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(3) 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.

For the third condition can arise in two different ways:

    The stevedore named Transient is configured as -sfile or -spersistent (The default is -smalloc)
    The default stevedore is -sfile or -spersistent and the synthetic object is given a TTL larger than the shortlived parameter (default: 10 seconds.)

Affected versions: 4.1.0 to 5.2.0

Upstream patch:

https://github.com/varnishcache/varnish-cache/commit/176f8a075a

External References:

https://varnish-cache.org/security/VSV00002.html