默默新聞08

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Nmap is a free utility tool for network discovery, port scanning and security auditing, even though we can use it for more than that but in this article we will learn how to do these three things with nmap.

The original author of nmap is Gordon Lyon (Fyodor). Nmap is licensed under GPL v2 and has available ports in many different languages. Nmap is available for Linux, Windows, and Mac OS X. You can download your copy of nmap from their website.

Lets get started with nmap.

When performing pentests we always look for networks we are going to attack. We need to identify live hosts on the network so that we can attack them. There are plenty of tools available for finding live hosts on a network but nmap is one of the best tools for doing this job.

Lets start with simple host (target) discovery scans i,e scans that will tell us which ip address is up on our target network. Those ip addresses which are up on our target network are the ones that are assigned to a device connected on our target network. Every device on the network is going to have a unique ip address.
To perform a simple host discovery scan we use the following command

nmap -v -sn 10.10.10.0/24

flags we used in the above command are
-v for verbose output
-sn to disable port scan (we don't want to scan for ports right now)

Following the flags is the ip address of the target network on which we want to look for live hosts. The /24 at the end of the ip address is the CIDR that specifies the subnet of the network on which we are looking for live hosts.

After running the above command you should get a list of live hosts on your target network.
If you just want to know the list of ip addresses your command is going to scan, you can use the -sL flag of the nmap like this.

nmap -sL 10.10.10.0/24

this command will simply output the list of ip addresses to scan.

We sometimes want to do dns resolution (resolving ip addresses to domain names) when performing our network scans and sometimes we don't want dns resolution. While performing a host discovery scan with nmap if we want to perform dns resolution we use -R flag in our command like this:

nmap -v -sn -R 10.10.10.0/24

And if we don't want to perform dns resolution of hosts during our scan we add the -n flag to our command like this:

nmap -v -sn -n 10.10.10.0/24

After we have discovered the hosts that are up on our target network, we usually put the ip addresses of these hosts into a file for further enumeration.

Next step in our enumeration would be to detect which operating system and which ports are running on these live hosts, for that we run this command:

nmap -O -v 10.10.10.119

here we use -O (capital o not zero) for operating system detection and by default nmap performs SYN Scan for port discovery. However nmap scans for 1000 ports only by default of a particular host.

To make nmap go over a list of ip addresses in a file we use -iL flag like this:

nmap -O -v -iL targetlist

where targetlist is the name of the file which contains ip addresses that we want to perform port scan on.

To make nmap scan all the ports of a target we use the -p flag like this:

nmap -p- -v 10.10.10.121

We can also specify a range of ports using the -p flag like this:

nmap -p1-500 -v 10.10.10.121

here 1-500 means scan all the ports from 1 to 500.

We can use a number of scan techniques to discover open ports on our network but I will only discuss some of them for brevity.

We can perform a TCP SYN scan using nmap with -sS flag like this:

nmap -sS -v 10.10.10.150

We have also flags for TCP connect and ACK scans which are -sT -sA

nmap -sT -v 10.10.10.150

nmap -sA -v 10.10.10.150

We can also perform UDP scan as well instead of TCP scan using -sU flag

nmap -sU -v 10.10.10.150

We can perform TCP Null, FIN, and Xmas scans using the flags -sN, -sF, -sX

nmap -sN -v 10.10.10.150

nmap -sF -v 10.10.10.150

nmap -sX -v 10.10.10.150

If you don't know what these scans are then please visit Port Scanning Techniques and Algorithms for explanation.

After discovering the open ports on our target host, we want to enumerate what services are running on those open ports. To enumerate services and versions information on open ports we use the -sV flag like this:

nmap -sV -v 10.10.10.118

This should give us information about what services are running on what ports and what versions of those services are running on the target host.

nmap has an interesting feature called NSE nmap scripting engine. It allows users to write their own scripts, using the Lua programming language, to automate a wide variety of networking tasks. nmap ships with a diverse set of scripts which are very helpful to enumerate a target. To use the nmap default set of scripts while enumerating the target, we use the -sC flag like this:

nmap -sC -sV -v 10.10.10.118

We can also save the results of our nmap scans to a file using the -o flag like this

nmap -sC -sV -v -oA defaultscan 10.10.10.119

here -oA tells the nmap to output results in the three major formats at once and defaultscan is the name of the file that will be prepended to all the three output files.

