Circular fragments in GraphQL refer to a situation where fragments in GraphQL queries reference each other, leading to a circular reference. This can cause issues such as infinite loops, resource exhaustion, and denial of service (DoS) attacks. An attacker could exploit this to overwhelm the GraphQL server, making it unavailable or slow for legitimate users.

Security Impact of Circular Fragments in GraphQL:

**1. Denial of Service (DoS):** Circular fragment references can cause the server to endlessly resolve queries, consuming excessive resources and eventually crashing or slowing down.
**2. Resource Exhaustion:** Infinite fragment loops can lead to high CPU and memory usage, causing resource exhaustion, which degrades the performance of the application or server.
**3. Query Complexity Exploitation:** Attackers can craft complex queries with circular fragments to exploit the lack of depth or recursion limits in the GraphQL server.

A basic example of a circular fragment might look like this:

```graphql
fragment UserFields on User {
  name
  ...MoreUserFields
}

fragment MoreUserFields on User {
  email
  ...UserFields
}
```

In this example, `UserFields` references `MoreUserFields`, and `MoreUserFields` references `UserFields`, creating a circular reference.



