Troubleshooting Tip: DLP credit card built-in type not matching ChatGPT Transactions

To reproduce this issue, a policy must be applied to ChatGPT traffic with a DLP security profile and SSL deep inspection enabled. The expected behavior is that the system filters posts containing credit card numbers by leveraging the built-in credit card data type to match the traffic, including cases where the credit card numbers are presented as a list separated by new lines.

Steps to reproduce:

• A DLP policy applied to ChatGPT traffic is needed. This policy must have SSL deep inspection enabled and the dlp profile applied:

• A DLP security profile configuration with a sensor configured with a dictionary matching DLP built in datatype:

• A ChatGPT post containing credit card numbers separated with new lines:

• Notice that the post was not blocked by DLP, even though it contains multiple valid credit card numbers.

Root Cause:

Upon analysis, it was found that this behavior is related to how ChatGPT formats data before sending it to the server.

• ChatGPT submits the content as JSON payloads.

• In decrypted HTTP POST requests, the data appears as a JSON string formatted this way, with the \n separator embedded in the string:

are these credit card numbers?\n4111111111111111\n4111 1111 1111 1111\n5555555555554444\n....

• The built-in Credit Card data type uses this regular expression with word boundaries (\b):
  
  

\b([2-6]{1}\d{3})[- ]?(\d{4})[- ]?(\d{2})[- ]?(\d{2})[- ]?(\d{2,4})\b

• Since the newline character \n is embedded into the string, it does not constitute a word boundary, so the regex fails to match credit card numbers.

Solution / Workaround:

To address this, a custom dictionary data type can be created for credit card detection without using the \b boundary included in the built-in data type.

This ensures that credit card numbers within ChatGPT traffic are detected correctly, even when separated by new lines.

Example configuration with the fix:

With this configuration, traffic is matched by DLP sensor and it is blocked correctly:

CLI Configuration with solution applied:

config firewall policy     edit 24         set name "chatgpt"         set srcintf "fortilab-local-zone"         set dstintf "virtual-wan-link"         set action accept         set srcaddr "all"         set dstaddr "chatgpt"         set schedule "always"         set service "ALL"         set utm-status enable         set inspection-mode proxy         set ssl-ssh-profile "deep-inspection"         set dlp-profile "block-cc-profile"         set application-list "default"         set logtraffic all         set nat enable     next end  config dlp profile     edit "block-cc-profile"         set feature-set proxy         config rule             edit 1                 set name "block-cc"                 set type message                 set proto http-post                 set filter-by sensor                 set sensor "cc"                 set action block             next         end     next end  config dlp sensor     edit "cc"         config entries             edit 1                 set dictionary "custom-cc"             next         end     next end  config dlp dictionary     edit "custom-cc"         config entries             edit 1                 set type "regex"                 set pattern "/([2-6]{1}\\d{3})[- ]?(\\d{4})[- ]?(\\d{2})[- ]?(\\d{2})[- ]?(\\d{2,4})/m"                 set repeat enable             next         end     next end

Important considerations:

• The modified regex is less restrictive than the built-in Credit Card data type.
• This may result in false positives in some environments.
• This workaround is intended specifically for ChatGPT transactions. Other applications or websites that format strings differently may not require this adjustment.

• In lower-end (with 2 GB of RAM or less) FortiGate models with versions v7.6.x, the DLP (Data Loss Prevention) option may not be visible in the GUI under Security Profiles or Feature Visibility.

   

  • In this case, the DLP configuration can still be accessed via a direct URL: https://<firewall-ip>/utm/dlp

     

  • If multiple VDOMs are enabled, specify the VDOM in the URL like this: https://<firewall-ip>/utm/dlp?vdom=<vdom_name>

  •  

    For example: https://10.9.18.44/utm/dlp?vdom=root

     

Related documents:

Protecting GenAI Access using DLP

Technical Tip: DLP profile is not blocking credit card info when outlook desktop app is using HTTPS

Technical Tip: How DLP utilizes Luhn Verification for Credit Card Check along with hyperscan matching engine