Exploitalert - Database of Exploits

Western Digital My Cloud Buffer Overflow

CVE	Category	Price	Severity
CVE-2018-7171	CWE-119	$10,000	Critical

Author	Risk	Exploitation Type	Date
Unknown	High	Remote	2017-03-08

CVSS	EPSS	EPSSP
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H	0.02192	0.50148

CVSS vector description

Metric	Value	Metric Description	Value Description
Attack vector	Network	AV	The vulnerable system is bound to the network stack and the set of possible attackers extends beyond the other options listed below, up to and including the entire Internet. Such a vulnerability is often termed “remotely exploitable” and can be thought of as an attack being exploitable at the protocol level one or more network hops away (e.g., across one or more routers). An example of a network attack is an attacker causing a denial of service by sending a specially crafted TCP packet across a wide area network (e.g., CVE-2004-0230).
Attack Complexity	High	AC	The successful attack depends on the evasion or circumvention of security-enhancing techniques in place that would otherwise hinder the attack. These include: Evasion of exploit mitigation techniques. The attacker must have additional methods available to bypass security measures in place. For example, circumvention of address space randomization (ASLR) or data execution prevention must be performed for the attack to be successful. Obtaining target-specific secrets. The attacker must gather some target-specific secret before the attack can be successful. A secret is any piece of information that cannot be obtained through any amount of reconnaissance. To obtain the secret the attacker must perform additional attacks or break otherwise secure measures (e.g. knowledge of a secret key may be needed to break a crypto channel). This operation must be performed for each attacked target.
Privileges Required	None	PR	The attacker is unauthenticated prior to attack, and therefore does not require any access to settings or files of the vulnerable system to carry out an attack.
User Interaction	None	UI	The vulnerable system can be exploited without interaction from any human user, other than the attacker. Examples include: a remote attacker is able to send packets to a target system a locally authenticated attacker executes code to elevate privileges
Scope	Unchanged	S	An exploited vulnerability can only affect resources managed by the same security authority. In the case of a vulnerability in a virtualized environment, an exploited vulnerability in one guest instance would not affect neighboring guest instances.
Confidentiality	High	C	There is total information disclosure, resulting in all data on the system being revealed to the attacker, or there is a possibility of the attacker gaining control over confidential data.
Integrity	High	I	There is a total compromise of system integrity. There is a complete loss of system protection, resulting in the attacker being able to modify any file on the target system.
Availability	High	A	There is a total shutdown of the affected resource. The attacker can deny access to the system or data, potentially causing significant loss to the organization.

https://cxsecurity.com/ascii/WLB-2017030080

Western Digital My Cloud Buffer Overflow------------------------------------------------------------------------ Stack-based buffer overflow in Western Digital My Cloud allows for remote code execution ------------------------------------------------------------------------ Remco Vermeulen, January 2017 ------------------------------------------------------------------------ Abstract ------------------------------------------------------------------------ It was discovered that the Western Digital My Cloud is vulnerable to a stack-based buffer overflow in the authentication mechanism. By exploiting this vulnerability it is possible for an unauthenticated attacker to run arbitrary code with root privileges. ------------------------------------------------------------------------ Tested versions ------------------------------------------------------------------------ This vulnerability was successfully verified on a Western Digital My Cloud model WDBCTL0020HWT running firmware version 2.21.126. This issue isn't limited to the model that was used to find this vulnerability since most of the products in the My Cloud series share the same (vulnerable) code. ------------------------------------------------------------------------ Fix ------------------------------------------------------------------------ There is currently no fix available. ------------------------------------------------------------------------ Details ------------------------------------------------------------------------ https://www.securify.nl/advisory/SFY20170105/stack_based_buffer_overflow_in_western_digital_my_cloud_allows_for_remote_code_execution.html The previous My Cloud firmware version 2.21.119 was susceptible to an authentication bypass. To solve this an additional check was added that verifies if an admin user is actually logged in. The authentication check is performed in the login_check() function found in the /web/lib/login_checker.php file. /* ret: 0: no login, 1: login, admin, 2: login, normal user */ function login_check() { $ret = 0; if (isset($_SESSION['username'])) { if (isset($_SESSION['username']) && $_SESSION['username'] != "") $ret = 2; //login, normal user if ($_SESSION['isAdmin'] == 1) $ret = 1; //login, admin } else if (isset($_COOKIE['username'])) { if (isset($_COOKIE['username']) && $_COOKIE['username'] != "") $ret = 2; //login, normal user if ($_COOKIE['isAdmin'] == 1) $ret = 1; //login, admin if (wto_check($_COOKIE['username']) === 0) //wto check fail $ret = 0; } return $ret; } To prevent the authentication bypass from occuring another check is performed by calling the wto_check() function with the value of the username cookie to check if a user is actually logged in. The wto_check() function calls the wto binary as shown in the following code fragment. /* return value: 1: Login, 0: No login */ function wto_check($username) { if (empty($username)) return 0; exec(sprintf("wto -n \"%s\" -i '%s' -c", escapeshellcmd($username), $_SERVER["REMOTE_ADDR"]), $login_status); if ($login_status[0] === "WTO CHECK OK") return 1; else [...] } The username is escaped with the escapeshellcmd argument to prevent command injection (which is not sufficient as pointed out by Zenofex). Analysis of the wto binary shows that it is susceptible to a buffer overflow when it copies the provided username to the stack using the strcpy() function without performing any input checking. To parse the provided parameters the wto binary uses the getopt() function (provided by libc). The follow listing shows the case called when the optstring equals 'n'. The optarg contains a pointer to our provided input and this is directly passed to the strcpy() function as the source parameter via R1. The destination pointer is provided via R0 and this is a pointer to the stack. .text:00010FC0 get_username .text:00010FC0 .text:00010FC0 LDR R3, =(optarg_ptr - _GLOBAL_OFFSET_TABLE_) ; jumptable 00010F48 case 13 .text:00010FC4 LDR R3, [R7,R3] .text:00010FC8 ADD R0, SP, #0xE0+var_A0 .text:00010FCC LDR R1, [R3] .text:00010FD0 BL strcpy .text:00010FD4 B parse_args .text:00010FD4 Proof of concept Sending a username consisting of 160 characters is enough to overwrite the saved LR register on the stack and to get control of the program counter (PC). The following core dump shows that besides controlling the value of LR (and therefore PC) we can set the values of R4-R8 and R10: Core was generated by `wto -n AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA'. Program terminated with signal SIGSEGV, Segmentation fault. #0 0x41414140 in ?? () (gdb) info reg r0 0x0 0 r1 0x0 0 r2 0x0 0 r3 0xb68b526c 3062583916 r4 0x41414141 1094795585 r5 0x41414141 1094795585 r6 0x41414141 1094795585 r7 0x41414141 1094795585 r8 0x41414141 1094795585 r9 0x0 0 r10 0x41414141 1094795585 r11 0x0 0 r12 0x0 0 sp 0xbef6fbd8 0xbef6fbd8 lr 0x41414141 1094795585 pc 0x41414140 0x41414140 cpsr 0x60000030 1610612784 (gdb)

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