[lacatoire]

SNMP::setSecurity() stored a pointer into the $contextName string instead of copying it, so the next request read freed memory once the string was released. Copy it and free it with the session.

To reproduce

$s = new SNMP(SNMP::VERSION_3, '127.0.0.1', 'adminMD5AES', 500000, 1);
$ctx = str_repeat('c', 5) . random_int(10, 99);
$s->setSecurity('authPriv', 'MD5', 'test1234', 'AES', 'test1234', $ctx);
unset($ctx);
var_dump(@$s->get('.1.3.6.1.2.1.1.4.0'));

Before (ASAN build): heap-use-after-free in strdup, called from snmp_sess_add_ex

After: bool(false)