Tags: aes-ecb web
Rating:
This challenge was notably different from the standard web challenges I'm familiar with, as it required knowledge of `AES` vulnerabilities in `ECB` mode.
In this case, the source code contained a particularly suspicious section of code:
```py
def parse(self, c):
d = {}
if c is None:
return d
for p in c.split("&"):
try:
k,v = p.split("=")
if not k in d:
d[k]=v
except:
pass
return d
def new_session(self, r):
id = secrets.token_hex(4)
c = f"_id={id}&admin=0&color=ffff00&"
return self._c(c)
def _c(self, v):
try:
v = v.encode()
while len(v) % 16 != 0:
v += b'\x41'
return AES.new(self.k,1).encrypt(v).hex()
except:
return None
```
After looking at this code for a while, I noticed that it was possible to easily encrypt arbitrary blocks that, if crafted correctly, could be mixed together to create a cookie with admin privileges. \
At this point, what I did was fill the portion of the cookie that I couldn't modify myself, `_id={id}&admin=0&color=` (where id is a string of 4 * 2 hexadecimal characters), with characters at the end to make its length divisible by 16 (in other words, full blocks). Then, I wrote `admin=1` in the next block. This way, I could shift the last block to the beginning and overwrite the original cookie to obtain the flag.
