A wrong if statement in the varnishd source code means that synthetic objects in stevedores which over-allocate, may leak up to page size of data from a malloc memory allocation.In a unpredictable percentage of the cases where this condition arises, a segmentation fault will happen instead. All the following conditions are required to trigger the problem: A -sfile or -spersistent stevedore must be configured A synthetic object must be created in vcl_backend_error{}The synthetic object ends up in the file or persistent stevedore. Affected Versions: 4.1.0 to 5.2.0 Fixed In: varnish 4.1.9, varnish 5.2.1