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Executive Summary

Researchers observed evidence of a sophisticated espionage operation linked to Midnight Blizzard, detailing how the threat group utilized Anthropic's Claude AI capabilities to systematically modify and redeploy flagged malware implants. Initial findings from ReversingLabs (RL) predated public disclosures by both Microsoft and Anthropic; RL first tracked samples on July 6 and July 10, 2026. The campaign involved a PowerShell stager and a Go backdoor designed for foothold establishment and persistence. While initial vendor reports provided core indicators like hashes and domains, RL's deep analysis uncovered additional intelligence, including specific lure files, server interactions, and unique beaconing labels not present in the public appendices. Attribution remains complex, with RL designating the cluster as "Midnight Blizzard-linked" rather than confirming one specific attribution, while demonstrating that subtle behavioral differences—such as those introduced during rebuilding attempts—allowed for tracing multiple distinct malware builds tied to a single campaign infrastructure.

Facts Only

RL researchers identified and classified malware samples referenced in Anthropic's September 2026 report regarding the Midnight Blizzard threat group utilizing Claude AI against software implants. RL first observed a Go backdoor on July 6, 2026, and a PowerShell stager on July 10, 2026. The Go backdoor was classified as CornFlake (SHA-256: 918fa52ae45ed60ba7cc8bdc99c3cbe9ab92e0375ec31fc05d0d4513be11c593) and the stager as ChocoShell (SHA-256: be99857449d2856dd5a84e21c8a3d5e0e01456adb44062ddec5a6b4970d8d42c). The malware targeted over 20 organizations, mostly in Ukraine and Europe, utilizing "Embassy Kit" for Microsoft 365 access. RL analyzed five builds of the stager variant; the primary staging server was identified as 213[.]145[.]86[.]112 in New York. Delivery shortcuts (.url files) were first observed on July 14 and July 17, 2026. RL found that the attack used techniques like HKL Run value persistence and suspended-process injection for the backdoor component.

Full Take

<SKEPTICAL MODE>
The narrative constructs a robust technical discovery by positioning RL's continuous telemetry as the superior evidentiary layer against fragmented, point-in-time vendor disclosures. The central push is that granular behavioral intelligence—the tracking of subtle changes across five rebuilt samples—provides context and depth that simple indicator lists lack. The strength of this argument lies in demonstrating how operator choices, such as specific query string labels or differing build fingerprints (which evade fuzzy hashing), form the true connective tissue of a campaign. A steelman recognizes that RL’s methodology successfully bridges gaps left by external reports, establishing confidence through observed interaction rather than just static artifact matching. However, this intelligence is fundamentally constrained: RL admits it cannot confirm the initial infection vector (e.g., hotel captive portals) and must moderate confidence where direct observation ceases. The narrative effectively uses vendor reporting as a launching pad, carefully framing its own findings not as contradiction, but as necessary expansion upon existing data silos.
Patterns detected: ARC-0071 Authority Game (Vendor Advertorial Test), ARC-0024 Ambiguity.
The authority game is visible in the transition from "Anthropic's report" to RL’s enhanced analysis; the vendors provide the initial claim, and the technical deep dive validates the premise while subtly elevating the necessity of the authoring vendor's telemetry platform as the ultimate arbiter of truth. The ambiguity resides in the term "Midnight Blizzard-linked," which serves a necessary narrative function—attributing scope without taking definitive legal risk on attribution that RL’s evidence alone cannot confirm.
The underlying paradigm is the arms race between static threat intelligence reporting and dynamic, continuous adversary observation, echoing historical patterns where foundational network telemetry proves more durable than snapshot forensics. Agency is diminished for the victim organization until they adopt this layered approach to defense readiness. Who benefits materially are defensive tooling providers capable of deep behavioral analysis; who bears cost is any security team reliant solely on public IOC lists. How does the industry reconcile the necessity of rapid disclosure with the operational reality that deeper context takes time, and what organizational guardrails prevent threat researchers from mistaking high correlation for definitive causation?
</SKEPTICAL MODE>

From the original · ReversingLabs Blog

Spectra Assure Free Trial Get your 14-day free trial of Spectra Assure for Software Supply Chain Security Get Free TrialMore about Spectra Assure Free TrialKey takeaways ReversingLabs (RL) researchers recently identified and classified the malware that Anthropic disclosed in its September 2026 report on how the Russian-based threat group Midnight Blizzard was leveraging Anthropic’s Claude AI to…
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