CWE-515 is a covert storage channel: information moves through bits that one program sets and another reads. What separates it from normal operation is that the bits carry encoded information.
How it happens
MITRE says these channels occur when out-of-band data is stored in messages for the purpose of memory reuse. Covert channels are usually classed as storage or timing channels. A file meant to hold only audit information that is used to carry passwords is one example, using the file name or status bits that all users can read. Steganography, hiding a message so only the recipient knows it exists, is another.
Sloppy messages cause an accidental version. Reserved fields and padding sent with leftover data can reveal details about the process that sent them.
What an attacker gains
Per MITRE, a covert storage channel can give attackers important information about the system. Extra data in messages may tip off listeners about the process and hardware platform behind them.
How to fix it
MITRE's mitigation is to set all reserved fields to zero before messages are sent and to include no unnecessary information.
struct msg m;
memset(&m, 0, sizeof m); /* clears reserved fields and padding */
m.type = MSG_PING;
send(sock, &m, sizeof m, 0);
Quick check
What simple coding habit reduces accidental covert storage channels?
Zero reserved fields and send only the data the message needs.
