Upload Handling in iWAF
While handling large requests, such as a file upload, certain requests can be blocked with a 413 Request Entity Too Large message.
Avi Vantage could trigger an alert or block uploads for requests such as the following:
- File size exceeding the limit set under the
Client Post Body Sizefield in the virtual service HTTP profile.
- Random match of
System-Default-Policyrules for binary upload data, which will result in WAF blocking or flagging the request, based on the mode.
- Exceeding regex match limit with rules running too long on the input, which will result in WAF terminating the execution.
Prior to Avi Vantage version 18.2.2, if the content-length header of a request is larger than the Maximum file upload size value configured in the WAF profile, then the request will be blocked with a 413 message irrespective of the value configured in the Client Post Body Size field of the HTTP profile.
Starting with Avi Vantage version 18.2.2, large requests (such as a regular file upload) are handled differently as these requests will be processed partially as explained below.
Parameters Handling Large Uploads
The following two configuration parameters are considered for large file uploads:
- Client Post Body Size: In Avi UI, navigate to Templates > Profiles > Application, to configure the HTTP policy. Under the DDoS tab, the Client Post Body Size field defines the maximum body size of a client request.
This value limits the size of a client POST as a part of a single HTTP request. In case of a WAF bypass rule, this value is overridden and not considered. Otherwise, if the value configured is lesser than the Maximum client request size configured under WAF Profile, then the buffering will fail with an 413 error message in the proxy before reaching the iWAF. To mitigate this, update the value configured under Client Post Body Size to be higher than the value configured under Maximum client request size.
Note: If the Client Post Body Size is set to a default value of zero, which refers to no-limit, then this value will always be greater than the Maximum file upload size limit.
- Maximum client request size: In Avi UI, navigate to Templates > WAF > WAF Profile, to configure a WAF Profile. Under the Settings > Other Settings section, the Maximum client request size field defines the maximum allowed size for the client request body to be scanned by WAF. If the client request size is larger than the value configured in this field, then the partial body as allowed per the defined size is scanned through WAF.
If WAF rejects the request due to a System-Default-Policy rule match, then the rest of the request body is discarded. If WAF allows the request, then the rest of the body is streamed in the backend.
Ensure that the checkbox for Ignore request body parsing errors due to partial scanning is selected, to avoid any errors triggered by partial scanning.
- This is supported for both HTTP 1.0 and HTTP 2.0.
- This is supported only for content-length request and not for chunked encoded POST.
- If WAF is enabled in detection mode and a chunked encoded POST is received, then that POST will be rejected if the size of the POST is greater than the Max Client Request Size defined under WAF profile.
The following are a few examples for specific uploads and the corresponding WAF log entries:
- Request is denied with a 413 message as the client request size exceeds the maximum value configured in HTTP profile.
- As WAF did not inspect the request, WAF status is PASSED.
This error can be mitigated by increasing the value of Client Post Body Size.
- Size limit has been increased and so no limit was hit.
- Request is denied with a 403 message.
- Coincidentally, as parts of the PDF matched the WAF CRS rules, the request is rejected and the status is REJECTED.
You can bypass certain large requests or particular upload requests from going through WAF. Few file extensions are bypassed from the WAF check, as they are static content. For more information on configuring static extensions, refer to Configuring WAF Profile.
You can also bypass uploads completely using the Modsec bypass rules as explained in the section below.
Modsec Bypass Rules
The following are a few examples for modsec bypass rules. It is recommended to configure this using URLs. The ID should either be within the local range of 0 to 99.999 or a private reserved range, as explained at the link here. The numbers are choosen here to explain the example. Ensure unique rule IDs in your deployment.
SecRule REQUEST_URI "@rx /app/upload/" id:90001,phase:1,t:none,nolog,pass,ctl:ruleEngine=off
SecRule REQUEST_URI "@rx /app/upload/|/app/upload_two/|/app/upload_three/" id:90002,phase:1,t:none,nolog,pass,ctl:ruleEngine=off
This rule can be altered using other OPERATORS such as @contains, @startwith.
SecRule REQUEST_HEADERS:Content-Length “@gt 1048576” phase:1,id:90003,nolog,pass,ctl:ruleEngine=off
Note: It is recommended to configure rules using the URL, instead of Content-Length.
The rules provided should have a number within the Avi Vantage recommended range, as explained in the ModSecurity Handbook.
Frequently Asked Questions
What is the effect on malicious uploads ?
These are cases where an attacker might want to smuggle a malicious upload onto a server. Such an upload might contain malware, ransomware, viruses, other file based exploits (pdf reader exploits) among many others. It is recommended to use a virus malware scanning tool on the upload directory of the application to detect the attacks and mitigate them.
Will upload bypass affect the security of my application ?
If large binary data bypass is configured with the right scope of uploading requests or URLs, then iWAF will not be able to inspect the data and the impact will be minimal.
Is it not enough to create an exclude rule for the upload parameter?
Large uploads will be cached during traffic processing. Even when a part of the request is not needed and bypassed it will be cached until other parts of that request have been inspected. Therefore, excluding only the upload parameter will not help achieve the best result.