This is the end of this short tutorial see you next time.

References:
https://nmap.org/book/scan-methods-null-fin-xmas-scan.html

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蔡怡婷

Interators are usually implemented using signed integers like the typical "for (int i=0; ..." and in fact is the type used indexing "cstr[i]", most of methods use the signed int, int by default is signed.
Nevertheless, the "std::string::operator[]" index is size_t which is unsigned, and so does size(), and same happens with vectors.
Besides the operator[] lack of negative index control, I will explain this later.

Do the compilers doesn't warn about this?

If his code got a large input it would index a negative numer, let see g++ and clang++ warnings:

No warnings so many bugs out there...

In order to reproduce the crash we can load a big string or vector from file, for example:

I've implemented a loading function, getting the file size with tellg() and malloc to allocate the buffer, then in this case used as a string.
Let see how the compiler write asm code based on this c++ code.

So the string constructor, getting size and adding -2 is clear. Then come the operator<< to concat the strings.
Then we see the operator[] when it will crash with the negative index.
In assembly is more clear, it will call operator[] to get the value, and there will hapen the magic dereference happens. The operator[] will end up returning an invalid address that will crash at [RAX]

In gdb the operator[] is a allq 0x555555555180 <_znst7__cxx1112basic_stringicst11char_traitsicesaiceeixem plt="">

(gdb) i r rsi
rsi 0xfffffffffffefffe -65538

The implmementation of operator ins in those functions below:

(gdb) bt
#0 0x00007ffff7feebf3 in strcmp () from /lib64/ld-linux-x86-64.so.2
#1 0x00007ffff7fdc9a5 in check_match () from /lib64/ld-linux-x86-64.so.2
#2 0x00007ffff7fdce7b in do_lookup_x () from /lib64/ld-linux-x86-64.so.2
#3 0x00007ffff7fdd739 in _dl_lookup_symbol_x () from /lib64/ld-linux-x86-64.so.2
#4 0x00007ffff7fe1eb7 in _dl_fixup () from /lib64/ld-linux-x86-64.so.2
#5 0x00007ffff7fe88ee in _dl_runtime_resolve_xsavec () from /lib64/ld-linux-x86-64.so.2
#6 0x00005555555554b3 in main (argc=2, argv=0x7fffffffe118) at main.cpp:29

Then crashes on the MOVZX EAX, byte ptr [RAX]

Program received signal SIGSEGV, Segmentation fault.

0x00005555555554b3 in main (argc=2, argv=0x7fffffffe118) at main.cpp:29
29 cout << "penultimate byte is " << hex << s[i] << endl;
(gdb)

What about negative indexing in std::string::operator[] ?
It's exploitable!

In a C char array is known that having control of the index, we can address memory.
Let's see what happens with C++ strings:

The operator[] function call returns the address of string plus 10, and yes, we can do abitrary writes.

Note that gdb displays by default with at&t asm format wich the operands are in oposite order:

And having a string that is in the stack, controlling the index we can perform a write on the stack.

To make sure we are writing outside the string, I'm gonna do 3 writes:

See below the command "i r rax" to view the address where the write will be performed.

The beginning of the std::string object is 0x7fffffffde50.
Write -10 writes before the string 0x7fffffffde46.
And write -100 segfaults because is writting in non paged address.

So, C++ std::string probably is not vulnerable to buffer overflow based in concatenation, but the std::string::operator[] lack of negative indexing control and this could create vulnerable and exploitable situations, some times caused by a signed used of the unsigned std::string.size()

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