $"{parts[0]}.{parts[1]}";
-        // var signature = Encoding.ASCII.GetBytes(EncodingUtils.FromBase64UrlEncoded(parts[2]));
-        // var publicKey = KeyPair.FromPublicKey(claims.Issuer.AsSpan());
-        //
-        // if (!publicKey.Verify(Encoding.ASCII.GetBytes(signingInput), signature))
-        // {
-        //     throw new NatsJwtException("JWT signature verification failed");
-        // }
+        var signingInput = $"{parts[0]}.{parts[1]}";
+        var signature = DecodeSignature(parts[2]);
+        var publicKey = KeyPair.FromPublicKey(claims.Issuer.AsSpan());
+
+        if (!publicKey.Verify(Encoding.ASCII.GetBytes(signingInput), signature))
+        {
+            throw new NatsJwtException("JWT signature verification failed");
+        }
+
         return claims;
     }
 
@@ -447,4 +448,17 @@ private static string Serialize<T>(T data, JsonTypeInfo<T> typeInfo)
         JsonSerializer.Serialize(jsonWriter, data, typeInfo);
         return EncodingUtils.ToBase64UrlEncoded(writer.WrittenMemory.ToArray());
     }
+
+    /// <summary>
+    /// Can't use EncodingUtils.FromBase64UrlEncoded as not backward compatible with golang code.
+    /// </summary>
+    /// <param name="signature">JWT signature.</param>
+    /// <returns>base64 decoded JWT signature.</returns>
+    private static byte[] DecodeSignature(string signature)
+    {
+        signature = signature.Replace('-', '+').Replace('_', '/');
+        var pd = signature.Length % 4; // ensure length divisible by 4..pad with ==
+        signature = signature + "====".Substring(0, pd);
+        return Convert.FromBase64String(signature);
+    }
 }