JA3 Fingerprint Plugin

Description

The JA3 fingerprint plugin calculates JA3 fingerprints for incoming SSL traffic. “JA3” is a method for creating SSL/TLS client fingerprints by concatenating values in the TLS Client Hello and hashing the result using MD5 to produce a 32 character fingerprint. A particular instance of malware tends to use the same encryption code/client, which makes it an effective way to detect malicious clients even when superficial details are modified. More info about JA3 is available here.

The calculated JA3 fingerprints are then appended to upstream request in the field X-JA3-Sig (to be processed at upstream). If multiple duplicates exist for the field name, it will append to the last occurrence; if none exists, it will add such a field to the headers. The signatures can also be logged locally.

Plugin Configuration

ja3_fingerprint can be used as a global/remap plugin and is configured via plugin.config or remap.config.

--ja3raw

This option cause the plugin to append the field X-JA3-Raw to proxy request. The field value is the raw JA3 fingerprint.

By default this is not enabled.

--ja3log

This option enables logging to the file ja3_fingerprint.log in the standard logging directory. The format is

[time] [client IP] [JA3 string] [JA3 hash]

By default this is not enabled.

--modify-incoming

As described above, typically, the X-JA3-Sig header is added to the request as it is sent upstream so that any upstream hosts can make use of it. The --modify-incoming option instructs the plugin to add the X-JA3-Sig header field earlier on the incoming request so that other plugins can make use of it while processing the request. To be precise, --modify-incoming adds the X-JA3-Sig header field in the TS_HTTP_READ_REQUEST_HDR_HOOK hook rather than the default TS_HTTP_SEND_REQUEST_HDR_HOOK hook.

This option is only useful when the plugin is configured as a global plugin. By default this is not enabled.

Requirement

This requires OpenSSL 1.0.1, 1.0.2, or OpenSSL 1.1.1 or later. OpenSSL 1.1.0 will not work due to API changes with regard to opaque structures.

There is a potential issue with very old TLS clients which can cause a crash in the plugin. This is due to a bug in OpenSSL which should be fixed in a future release.