Remanent Data Readable after Memory Erase
Confidential information stored in memory circuits is readable or recoverable after being cleared or erased.
Description
Data remanence occurs when stored, memory content is not fully lost after a memory-clear or -erase operation. Confidential memory contents can still be readable through data remanence in the hardware.
Data remanence can occur because of performance optimization or memory organization during 'clear' or 'erase' operations, like a design that allows the memory-organization metadata (e.g., file pointers) to be erased without erasing the actual memory content. To protect against this weakness, memory devices will often support different commands for optimized memory erase and explicit secure erase.
Data remanence can also happen because of the physical properties of memory circuits in use. For example, static, random-access-memory (SRAM) and dynamic, random-access-memory (DRAM) data retention is based on the charge retained in the memory cell, which depends on factors such as power supply, refresh rates, and temperature.
Other than explicit erase commands, self-encrypting, secure-memory devices can also support secure erase through cryptographic erase commands. In such designs, only the decryption keys for encrypted data stored on the device are erased. That is, the stored data are always remnant in the media after a cryptographic erase. However, only the encrypted data can be extracted. Thus, protection against data recovery in such designs relies on the strength of the encryption algorithm.
Demonstrations
The following examples help to illustrate the nature of this weakness and describe methods or techniques which can be used to mitigate the risk.
Note that the examples here are by no means exhaustive and any given weakness may have many subtle varieties, each of which may require different detection methods or runtime controls.
Example One
Consider a device that uses flash memory for non-volatile-data storage. To optimize flash-access performance or reliable-flash lifetime, the device might limit the number of flash writes/erases by maintaining some state in internal SRAM and only committing changes to flash memory periodically.
The device also supports user reset to factory defaults with the expectation that all personal information is erased from the device after this operation. On factory reset, user files are erased using explicit, erase commands supported by the flash device.
In the given, system design, the flash-file system can support performance-optimized erase such that only the file metadata are erased and not the content. If this optimized erase is used for files containing user data during factory-reset flow, then device, flash memory can contain remanent data from these files.
On device-factory reset, the implementation might not erase these copies, since the file organization has changed and the flash file system does not have the metadata to track all previous copies.
A flash-memory region that is used by a flash-file system should be fully erased as part of the factory-reset flow. This should include secure-erase flow for the flash media such as overwriting patterns multiple times followed by erase.
See Also
Weaknesses in this category are related to resource lifecycle management.
This view (slice) covers all the elements in CWE.
This view (slice) lists weaknesses that can be introduced during implementation.
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