spomky-labs/otphp: Unbounded digits parameter in a provisioning URI triggers an uncaught DivisionByZeroError in OTP generation

Summary

The digits parameter parsed from a provisioning URI is validated only with a lower bound ($value > 0) and has no upper bound (src/OTP.php:353-357). OTP generation computes $code % (10 ** $this->getDigits()) (src/OTP.php:283). When digits is large enough that 10 ** digits overflows PHP's integer range and the (int) cast yields 0 (around digits >= 40 on 64-bit PHP 8.x), the modulo operand becomes 0 and PHP raises a DivisionByZeroError.

Impact

OTPHP\Factory::loadFromProvisioningUri() forwards the attacker-controlled digits query value to setParameter('digits', $value), so a hostile URI such as otpauth://totp/Alice?secret=JBSWY3DPEHPK3PXP&digits=50 produces an OTP object whose at(), now(), and verify() all throw DivisionByZeroError. Because DivisionByZeroError extends Error (not Exception), callers that guard OTP generation with a catch (\Exception) do not catch it, turning a malformed URI into an unhandled fatal error (denial of service of the verification path).

Measured threshold on PHP 8.3: digits = 30 works, digits >= 40 throws DivisionByZeroError: Modulo by zero.

Affected component

• src/OTP.php:353-357 — digits parameter callback (no upper bound)
• src/OTP.php:283 — $code % (10 ** $this->getDigits())

Proof of concept

use OTPHP\Factory;
use OTPHP\InternalClock;

$otp = Factory::loadFromProvisioningUri(
    'otpauth://totp/Alice?secret=JBSWY3DPEHPK3PXP&digits=50',
    new InternalClock()
);
$otp->at(0); // DivisionByZeroError: Modulo by zero (escapes catch (\Exception))

Remediation

Enforce a sane upper bound on digits in the parameter validation callback (e.g. reject values above 8–10, the practical range for OTPs) so that an out-of-range value is rejected with a documented exception instead of producing an object that fails later with an uncatchable Error.