Source Link: https://www.ncsc.gov.uk/news/indicators-of-compromise-for-malware-used-by-apt28
This NCSC report directly links the IP address "139.5.177.205" to APT28. The report states that this IP address has been used as a Command and Control (C2) server for the X-AGENT malware, a tool associated with the APT28 threat group (also known as Fancy Bear, Sofacy, Sednit, and STRONTIUM).
Deciphering the Sigil and Revealing the Cabal:
By matching the "Shadowed Sigil" IP address "139.5.177.205" to the information in the NCSC report, we can confidently identify the "notorious cabal of shadow mages" as APT28.
Therefore, the "APTNumber" in the flag corresponds to the numerical identifier of the APT28 group.
Flag:
HTB{APT28}
Conclusion:
"The Shadowed Sigil" challenge was a straightforward exercise in threat intelligence analysis. It required recognizing the IP address as an indicator of compromise and leveraging external threat intelligence resources to link it to a known APT group. The challenge highlights the importance of threat intelligence in cybersecurity investigations and how seemingly simple indicators can point to sophisticated threat actors.