huntrule

Proxy User-Agent Matching APT40 Dropbox Tool on api.dropbox.com

Alerts on proxy requests using a fixed Chrome 36 user-agent to api.dropbox.com.

FreeReviewedSigma · High · v5
Category
proxy
Author
Thomas Patzke (SigmaHQ), DRL 1.1
Published
2019-11-12
Updated
2026-07-31
AnalysisDetection

ATT&CK techniques

C2 → Exfiltration

  1. Recon

  2. Resource Dev

  3. Initial Access

  4. Execution

  5. Persistence

  6. Priv Esc

  7. Defense Evasion

  8. Cred Access

  9. Discovery

  10. Lateral Movement

  11. Collection

  12. C2

  13. Exfiltration

  14. Impact

What it detects

This rule flags proxy traffic where the User-Agent exactly matches a specific Windows/Chrome 36 string while the destination host is api.dropbox.com. Such mismatched or overly specific tool-like User-Agents can indicate command-and-control or data-exfiltration activity disguised as legitimate Dropbox access. It relies on proxy telemetry capturing destination host and the full client User-Agent string.

Reporting behind it

Related detections9 linkedT1071.001 — drag to rearrange
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Malicious HarborWatch RAT Command and Control Beacon by User Agent (via proxy)
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Malicious BadIIS C2 Communication via lwxatisme User-Agent
Possible PS1Bot C2 Beacon With Drive Serial URI Pattern (via proxy)
Suspicious WarmCookie C2 Beacon With Fixed Firefox User-Agent
Proxy User-Agent Matching APT40 Dropbox Tool on api.dropbox.com
Pivot detection · T1071.001 · 9 related

Changelog

v5
  1. v5
    Candidate ingested via manual entry.2026-07-31
  2. v4
    Candidate ingested via manual entry.2026-07-31
  3. v3
    Candidate ingested via manual entry.2026-07-31
  4. v2
    Candidate ingested via manual entry.2026-07-31
  5. v1
    No changelog recorded for this version.2026-07-30

Detection content is published as a reviewed draft. Tune thresholds and exclusions against your own telemetry before enabling this rule for alerting.