Methods, systems, and apparatus, including computer programs encoded on computer storage media, for memory address pinning. One of the methods includes loading a software module into a sandbox environment; receiving, a message from the software module to a recipient, the message includes a memory address; determining whether to pin the memory address; and passing the message to an address pinning unit which replaces at least a portion of the memory address with at least a portion of a specified replacement address, when it is determined to pin the memory address, and passes the modified message to be delivered to the recipient.