TLDR

In early 2021, we reported a few security issues to Cisco related to their RV34X series of routers, two of which have been recently patched. The issues in question were an authentication bypass and system command injection, both in the web management interface. These can be chained together to achieve unauthenticated command execution. Cisco has released an advisory, and assigned CVE IDs as follows:

• RV34X /upload Authorization Bypass Vulnerability (CVE-2021-1472)
• RV34X OS Command injection in Cookie string (CVE-2021-1473)

The issues have been fixed in firmware version 1.0.03.21 in the RV34X series. Cisco has noted that the RV26X and RV16X series are also affected by the authentication bypass issue, and has released firmware version 1.0.01.03 to address this. This post contains a root cause analysis for these bugs. Enjoy! [caption id="attachment_4060" align="aligncenter" width="719"] © Cisco[/caption]

Affected vendor & product	Cisco Small Business RV Series Router (www.cisco.com)
Vulnerable version	RV34X 1.0.3.20 & below, RV16X/RV26X 1.0.01.02 & below.
Fixed version	RV34X series: 1.0.03.21. RV16X/RV26X: 1.0.01.03.
CVE IDs	CVE-2021-1472, CVE-2021-1473
Impact	5.3 (medium) CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N 8.8 (high) CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:L
Credit	T. Shiomitsu, IoT Inspector Research Lab

RV34X/RV26X/RV16X /upload Authorization Bypass Vulnerability (CVE-2021-1472)

While Cisco has noted that this issue affects other devices, I'll only go over the specifics of how it affects the RV34X series here. On RV34X devices, the web management interface is served by nginx on port 443. nginx is configured (by files in /etc/nginx/) so that requests made to the URIs /upload, /form-file-upload and /api/operations/ciscosb-file:form-file-upload are all passed to a CGI binary called upload.cgi. Depending on which URI is requested, the behavior of upload.cgi is slightly different. In firmware revisions earlier than 1.0.3.20, there was no real attempt to restrict access to these upload.cgi-related endpoints. In fact, a set of command injection issues from late 2020 affecting the RV34X series were initially disclosed as post-authentication issues but later revised to reflect the fact that these could be exploited pre-authentication (after a very charitable and publicly-uncredited researcher - cough cough - tipped off the Cisco PSIRT). These were tracked by the ZDI as ZDI-20-1100 and ZDI-20-1101, and you can see the Cisco advisory here. While the ZDI advisories have not been updated and still show the initial lower CVSS rating – the Cisco advisory and CVSS scores have been updated to reflect the pre-authentication nature of the bugs. In 1.0.03.20, an authentication check was implemented. This was written into nginx configuration, which you can see here: The attempt here appears to be to check that some Authorization header is set, and/or that a file exists in the /tmp/websession/token/ folder with the same name as the request sessionid cookie. Then a user is assumed to be authorized. Unfortunately, there’s a fatal flaw in this fix. The logic is such that any non-null Authorization header would set $deny to “0”. So, sending literally any valid-looking Authorization header as part of a request to /upload will bypass the authorization check.

RV34X OS Command injection in Cookie string (CVE-2021-1473)

Once we have bypassed authentication, it’s then possible to interact directly with the /upload endpoint. Requests made to this endpoint are passed directly to the upload.cgi binary by the nginxuwsgi CGI configuration. Within the main() function in upload.cgi, the HTTP_COOKIE environmental variable is read, and the value from the sessionid cookie is extracted using a simple series of strtok_r and strstr. This specific sessionid-reading logic is notable because, due to the strtok_r call, it’s not possible to use “;” characters in any injection, as it will prematurely terminate the injection string. In pseudocode, it looks like this:

if (HTTP_COOKIE != (char *)0x0) { 
     StrBufSetStr(cookie,HTTP_COOKIE); 
     cookie = StrBufToStr(cookie); 
     cookie = strtok_r(cookie, ";", &saveptr); 
     while (cookie != 0x0) { 
       cookie = strstr(cookie, "sessionid="); 
       if (cookie != 0x0) { 
         sessionid_cookie_value = pathparam_ + 10; 
       } 
     } 
   }

Because our HTTP request is made to the /upload URI, the main() function in upload.cgi calls a function at 000124a4, which I’ve named handle_upload(). This function takes a pointer to the sessionid cookie value as its first argument. void handle_upload(char *sessionId, char *destination, char *option, char *pathparam, char *fileparam, char *cert_name, char *cert_type, char *password)  It also takes several other arguments, each of which are populated by the multipart request parsing that takes place in the main() function. The names I’ve given these arguments roughly align with the names of the parameters that this multipart ingesting logic looks for.

Depending on what string is passed as the pathparam parameter, slightly different code paths will be taken, which means that slightly different checks must be bypassed to be able to reach the vulnerable code. In this example, I am using a request with the pathparam set to “Configuration”, so the pseudocode I'm showing reflects this.

Within handle_upload(), a curl command is constructed with a call to sprintf, the resulting buffer of which is then passed directly to popen:

ret = strcmp(pathparam, "Configuration"); 
 if (ret == 0) { 
   config_json = upload_Configuration_json(destination,fileparam); 
   if (config_json != 0) { 
     post_data = json_object_to_json_string(config_json); 
     sprintf(command_buf, "curl %s --cookie \'sessionid=%s\' -X POST -H \'Content-Type: application/json\' -d\'%s\' ", jsonrpc_cgi, sessionId , post_data); 
     debug("curl_cmd=%s",command_buf); 
     __stream = popen(command_buf, "r"); 
     if (__stream != (FILE *)0x0) { 
       [...snip...] 
   }

The sessionid cookie value that we have passed in our request is passed directly into this sprintf() call. With a crafted sessionid value, we would therefore be able to inject arbitrary commands into this command buffer. This will run the command with the privileges of the upload.cgi binary which, in this case, is www-data.

Key Takeaways

Logic bugs can be quite easy to introduce, and sometimes tricky to identify. Authentication can be difficult to implement well, especially when multiple authorization methods might be accepted. As higher-end embedded devices start to use more common server software components (for purposes they were not necessarily intended for), there are often more layers of complexity introduced - thicker web servers requiring more precise configuration, CGI binaries, middleware gluing things together. Each layer introduces opportunities for mis-configuration, which could lead to security issues.

Timeline

2021-01-02: Initial disclosure made to Cisco PSIRT. 2021-01-07: Confirmation of receipt of disclosure from Cisco PSIRT. 2021-01-27: Confirmation that issue is valid from Cisco PSIRT. 2021-02-12: Update from Cisco PSIRT. 2021-03-23: We contact Cisco PSIRT for timeline update and CVE IDs. 2021-03-23: Cisco PSIRT respond giving us timeline and CVE IDs. 2021-04-07: Cisco release advisory.