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I would like to send a salve to my friend noob at Rivendel in Brazilian company hahaha

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Inspired by James Kettle's great OWASP AppSec Europe talk on CORS misconfigurations, we decided to fiddle around with CORS security issues a bit. We were curious how many websites out there are actually vulnerable because of dynamically generated or misconfigured CORS headers.

The issue: CORS misconfiguration

Cross-Origin Resource Sharing (CORS) is a technique to punch holes into the Same-Origin Policy (SOP) – on purpose. It enables web servers to explicitly allow cross-site access to a certain resource by returning an Access-Control-Allow-Origin (ACAO) header. Sometimes, the value is even dynamically generated based on user-input such as the Origin header send by the browser. If misconfigured, an unintended website can access the resource. Furthermore, if the Access-Control-Allow-Credentials (ACAC) server header is set, an attacker can potentially leak sensitive information from a logged in user – which is almost as bad as XSS on the actual website. Below is a list of CORS misconfigurations which can potentially be exploited. For more technical details on the issues read the this fine blogpost.

Misconfiguation	Description
Developer backdoor	Insecure developer/debug origins like JSFiddler CodePen are allowed to access the resource
Origin reflection	The origin is simply echoed in ACAO header, any site is allowed to access the resource
Null misconfiguration	Any site is allowed access by forcing the `null` origin via a sandboxed iframe
Pre-domain wildcard	`notdomain.com` is allowed access, which can simply be registered by the attacker
Post-domain wildcard	`domain.com.evil.com` is allowed access, can be simply be set up by the attacker
Subdomains allowed	`sub.domain.com` allowed access, exploitable if the attacker finds XSS in any subdomain
Non-SSL sites allowed	An HTTP origin is allowed access to a HTTPS resource, allows MitM to break encryption
Invalid CORS header	Wrong use of wildcard or multiple origins,not a security problem but should be fixed

The tool: CORStest

Testing for such vulnerabilities can easily be done with curl(1). To support some more options like, for example, parallelization we wrote CORStest, a simple Python based CORS misconfiguration checker. It takes a text file containing a list of domain names or URLs to check for misconfigurations as input and supports some further options:

usage: corstest.py [arguments] infile

positional arguments:
  infile         File with domain or URL list

optional arguments:
  -h, --help     show this help message and exit
  -c name=value  Send cookie with all requests
  -p processes   multiprocessing (default: 32)
  -s             always force ssl/tls requests
  -q             quiet, allow-credentials only
  -v             produce a more verbose output

CORStest can detect potential vulnerabilities by sending various Origin request headers and checking for the Access-Control-Allow-Origin response. An example for those of the Alexa top 750 websites which allow credentials for CORS requests is given below.

Evaluation with Alexa top 1 Million websites

To evaluate – on a larger scale – how many sites actually have wide-open CORS configurations we did run CORStest on the Alexa top 1 million sites:

$ git clone https://github.com/RUB-NDS/CORStest.git && cd cors/
$ wget -q http://s3.amazonaws.com/alexa-static/top-1m.csv.zip
$ unzip top-1m.csv.zip
$ awk -F, '{print $2}' top-1m.csv > alexa.txt
$ ./corstest.py alexa.txt

This test took about 14 hours on a decent connection and revealed the following results:

Only 29,514 websites (about 3%) actually supported CORS on their main page (aka. responded with Access-Control-Allow-Origin). Of course, many sites such as Google do only enable CORS headers for certain resources, not directly on their landing page. We could have crawled all websites (including subdomains) and fed the input to CORStest. However, this would have taken a long time and for statistics, our quick & dirty approach should still be fine. Furthermore it must be noted that the test was only performed with GET requests (without any CORS preflight) to the http:// version of websites (with redirects followed). Note that just because a website, for example, reflects the origin header it is not necessarily vulnerable. The context matters; such a configuration can be totally fine for a public sites or API endpoints intended to be accessible by everyone. It can be disastrous for payment sites or social media platforms. Furthermore, to be actually exploitable the Access-Control-Allow-Credentials: true (ACAC) header must be set. Therefore we repeated the test, this time limited to sites that return this header (see CORStest -q flag):

$ ./corstest.py -q alexa.txt

This revealed even worse results - almost half of the websites supporting ACAO and ACAC headers contained a CORS misconfigurations that could be exploited directly by a web attacker (developer backdoor, origin reflection, null misconfig, pre-/post-domain wildcard):

The Impact: SOP/SSL bypass on payment and taxpayer sites

Note that not all tested websites actually were exploitable. Some contained only public data and some others - such as Bitbucket - had CORS enabled for their main page but not for subpages containing user data. Manually testing the sites, we found to be vulnerable:

A dozen of online banking, bitcoin and other payment sites; one of them allowed us to create a test account so we were able to write proof-of-concept code which could actually have been used to steal money
Hundred of online shops/e-commerce sites and a bunch of hotel/flight booking sites
Various social networks and misc sites which allow users to log in and communicate
One US state's tax filing website (however, this one was exploitable by a MitM only)

We informed all sites we manually tested and found to be vulnerable. A simple exploit code example when logged into a website with CORS origin reflection is given below.

