Integer Overflow (Web Applications)

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This page focuses on how integer overflows/truncations can be abused in web applications and browsers. For exploitation primitives inside native binaries you can continue reading the dedicated page:

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../../binary-exploitation/integer-overflow.md {{#endref}}

1. Why integer math still matters on the web

Even though most business-logic in modern stacks is written in memory-safe languages, the underlying runtime (or third-party libraries) is eventually implemented in C/C++. Whenever user-controlled numbers are used to allocate buffers, compute offsets, or perform length checks, a 32-bit or 64-bit wrap-around may transform an apparently harmless parameter into an out-of-bounds read/write, a logic bypass or a DoS.

Typical attack surface:

1. Numeric request parameters – classic id, offset, or count fields.
2. Length / size headers – Content-Length, WebSocket frame length, HTTP/2 continuation_len, etc.
3. File-format metadata parsed server-side or client-side – image dimensions, chunk sizes, font tables.
4. Language-level conversions – signed↔unsigned casts in PHP/Go/Rust FFI, JS Number → int32 truncations inside V8.
5. Authentication & business logic – coupon value, price, or balance calculations that silently overflow.

2. Recent real-world vulnerabilities (2023-2025)

3. Testing strategy

3.1 Boundary-value cheat-sheet

Send extreme signed/unsigned values wherever an integer is expected:

-1, 0, 1,
127, 128, 255, 256,
32767, 32768, 65535, 65536,
2147483647, 2147483648, 4294967295,
9223372036854775807, 9223372036854775808,
0x7fffffff, 0x80000000, 0xffffffff

Other useful formats:

• Hex (0x100), octal (0377), scientific (1e10), JSON big-int (9999999999999999999).
• Very long digit strings (>1kB) to hit custom parsers.

3.2 Burp Intruder template

§INTEGER§
Payload type: Numbers
From: -10 To: 4294967300 Step: 1
Pad to length: 10, Enable hex prefix 0x

3.3 Fuzzing libraries & runtimes

• AFL++/Honggfuzz with libFuzzer harness around the parser (e.g., WebP, PNG, protobuf).
• Fuzzilli – grammar-aware fuzzing of JavaScript engines to hit V8/JSC integer truncations.
• boofuzz – network-protocol fuzzing (WebSocket, HTTP/2) focusing on length fields.

4. Exploitation patterns

4.1 Logic bypass in server-side code (PHP example)

$price = (int)$_POST['price'];          // expecting cents (0-10000)
$total = $price * 100;                  // ← 32-bit overflow possible
if($total > 1000000){
    die('Too expensive');
}
/* Sending price=21474850 → $total wraps to ‑2147483648 and check is bypassed */

4.2 Heap overflow via image decoder (libwebp 0-day)

The WebP lossless decoder multiplied image width × height × 4 (RGBA) inside a 32-bit int. A crafted file with dimensions 16384 × 16384 overflows the multiplication, allocates a short buffer and subsequently writes ~1GB of decompressed data past the heap – leading to RCE in every Chromium-based browser before 116.0.5845.187.

4.3 Browser-based XSS/RCE chain

1. Integer overflow in V8 gives arbitrary read/write.
2. Escape the sandbox with a second bug or call native APIs to drop a payload.
3. The payload then injects a malicious script into the origin context → stored XSS.

5. Defensive guidelines

1. Use wide types or checked math – e.g., size_t, Rust checked_add, Go math/bits.Add64.
2. Validate ranges early: reject any value outside business domain before arithmetic.
3. Enable compiler sanitizers: -fsanitize=integer, UBSan, Go race detector.
4. Adopt fuzzing in CI/CD – combine coverage feedback with boundary corpora.
5. Stay patched – browser integer overflow bugs are frequently weaponised within weeks.

References

• NVD CVE-2023-4863 – libwebp Heap Buffer Overflow
• Google Project Zero – "Understanding V8 CVE-2024-0519"