CVE-2026-67277
MikroTik RouterOS Missing Authentication for Critical Function Vulnerability
MikroTik RouterOS
RouterOS accepts a "related" btest connection before the corresponding primary session has completed authentication. An unauthenticated client can use this state to start an IPv4 UDP test. With "random-data=false", the sender transmits an uninitialized tail from a kernel packet buffer. A separate unchecked, inverted packet-size interval causes unsigned integer underflow, anomalously large fragmented output, and can restart the RouterOS kernel. This issue was fixed in versions: 6.49.21 (Long-term), 7.23.4 (Long-term) and 7.24.2 (Stable)
Exploitation status
CISA Known Exploited Vulnerabilities
Listed by CISA on 10 September 2026. US federal agencies were required to remediate it by 13 September 2026.
EPSS — exploitation probability
FIRST puts the probability of exploitation activity in the next 30 days at 0.9%, which is higher than 57% of all scored vulnerabilities. EPSS is a forecast of activity, not a measure of severity, and it is rescored daily.
CVSS vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:H
Frequently asked questions
Is CVE-2026-67277 being exploited by ransomware groups?
No ransomware group in this database is currently recorded as exploiting CVE-2026-67277.
What does CVE-2026-67277 affect?
CVE-2026-67277 affects MikroTik RouterOS. RouterOS accepts a "related" btest connection before the corresponding primary session has completed authentication. An unauthenticated client can use this state to start an IPv4 UDP test. With "random-data=false", the sender transmits an uninitialized tail from a kernel packet buffer. A separate unchecked, inverted packet-size interval causes unsigned integer underflow, anomalously large fragmented output, and can restart the RouterOS kernel. This issue was fixed in versions: 6.49.21 (Long-term), 7.23.4 (Long-term) and 7.24.2 (Stable)
How severe is CVE-2026-67277?
CVE-2026-67277 is rated HIGH with a CVSS base score of 8.2. EPSS puts the probability of exploitation in the next 30 days at 0.9%, higher than 57% of all scored vulnerabilities. It is listed in the CISA Known Exploited Vulnerabilities catalog, with a federal remediation deadline of 13 September 2026.
How should organisations respond to CVE-2026-67277?
Apply the vendor patch for MikroTik RouterOS as the first priority, and treat any internet-facing instance as the most urgent. Because this vulnerability is associated with ransomware activity, also review whether the affected systems were reachable before patching, rather than assuming that patching alone closes the incident.