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代做INATION 2019帮做R实验、Java作业代写

UNIVERSITY OF LONDON
BSc EXAMINATION 2019
For Internal Students of
Royal Holloway
DO NOT TURN OVER UNTIL TOLD TO BEGIN
IY2840: Computer and Network Security
IY2840R: Computer and Network Security – PAPER FOR RESIT
CANDIDATES
Time Allowed: TWO hours
Answer ALL questions
Calculators are NOT permitted
c©Royal Holloway, University of London 2019
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IY2840/IY2840R
1. MECHANISMS FOR ACCESS CONTROL
(a) Anderson proposed the concept of a Reference Monitor in 1972.
i. Describe what is meant by the concept of a Reference Monitor. [6 marks]
ii. List three properties that a Reference Monitor must have. [3 marks]
iii. For each of the three properties above, explain why the Reference Mon-
itor would not work if it did not have this property. [6 marks]
(b) In the MULTICS hardware security architecture, the Reference Monitor was
implemented by means of a protection ring architecture. What is a protection
ring architecture and how does it provide access control? [6 marks]
(c) Call gates allow controlled access between protection rings. In the following
diagram, arrows 1 and 2 illustrate attempts by a piece of code to access a
piece of data, and arrows 3 and 4 illustrate attempts to transfer control via a
call gate.
Ring 0
Ring 1
Ring 2
Ring 3
D DATA
C CODE
G CALL GATE
C
D
2
C
D
1
C
G
C
4
4
C
G
C
33
Which of the four attempts to access data or transfer control are allowed?
Which ones are not allowed? (No explanation necessary.) [4 marks]
2. HARDWARE ACCESS CONTROL
The Intel 80386 architecture implements concepts from the MULTICS protection
ring architecture.
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IY2840/IY2840R
(a) This question is on the technical realization of protection rings on the 80386.
Describe details specific to the 80386 (that is, details that could be different
for other CPUs) that are related to protection rings.
You can base your description on the following topics (other topics are also
possible): How and where are ring numbers encoded? Which technical
component makes the access control decision? Which operation is required
to make such a decision? How is the controlled invocation mechanism im-
plemented? [8 marks]
(b) Many modern CPUs offer four protection rings, but most current operating
systems, including Windows and Linux, only use two. Why do you think this
is the case? [4 marks]
3. UNIX ACCESS CONTROL
Assume Alice, Bob, and Charlie have accounts on the same UNIX system. Their
usernames are alice, bob and charlie, respectively.
• Alice is a member of groups staff and cdrom.
• Alice is not a member of group admin.
• Charlie is a member of group admin.
• Charlie is not a member of groups staff and cdrom.
Assume a directory on the system that contains six files (file1–file6) with file
mode and ownership displayed as follows:
$ stat --format="%a %A %U/%G %n" file[1-6]
0466 -r--rw-rw- alice/staff file1
0442 -r--r---w- root/cdrom file2
0777 -rwxrwxrwx root/admin file3
0044 ----r--r-- alice/staff file4
0424 -r---w-r-- root/cdrom file5
0204 --w----r-- root/admin file6
Here, the left-most column (%a) lists the file mode in octal, the second column (%A)
lists the file mode in ASCII, the third column (%U/%G) shows the user identifier and
the group identifier of the file owner in combined form (in UID/GID format), and
the right-most column shows the name of the file.
(a) List all files that . . .
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IY2840/IY2840R
i. user alice can open for writing. [2 marks]
ii. user alice can open for reading. [2 marks]
iii. user root (with UID = 0) can open for reading. [2 marks]
iv. user charlie can open for writing. [2 marks]
(b) Which of the six files has the set-user-id (SUID) bit set? [2 marks]
(c) The directory in which the six files reside has the sticky bit set. More pre-
cisely, the file mode is as follows:
$ stat --format="%a %A %U/%G %n" .
1777 drwxrwxrwt root/root .
Which two users can delete file1? [2 marks]
4. SOFTWARE SECURITY and SHELLCODE
The following assembly listing shows a shellcode that is functional for Linux on the
Intel 80386. It is similar to the ones that were considered in the IY2840 lectures
and labs.
1 jmp ahead
2
3 back:
4 popl %ebx
5 movl $0x0, %eax
6 movl %ebx, 0x8(%ebx)
7 leal 0x8(%ebx), %ecx
8 movb %al, 0x7(%ebx)
9 movl %eax, %edx
10 movl %eax, 0xc(%ebx)
11 movb $0xb, %al
12 int $0x80
13
14 ahead:
15 call back
16 .string "/bin/sh"
(a) If a shellcode is executed by the CPU (starting with the instruction in line 1),
what do you expect to happen? [2 marks]
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IY2840/IY2840R
(b) What is the number of the execve system call in Linux? You can answer in
decimal or hexadecimal, but please indicate which one you use. [2 marks]
(c) What is the effect of line 8? (That is, what is its function in the shellcode?)
[2 marks]
(d) What purpose do the call (line 15) and pop (line 4) instructions serve? (That
is, what is their function in the shellcode?) [4 marks]
(e) The shellcode would stop working if line 1 was replaced by call back (that
is, a call to line 3). However it could be repaired by inserting an additional
instruction between lines 4 and 5. Which instruction would that be, and
which registers would it affect? (No need to be precise with the quantities,
and no need to avoid null bytes in the opcodes.) [4 marks]
(f) A shellcode can be augmented by a so-called NOP sled. Explain the NOP
sled concept. Explain in particular: What does it consist of, where is it placed
in relation to the shellcode, what is its purpose, and how large would it opti-
mally be. [5 marks]
5. OPERATING SYSTEM SECURITY
In a UNIX environment, applications might be vulnerable to one or more of the
following attacks: (1) attack via environment variable, (2) attack via symlink, (3) at-
tack by exploiting a race condition (‘TOCTOU’), (4) attack by command injection.
Pick one of the four attacks (indicate which one) and give a concrete example for
how it could be conducted. Likely your attack applies only in specific cases or as-
sumes specific programming errors; say which conditions these are. Which prop-
erty of the UNIX environment is relevant and exploited? Also mention what the
outcome of the attack is: What did the attacker gain from conducting it? [14 marks]
6. WEB SECURITY
Cross-Site Scripting (XSS) is a widespread problem affecting a number of web
services.
(a) State the main vulnerability that leads to XSS attacks. [2 marks]
(b) Briefly describe the general principle of XSS attacks. Which security policy
is both evaded and exploited in such attacks. [8 marks]
(c) Describe the difference between a Stored XSS attack and a Reflected XSS
attack. [8 marks]
END
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