Require some example for from_base64 keyword

Rules
1

Hi all.

I am attempting to write rules to detect the deserialization attack payload below, which is encoded with base64. I noticed that the Suricata team now recommends using from_base64 instead of the base64_decode + base64_data keywords. However, after writing the rule and testing it, the rule did not generate any alerts.

alert http any any -> $BUSSINESS_NET any (msg:"deserialization attack"; flow:to_server,established; http.uri; content:"/Kingdee.BOS.ServiceFacade.ServicesStub.DevReportService.GetBusinessObjectData.common.kdsvc"; http.request_body; content:"|7b 22|ap0|22 3a|"; from_base64:bytes 15884,offset 1,mode rfc4648; content:"This|20|program|20|cannot|20|be|20|run|20|in|20|DOS|20|mode."; classtype:web-application-attack; sid:0100132; gid:1; priority:1; rev:1;)

POST /Kingdee.BOS.ServiceFacade.ServicesStub.DevReportService.GetBusinessObjectData.common.kdsvc HTTP/1.1
host: www.test.com
accept: */*
accept-encoding: gzip, deflate
connection: keep-alive
user-agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 12_10) AppleWebKit/600.1.25 (KHTML, like Gecko) Version/12.0 Safari/1200.1.25
content-type: text/json
Content-Length: 15911

{"ap0": 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"3"}
2

Hi, there!

Thank you for the report. I have a few questions for you.

  1. Do you see alerts with base64_decode + base64_data keywords?
  2. Which version are you testing here?

There have been several updates on the base64 front lately.
Also, could you please share a pcap?

3

Hi, thanks for your reply. actually I did a test by using base64_encode+ base64_data. and there weren’t any alert.
My suricata version: 8.0.0-dev (30806ce2b 2024-10-21)
Here is the pcap of my test
deserialization_attack.pcap (17.6 KB)

4

Thanks for sharing the extra info!

Since you’ve mentioned that the rule isn’t alerting with base64_decode + base64_data: one step that you could do is to build your rule incrementally. Start with a very simple version that you know alerts, and then add more elements to it – this is just a sanity check to reduce the notice of other aspects that may be impacting on the alert triggering.

5

Change the offset value to 9

The from_base64 transform receives a buffer starting with {"ap0": "AA..."}. Using offset positions the decode starting point to AA...

2 Likes
6

Cool, It works. Thanks for all your help.

1 Like