CVE-2026-67277

CVE-2026-67277 is a missing-authentication flaw (CWE-306) in the bandwidth-test (btest) service of MikroTik RouterOS: the service accepts a "related" btest connection before the primary session has completed authentication, so an unauthenticated remote client can start an IPv4 UDP bandwidth test. When the test runs with "random-data=false", the sender transmits an uninitialized tail from a kernel packet buffer, leaking a small amount of kernel memory, and a separate unchecked, inverted packet-size interval causes an unsigned integer underflow that produces anomalously large fragmented output capable of restarting the RouterOS kernel. An unauthenticated network attacker therefore gains limited information disclosure and, more significantly, a remote denial of service through repeated router crashes, with no credentials or user interaction required (CVSS 4.0: 8.8 High). Any MikroTik device running an unpatched RouterOS version with the bandwidth-test server enabled and reachable from an untrusted network is affected. No public PoC or CISA KEV listing exists for this CVE and EPSS is low (0.4%), but it is part of the batch of RouterOS fixes in MikroTik advisory AV26-887 that attackers have recently chained to hijack routers, so in-the-wild exploitation of the flaw set is reported.

What to do: Upgrade to RouterOS 6.49.21 (Long-term), 7.23.4 (Long-term), or 7.24.2 (Stable), matching your release channel. Until patched, disable the Bandwidth Test Server (btest) or restrict access to it with firewall rules to trusted hosts only. Given reports of recent RouterOS flaws being chained to hijack routers, also review devices for signs of compromise such as unexpected configuration changes or unfamiliar scheduled tasks.