A format string vulnerability is a bug where user input is passed as the format argument to printf, scanf, or another function in that family.

March 5, 2025 - void blackbox(char *input) { printf(input); // Format string vulnerability here if (brocode == 0x4ADF070F) { system("cat flag.txt"); // Goal: Trigger this branch } else { puts("You cannot enter as brocode"); } } int main() { char name[1048]; setbuf(stdout, NULL); setbuf(stdin, NULL); setbuf(stderr, NULL); printf("Enter name: "); fgets(name, sizeof(name), stdin); blackbox(name); return 0; }

April 12, 2026 - If printf("%s", buf) is used instead, the input is always treated as a plain string and no specifiers are interpreted. Send %x specifiers to dump stack values as hex integers. Each %x consumes one word from the stack: ... Use %p instead of %x to get pointer-width output with a 0x prefix, which is cleaner on 64-bit systems: ... If you want to read from a specific stack offset without cycling through all the preceding ones, use the positional argument syntax %N$x where N is the index: $ echo '%6$x' | ./vulnerable # read the 6th stack word

It's easy to understand that we're going to use the format string vulnerability to hijack the return address of the program to the address we want to execute. Here we take [three white hat-pwnme_k0] (https://github.com/ctf-wiki/ctf-challenges/tree/master/pwn/fmtstr/three white hats-pwnme_k0) as an example for analysis.

February 16, 2023 - When we check the function called vuln() we know that vulnerability occurs however. We should consider on printf(story) because there is not parameters given. ... The first sample interprets story as a format string, and parses any formatting instructions it may contain.

May 23, 2024 - This is a type of vulnerability where submitted data of an input string is evaluated as an argument to an unsafe use of e.g., a printf() function by the application, resulting in the ability to read and/or write to memory.

For example, '%s' indicates that we will output a string argument.

March 19, 2021 - # We now use the format string vulnerability to write and execute a ropchain # Overwrite EIP with whatever we want and use it to leak LIBC. # In order to leak libc we execute puts with a function's GOT entry address as an argument. # This way puts will print out, as a string, the address of ...

) = 6 puts("Input 2:"Input 2: ) = 9 read(0BBBB , "BBBB\n", 128) = 5 puts("\nInput 3:" Input 3: ) = 10 read(0CCCC , "CCCC\n", 128) = 5 printf("CCCC\n"CCCC ) = 5 free(0x55db2ad8d2a0) = <void> puts("\nLol how could inputting strings"... Lol how could inputting strings stop the central finite curve. ) = 64 +++ exited (status 0) +++ ``` It is clear that there is a format string vulnerability on the third input.

Decompiling ``main2()``, we can see that it prints a global buffer ``fake_flag``, again introducing another format string vulnerability. We can see that the real flag is read into the ``local_38`` buffer. The idea for the solution would be to first use ``%n`` to rewrite ``fake_flag`` to ``%s``, so it prints out the real flag.

This program is vulnerable to a format string attack! Try supplying a format string to overwrite a global variable and get a shell!

This program is clearly vulnerable to a format string attack. Further, to get to the winning path it checks a global variable against the value 0xCCC31337.

Format string vulnerabilities occur when a program passes user-controlled data as the format string parameter to functions like printf(), fprintf(), sprintf(), etc. These functions interpret format specifiers (such as %s, %d, %x, %n) within the format string to determine how to process subsequent ...

CTF events / redpwnCTF 2019 / Tasks / Black Echo / Writeup · by bytevsbyte / beerpwn · Rating: 5.0 · # WRITEUP BLACK ECHO (blind format string) ## __Author:__ bytevsbyte @ beerpwn team · There wasn't any executable to download, just a service running on the server.\ When I tried to connect with netcat and send some bytes, the server replied echoing my input.\ Then I sent some "%p" to check if it was a format string vulnerability.

December 5, 2020 - #include <stdio.h> #include <stdlib.h> #include <string.h> int main(int argc, char **argv){ int i = 1; char buffer[64]; snprintf(buffer, sizeof buffer, argv[1]); buffer[sizeof (buffer) - 1] = 0; printf("Change i's value from 1 -> 500. "); if(i==500){ printf("GOOD\n"); setreuid(geteuid(),geteuid()); system("/bin/sh"); } printf("No way...let me give you a hint!\n"); printf("buffer : [%s] (%d)\n", buffer, strlen(buffer)); printf ("i = %d (%p)\n", i, &i); return 0; } Hi! I'm working on ctf exercise(Overthewire 5 level, Narnia).

June 11, 2020 - Offensive Security Engineer, I blog about Cyber security, CTF writeup, Programming, Blockchain and more about tech. born and raised in indonesia, currently living in indonesia Posts About ... did you know , we can write something on memory by using printf ? yes we can. in this post i will try to explain how printf works and how we can exploit format string vulnerability on printf()

October 14, 2023 - CTF Writeups WalkthroughCyberSecurity ... in which the vulnerable code contains an implementation of printf statement that takes the user input directly as an argument without input filtering or validation....