apollo-compiler is a query-based compiler for the GraphQL query language. Prior to 1.27.0, a vulnerability in Apollo Compiler allowed queries with deeply nested and reused named fragments to be prohibitively expensive to validate. Named fragments were being processed once per fragment spread in some cases during query validation, leading to exponential resource usage when deeply nested and reused fragments were involved. This could lead to excessive resource consumption and denial of service in applications. This vulnerability is fixed in 1.27.0.