GraphQL Armor Max-Depth Plugin Bypass via fragment caching

Summary

A query depth restriction using the max-depth can be bypassed if ignoreIntrospection is enabled (which is the default configuration) by naming your query/fragment __schema.

Details

In the countDepth function, we have the following code that calculates the depth of a used fragment:

    } else if (node.kind == Kind.FRAGMENT_SPREAD) {
      if (this.visitedFragments.has(node.name.value)) {
        return this.visitedFragments.get(node.name.value) ?? 0;
      } else {
        this.visitedFragments.set(node.name.value, -1);
      }
      const fragment = this.context.getFragment(node.name.value);
      if (fragment) {
        let fragmentDepth;
        if (this.config.flattenFragments) {
          fragmentDepth = this.countDepth(fragment, parentDepth);
        } else {
          fragmentDepth = this.countDepth(fragment, parentDepth + 1);
        }
        depth = Math.max(depth, fragmentDepth);
        if (this.visitedFragments.get(node.name.value) === -1) {
          this.visitedFragments.set(node.name.value, fragmentDepth);
        }
      }
    }

which will calculate the depth of the fragment used in the current node, store the value in this.visitedFragments and re-use it in the future to avoid re-calculating the depth for the same fragment.

The issue arises when the same fragment is used multiple times, at different depths. The current caching takes into account the depth of the first occurrence, which means if the fragment is re-used later in a higher depth, this cached value is not updated.

So, for example, sending the following query with a max depth of 6:

query {
  books {
    author {
      ...Test
    }
  }
  books {
    author {
      books {
        author {
          ...Test
        }
      }
    }
  }
}
fragment Test on Author {
  books {
    title
  }
}

The first use of Test fragment does not exceed the defined limit, and this depth will be cached.

In the second use, the fragment is reused in a greater depth, but the countDepth function will still use the depth cached, without accounting for the increased depth.

PoC

Max depth: 6

query {
  books {
    author {
      ...Test
    }
  }
  books {
    author {
      books {
        author {
          ...Test
        }
      }
    }
  }
}
fragment Test on Author {
  books {
    title
  }
}

Impact

This issue affects applications using the GraphQL Armor Depth Limit plugin.

Fix

This is fixed in PR#824. We now store only the additional depth contributed by the fragment and add it to the parent depth where the fragment is used (parentDepth).