Comment on page



Can you take advantage of misused malloc calls to leak the secret through this service and get the flag? Connect with nc 21899. Source.


  1. 1.
    nc 21899:
    login - login as a user
    print-flag - print the flag
    logout - log out
    quit - exit the program
    Enter your command:
    [anon]> login
    Please enter the length of your username
    Please enter your username
    Enter your command:
    [tes]> Invalid option
    login - login as a user
    print-flag - print the flag
    logout - log out
    quit - exit the program
    Enter your command:
    [tes]> print-flag
    Incorrect Access Code: ""
    Enter your command:
    [tes]> logout
    Enter your command:
    [anon]> quit
  2. 2.
    We need to login as a user which has an access code allowing them to print the flag. However, there is no API to set access codes. user struct:
    struct user {
    char *username;
    char access_code[ACCESS_CODE_LEN];
    char *files;
  3. 3.
    The program uses malloc instead of calloc. malloc allocates memory block of given size (in bytes) and returns a pointer to the beginning of the block. malloc doesn’t initialize the allocated memory. calloc allocates the memory and also initializes the allocated memory block to zero. Therefore, the memory set in line 73 (char *username = malloc(username_len+1);) can be reused. The values stored in the block of memory allocated by malloc will persist when malloc is called again since malloc does not overwrite them. More info about malloc vs calloc.
  4. 4.
    malloc in itself is not dangerous but this program has a vulnerability where it only initializes the username field of the user struct, which means the other members of the struct will contain leftover values from whatever the memory was previously used for.
  5. 5.
    Running the program allows us to create a user that has a username with the length of our choice. We can set the length of the username to sizeof(struct user), which is the length of the user structure (32 bytes = 8 for username pointer + 16 for size 2 array + 8 for files pointer). We can set this allocated username field as if it were a user structure. When we logout, the allocation is freed. However, we can log back in and the heap manager will probably provide us with the same buffer we just freed since the requested buffer size is equal to a previously freed buffer size (an optimization to reduce memory fragmentation). This new user will have the "leftover" access code and will be able to access the flag. This is possible because the previous user's username memory was freed (and was the last chunk to be freed), but not cleared.
  6. 6.
    Visual Description:
    1. 1.
      Allocate the first user:
      user struct
      char *username: pointer (that points to a "username buffer" with the username)
      char access_code: unknown leftover
      char *files: unknown leftover
    2. 2.
      Free first user
    3. 3.
      Allocate second user (Heap Manager provides "username buffer" as buffer for user struct):
      struct user (previously: "username buffer")
      char *username: pointer (that points to a newly allocated block of memory containing the new username)
      char access_code: ACCESS_CODE (leftover from "username buffer")
      char *files: bbbbbbbb (leftover from "username buffer")
  7. 7.
    Run the python USER=<username> PASSWORD=<password>:
    [*] '~/Documents/PicoCTF/Binary Exploitation/messy-malloc/auth'
    Arch: amd64-64-little
    RELRO: Partial RELRO
    Stack: Canary found
    NX: NX enabled
    PIE: No PIE (0x400000)
    FORTIFY: Enabled
    [+] Opening connection to on port 21899: Done
    [+] picoCTF{g0ttA_cl3aR_y0uR_m4110c3d_m3m0rY_ac0e0e6a}