wupco

Solving "includer's revenge" from hxp ctf 2021 without controlling any files

The challenge

The challenge was to achieve RCE with this file:

<?php ($_GET['action'] ?? 'read' ) === 'read' ? readfile($_GET['file'] ?? 'index.php') : include_once($_GET['file'] ?? 'index.php');

Some additional hardening was applied to the php installation to make sure that previously known solutions wouldn't work (for further information read this writeup from the challenge author).

I didn't solve the challenge during the competition - here is a writeup from someone who did - but since the idea I had differed from the techniques used in the published writeups I read (and I thought it was cool :D), here is my approach.

wupco

123;
    return 123;
}
extern void *opendir(const char *);
extern void *readdir(void *);
extern void *shmat(int, const void *, int);
typedef struct {
    ino_t d_ino;
    off_t d_off;
    unsigned short d_reclen;