The Reason: Copy & Paste and broken frameworks

We were further interested in reasons for CORS misconfigurations. Particularly we wanted to learn if there is a correlation between applied technology and misconfiguration. Therefore we used WhatWeb to fingerprint the web technologies for all vulnerable sites. CORS is usually enabled either directly in the HTTP server configuration or by the web application/framework. While we could not identify a single major cause for CORS misconfigurations, we found various potential reasons. A majority of dangerous Access-Control-* headers had probably been introduced by developers, others however are based on bugs and bad practices in some products. Insights follow:

Various websites return invalid CORS headers; besides wrong use of wildcards such as *.domain.com, ACAO headers which contain multiple origins can often be found; Other examples of invalid - but quite creative - ACAO values we observed are: self, true, false, undefined, None, 0, (null), domain, origin, SAMEORIGIN
Rack::Cors, the de facto standard library to enable CORS for Ruby on Rails maps origins '' or origins '*' into reflecting arbitrary origins; this is dangerous, because developers would think that '' allows nothing and '*' behaves according to the spec: mostly harmless because it cannot be used to make to make 'credentialed' requests; this config error leads to origin reflection with ACAC headers on about a hundred of the tested and vulnerable websites
A majority of websites which allow a http origin to CORS access a https resource are run on IIS; this seems to be no bug in IIS itself but rather caused by bad advises found on the Internet
nginx is the winner when it comes serving websites with origin reflections; again, this is not an issue of nginx but of dangerous configs copied from "Stackoverflow; same problem for Phusion Passenger
The null ACAO value may be based on programming languages that simply return null if no value is given (we haven't found any specific framework though); another explanation is that 'CORS in Action', a popular book on CORS, contains various examples with code such as var originWhitelist = ['null', ...], which could be misinterpreted by developers as safe
If CORS is enabled in the crVCL PHP Framework, it adds ACAC and ACAO headers for a configured domain. Unfortunatelly, it also introduces a post-domain and pre-subdomain wildcard vulnerability: sub.domain.com.evil.com
All sites that are based on "Solo Build It!" (scam?) respond with: Access-Control-Allow-Origin: http://sbiapps.sitesell.com
Some sites have :// or // as fixed ACAO values. How should browsers deal with this? Inconsistent at least! Firefox, Chrome, Safari and Opera allow arbitrary origins while IE and Edge deny all origins.

Related links

Puffadust

There are many goodware and malware developed in pascal, and we will see that the binary generated by the pascal compilers is fascinating, not only because the small and clean generated binaries, or the clarity of the pascal code, but also the good performance. In Linux we have Lazarus which is a good free IDE like Delphi and Kylix the free pascal IDE for windows.

The program:

program strtest;

var
cstr: array[0..10] of char;
s, s2: ShortString;

begin
cstr := 'hello world';
s := cstr;
s2 := 'test';

WriteLn(cstr + ' ' + s + ' ' + s2);
end.

We are going to compile it with freepascal and lazarus, and just the binary size differs a lot:

lazarus 242,176 btytes 845 functions
freepascal 32,256 bytes 233 functions
turbopascal 2,928 bytes 80 functions (wow)

And surprisingly turbopascal binaries are extremely light.
Lets start with lazarus:

Logically it imports from user32.dll some display functions, it also import the kernel32.dll functions and suspiciously the string operations of oleaut32.dll

And our starting point is a function called entry that calls the console initialization and retrieve some console configurations, and then start a labyrinth of function calls.

On functions 10000e8e0 there is the function that calls the main function.

I named execute_param2 because the second param is a function pointer that is gonna be executed without parameters, it sounds like main calling typical strategy.
And here we are, it's clearly the user code pascal main function.

What it seems is that function 100001800 returns an string object, then is called its constructor to initialize the string, then the string is passed to other functions that prints it to the screen.

This function executes the method 0x1c0 of the object until the byte 0x89 is a null byte.
What the hell is doing here?
First of all let's create the function main:

Simply right button create function:

After a bit of work on Ghidra here we have the main:

Note that the struct member so high like 0x1b0 are not created by default, we should import a .h file with an struct or class definition, and locate the constructor just on that position.

The mysterious function was printing byte a byte until null byte, the algorithm the compiler implemented in asm is not as optimized as turbopascal's.

In Windbg we can see the string object in eax after being created but before being initialized:

Just before executing the print function, the RCX parameter is the string object and it still identical:

Let's see the constructor code.
The constructor address can be guessed on static walking the reverse-cross-references to main, but I located it in debugging it in dynamic analysis.

The constructor reads only a pointer stored on the string object on the position 0x98.

And we have that the pointer at 0x98 is compared with the address of the literal, so now we know that this pointer points to the string.
The sentence *string_x98 = literal confirms it, and there is not memory copy, it only points reusing the literal.

Freepascal

The starting labyrinth is bigger than Lazarus so I had to begin the maze from the end, searching the string "hello world" and then finding the string references:

There are two ways to follow the references in Ghidra, one is [ctrl] + [shift] + F but there is other trick which is simply clicking the green references texts on the disassembly.

At the beginning I doubted and put the name possible_main, but it's clearly the pascal user code main function.

The char array initialization Is converted by freepascal compiler to an runtime initialization using mov instructions.

Reducing the coverage on dynamic we arrive to the writeln function:

EAX helds a pointer to a struct, and the member 0x24 performs the printing. In this cases the function can be tracked easily in dynamic executing the sample.

And lands at 0x004059b0 where we see the WriteFile, the stdout descriptor, the text and the size supplied by parameter.

there is an interesting logic of what happens if WriteFile() couldn't write all the bytes, but this is other scope.
Lets see how this functions is called and how text and size are supplied to figure out the string object.

EBX helds the string object and there are two pointers, a pointer to the string on 0x18 and the length in 0x18, lets verify it on windbg.

And here we have the string object, 0x0000001e is the length, and 0x001de8a68 is the pointer.

Thanks @capi_x for the pascal samples.

Puffadust Mobile Blog (BAK)

Wednesday, August 26, 2020

Leo's Noob

CORS Misconfigurations On A Large Scale

The issue: CORS misconfiguration

The tool: CORStest

Evaluation with Alexa top 1 Million websites

The Impact: SOP/SSL bypass on payment and taxpayer sites

The Reason: Copy & Paste and broken frameworks

Related links

Reversing Pascal String Object

Freepascal

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