The accidental addition of a data-structure sentinel can cause
serious programming logic problems.
Extended Description
Data-structure sentinels are often used to mark the structure of data. A
common example of this is the null character at the end of strings or a
special sentinel to mark the end of a linked list. It is dangerous to allow
this type of control data to be easily accessible. Therefore, it is
important to protect from the addition or modification of sentinels.
Time of Introduction
Architecture and Design
Implementation
Applicable Platforms
Languages
C
C++
Common Consequences
Scope
Effect
Availability
Generally this error will cause the data structure to not work
properly by truncating the data.
Likelihood of Exploit
High to Very High
Demonstrative Examples
Example 1
The following example assigns some character values to a list of
characters and prints them each individually, and then as a string. The
third character value is intended to be an integer taken from user input and
converted to an int.
The first print statement will print each character separated by a
space. However, if a non-integer is read from stdin by getc, then atoi
will not make a conversion and return 0. When foo is printed as a
string, the 0 at character foo[2] will act as a NULL terminator and
foo[3] will never be printed.
Potential Mitigations
Phase
Description
Implementation
Architecture and Design
Encapsulate the user from interacting with data sentinels. Validate
user input to verify that sentinels are not present.
Implementation
Proper error checking can reduce the risk of inadvertently introducing
sentinel values into data. For example, if a parsing function fails or
encounters an error, it might return a value that is the same as the
sentinel.
Requirements
Use a language or compiler that performs automatic bounds
checking.
Architecture and Design
Use an abstraction library to abstract away risky APIs. This is not a
complete solution.
Build and Compilation
Compiler-based canary mechanisms such as StackGuard, ProPolice, and
Microsoft Visual Studio /GS flag. Unless this provides automatic bounds
checking, it is not a complete solution.
Operation
Use OS-level preventative functionality. This is not a complete
solution.
Description
