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Given that most of these quirks are rooted in Microsoft's attempts to
maintain backward compatibility, websites running on Windows are basically
playing on hard mode. Even the WorstFit Attack [46] @splitline and I
published last year has stemmed from a technical debt that Windows has
carried for over twenty years - all just to support legacy ANSI encoding -
but we'll dive deeper into that later!
------[ 3.2 - Let's Make Windows Defender Angry!
Although we've talked plenty about Windows' weird behaviors, I guess if
you choose Microsoft, you'll just have to live with it. However, what's
even more surprising is that sometimes even Windows' built-in antivirus
can sneak up and hit you with a sucker punch! And that's exactly what
happened with AVOracle - an ingenious new technique from @icchy that can
turn literally any scan result into a side-channel oracle!
This technique first showed up in a challenge called "Gyotaku The Flag"
[47] at WCTF 2019. Since this is the second time we're mentioning WCTF, I
think it's worth giving a bit more context here. Unlike traditional CTFs,
WCTF uses a special "Belluminar" format [48]. The organizer invited the
world's top 10 CTF teams, asked each team to create two challenges, and
had them compete by solving each other's problems. And just like the name
"Belluminar" suggests - besides "Bellum" (Latin for "war"), the more
important part was the "Seminar" afterward. Each team had to give a
detailed presentation explaining their challenge design, which was then
evaluated by judges and other teams.
Since WCTF offered the largest prize pool at the time, figuring out "how
to design a good challenge" naturally became the key to winning the
competition. Designing a challenge that's fair - but not frustrating - and
still fun enough to impress the world's top CTF players (including
industry experts, pro hackers, and even multiple-time Pwn2Own champions)
is way harder than it sounds. But that's exactly why so many
groundbreaking hacking techniques made their debut at WCTF. For instance,
the ever-popular "Semicolon Trick" [49] actually originated from a
challenge I made for WCTF 2016, and was only later officially unveiled at
Black Hat USA 2018!
Though @icchy made a small slip-up while designing his WCTF challenge, it
didn't take anything away from its novelty. Later that same year, he
brought the technique back using PHP at TokyoWesterns CTF - proving that
AVOracle wasn't just an edge case; instead, it was indeed a new attack
that could adapt to different scenarios!
=> Here's what @icchy shared about how many teams solved his WCTF challs
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- 2017: 7dcs (Crypto, Web, Reverse, Pwn) -> 0 solved
- 2018: f (Forensics, Reverse, Web) -> 1 solved
- 2019: Gyotaku The Flag (Web, Misc) -> **everyone solved**
The entire AVOracle stems from how Windows Defender scans for malware - it
automatically emulates anything that "looks like JavaScript." Especially
since Defender evaluates the file as a whole, if a file includes both
"attacker-controlled" and "unknown" parts, the attacker can leverage the
controlled part to influence how Defender perceives the unknown section.
What's worse, if Defender flags the file as malicious, it'll automatically
delete it - letting attackers turn this file deletion into a side-channel
oracle to reveal the file content!
=> Let's pretend the following file is a valid EICAR so we don't make
Windows Defender *angry*! ;)
+-------------------------------------------------------------------------+
=> [1] Defender detects the EICAR string
$ cat eicar.com
EICAR-STANDARD-ANTIVIRUS-TEST-FILE!
$ ./mpclient eicar.com
[...]
EngineScanCallback(): Threat Virus:DOS/EICAR_Test_File identified.
|-------------------------------- [ next ] -------------------------------|
=> [2] Defender kindly *emulates* your file as JScript
$ cat sample.txt
var mal = "EICAR-STANDARD-ANTIVIRUS-TEST-FILE"
eval(mal + "!")
$ ./mpclient sample.txt
[...]
EngineScanCallback(): Threat Virus:DOS/EICAR_Test_File identified.
+-------------------------------------------------------------------------+
So, let's say you can store a secret in the SESSION file, but there's no
way to read it directly. Crafting the following structure gives you the
ability to check whether the first character of the secret is an `A`: if
it is, the file gets deleted; otherwise, it stays. I think this technique
is super creative - and in some ways, it's a perfect example of how CTF
helps push the boundaries of cybersecurity!