C Security Quiz (Short Version)
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Which of the following is true with respect to buffer overflows?
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Buffer overflows on the heap cannot be exploited to run arbitrary code.
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If a function is vulnerable to a buffer overflow due to large user input being put in a small fixed-size buffer, making the buffer 10 times as large as a “quick fix” will reduce the impact of the vulnerability.
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Buffer overflows can be used to alter the state and operation of the vulnerable application in an undetectable way.
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If code cannot be executed on the stack (e.g. through the use of the non-execute bit or DEP), attackers cannot run arbitrary code by exploiting a buffer overflow.
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Calling free() on the same memory address twice may crash the application, but will not lead to an exploitable buffer overflow.
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Do you think you know enough about security issues in C code? Can you think with the mind of a hacker?
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- 2.
Which of the following statements (in the area of protection against typical C/C++ vulnerabilities) is true?
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There is no reliable way to protect against format string vulnerabilities.
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Injected shellcode can be reliably identified by intrusion detection software.
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Proper use of secure integer libraries eliminates integer overflow vulnerabilities.
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Using data execution prevention, address space layout randomization and stack smashing protection at the same time provides complete protection against buffer overflow exploits.
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Using strncpy and strncat instead of strcpy and strcat guarantees error-free operation.
Correct Answer
A. Proper use of secure integer libraries eliminates integer overflow vulnerabilities.Explanation
a] NO; format string vulnerabilities are trivially avoided by e.g. #define printf(str) printf("%s",str)
b] NO; shellcode can be obfuscated, encrypted, or even masquerade as alphanumeric text.
c] YES; secure integer libraries either prevent overflows altogether or throw errors when an overflow is encountered.
d] NO; heap spraying, return-oriented programming, return-to-libc and similar techniques can be used to bypass these protections.
e] NO; strncpy/strncat do not add a trailing zero if the 'num' parameter specifying the number of characters to copy is greater than the length of source string. This can lead to a buffer overflow later when the string is read out.Rate this question:
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- 3.
This function is part of a program that is running on a 32-bit x86 system; the compiler does not change the order of variables on the stack. void function(char *input) { int i = 1; char buffer[8]; int j = 2; strcpy(buffer,input); printf("%x %x %s\n",i,j,buffer); } What is the minimum length of a string – passed to the function through the input parameter – that can crash the application?
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9
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10
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11
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12
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13
Correct Answer
A. 12Explanation
12 characters. Since the string is zero-terminated, it will be stored in a 13-byte array that is copied over the buffer, and the first byte of the EBP will be overwritten - causing the program to crash.Rate this question:
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- 4.
When dealing with Unicode user input in C, the following issues need to be taken into account:
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Unicode characters may be used to bypass black-list filtering
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In every encoding form, the size of Unicode characters may differ from each other
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The length() of a Unicode string may be different from its size()
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Unicode strings cannot be printed easily out on the screen
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Directional control characters such as U+202E may be exploited
Correct Answer(s)
A. Unicode characters may be used to bypass black-list filtering
A. In every encoding form, the size of Unicode characters may differ from each other
A. The length() of a Unicode string may be different from its size()
A. Directional control characters such as U+202E may be exploitedExplanation
a]YES; filtering may use a different Unicode conversation routine than the called function.
b]YES; the character representation length varies for different characters in UTF-8 encoding form.
c]YES; depending on the Unicode encoding being used, size may be up to 4x larger than length.
d]NO; every function has an Unicode pair, for example you can use wprintf instead of printf.
e]YES; if the user-provided string is concatenated with UI elements, it may be used to reverse built-in UI element text.Rate this question:
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- 5.
#define ll 12 char pwd[37], n[ll]; void s(char *u) {strncpy(n,u,ll); printf(n);} How would you fix the code above?
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Char pwd[37], n[ll+1];
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#define ll 13
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Void s(char *u) {strncpy(n,u,ll-1); printf(n);}
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Void s(char *u) {strncpy(n,u,11); printf(“%s”, n);}
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Void s(char *u) {strncpy(n,u,ll-1); cout
Correct Answer(s)
A. Void s(char *u) {strncpy(n,u,11); printf(“%s”, n);}
A. Void s(char *u) {strncpy(n,u,ll-1); coutExplanation
a]NO; this fixes the off-by-one error in the strncpy, but does not fix the printf vulnerability
b]NO; neither of the bugs is fixed in this way, just increases the string’s size by one
c]NO; this fixes the off-by-one error in the strncpy, but does not fix the printf vulnerability
d]YES; both the off-by-one error and the printf vulnerability are fixed, but hard-coding the number of characters to be copied may cause problems in the future
e]YES; both the off-by-one error and the printf vulnerability are fixedRate this question:
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Current Version
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Mar 21, 2023Quiz Edited by
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Sep 17, 2012Quiz Created by
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