Invalid Base64 payload for filemd5 alerts
Per previous conversation, Peter suggested that I open a new one for this issue.
Summary of the issue: While using a blacklist, containing hashes to be blocked using the filemd5 keyword. At times the payload, if enabled in base64 format is not seen as valid (base64 format).
If I manually try to decode it using: base64 -d (-D macos) tempfile, the following error is shown: "invalid input". This issue have not been seen with any other typs of alerts.
If I enable Suricata to drop the payload as ascii (payload-printable) the full payload is shown without errors for the same files.
Due to the nature of the blacklist it's mostly checksum for PE files, but during tests I have gotten the same issue with plain html files as well
OS: Ubuntu 16.04
Suricata 4.0.4 (PPA) in IPS mode
Attached is a pcap of the alert with the corresponding malicious file in the pcap (you can carve the file if you need to re-run the test, it's a live malware so don't run it =)) also a plain html file (part of the default page for Apache) testfile that generates the same error
Steps to re-create the issue:
- One Ubuntu VM with Suricata 4.0.4 (PPA)
- Install Apache, move the attached html file (testfile) to /var/www/html
- Enable eve logging with base64 payload set to enabled (payload: yes)
- Include the rule set files.rules in suricata.yaml
- Create a rule like this: drop http any any -> any any (msg:"Blacklisted File Blocked"; flow:established; filemd5:bl.md5; classtype:misc-activity; sid:666; rev:1;) in /etc/suricata/rules/files.rule
- Hash a file (for example the testfile attached (10f7309902d8b2c1d801440302baf76e), that you can upload to your monitored asset
- Include the file hash in the bl.md5 file
- Start Suricata with: suricata --simulate-ips -c /etc/suricata/suricata.yaml -i ens33
- Curl/Wget down the file <IP>/testfile
- Review the eve log and copy the base64 payload to a file
- Run base64 -d (or -D) <filename>