Affected devices do not contain an Immutable Root of Trust in Hardware. With this the integrity of the code executed on the device can not be validated during load-time. An attacker with physical access to the device could use this to replace the boot image of the device and execute arbitrary code.
https://cert-portal.siemens.com/productcert/pdf/ssa-482757.pdf
https://cert-portal.siemens.com/productcert/html/ssa-482757.html