CVE-2020-0787: Microsoft Windows Background Intelligent Transfer Service (BITS) Improper Privilege Management Vulnerability

CVE-2020-0787 is a privilege-elevation vulnerability in the Background Intelligent Transfer Service (BITS) on Microsoft Windows. BITS improperly handles symbolic links, which can allow an attacker who already has some access on a system to run code with SYSTEM-level privileges. This matters because elevation to SYSTEM is a common step after initial access, and the vulnerability has been used in ransomware activity. Defenders should treat it as a high-priority local privilege issue on Windows hosts and confirm exact scope and fixes against the vendor advisory.

How it works

The weakness combines improper privilege management (CWE-269) with issues around symbolic link handling (CWE-59). BITS is a Windows service used for background file transfers. When it does not correctly validate or constrain how symbolic links are resolved in its operations, a local attacker can influence paths or objects the service touches so that privileged actions occur in a context the attacker controls.

In practical terms, an actor who can already run code or place files on the host abuses the link-handling flaw so that BITS performs work that results in arbitrary code running as SYSTEM. Public detail on exact exploit mechanics is limited here; treat this as a local elevation path that turns limited access into full system control. Confirm technical specifics only from the Microsoft advisory for this CVE.

Am I affected? How to find it in your systems

BITS ships as part of Microsoft Windows and is commonly present on workstations and servers. Any Windows system that has not received the vendor update addressing CVE-2020-0787 should be considered potentially affected until proven otherwise. Inventory should focus on Windows endpoints and servers, especially those where local user or malware footholds are realistic.

• Build an inventory of Windows hosts (domain-joined and standalone) via your asset management, CMDB, or endpoint management tools, and note OS build and patch level.
• Confirm whether the security update that remediates CVE-2020-0787 is installed; match KB or build information only against the official Microsoft advisory—do not rely on guessed version ranges.
• Check that the BITS service exists and its state where relevant; absence of a particular feature does not automatically mean the host is out of scope without vendor confirmation.
• For exploitation signs, review endpoint detection and Windows security/application logs for unexpected BITS job activity, unusual process creations spawning from or interacting with bits-related components, sudden privilege changes to SYSTEM, and ransomware-related behaviors following local elevation. Correlate with EDR alerts for privilege escalation and symbolic-link or path-redirection abuse.

If telemetry is sparse, prioritize unpatched systems that allow local code execution by standard users or that are exposed to untrusted workloads.

How to remediate

Patch first. Apply the updates Microsoft released for this vulnerability exactly as described in the vendor advisory and per CISA’s required action: apply updates per vendor instructions. Use your standard patch pipeline (WSUS, Microsoft Update, ConfigMgr, Intune, or equivalent), verify installation, and reboot if the advisory requires it.

• After patching, validate that the expected update is present on a sample of hosts and that BITS continues to function for legitimate business use.
• Harden generally for this class: limit who can create symbolic links or write to locations BITS may use; enforce least privilege so standard users cannot easily stage elevation attempts; keep EDR and attack-surface reduction rules enabled for privilege-escalation techniques.
• Remove standing local admin rights where possible and monitor for abuse of transfer and background services.

Do not assume partial workarounds replace the official fix; confirm residual risk only against Microsoft’s guidance.

If you can't patch immediately

Reduce likelihood and impact until the vendor update can be deployed.

• Segment and restrict: isolate high-value or high-risk Windows hosts; limit lateral movement paths so a local elevation on one machine does not easily reach domain admins or critical servers.
• Tighten local privileges: remove unnecessary local administrator rights; restrict interactive logon and code-execution opportunities for low-privilege accounts.
• Monitor aggressively: alert on BITS job anomalies, unexpected SYSTEM process trees, symlink creation in sensitive paths, and post-elevation behaviors associated with ransomware. Ensure EDR coverage is healthy on unpatched hosts.
• Where policy allows, constrain or carefully control use of features that depend on BITS for non-essential workloads, and apply any vendor-supported temporary mitigations only after confirming them in the advisory—do not invent registry or service disablement steps.
• Virtual patching at the host or network layer has limited effect on a local privilege issue, so focus on containment, least privilege, and detection rather than perimeter WAF rules alone.

Schedule the official update as soon as operationally possible; compensating controls do not eliminate the underlying flaw.

If your data may have been exposed

Actively exploited elevation vulnerabilities, including those with known ransomware use, frequently appear in breach chains after an initial foothold. If you suspect compromise, isolate affected hosts, preserve forensic evidence, rotate credentials that may have been accessible to SYSTEM-level processes, and follow your incident-response plan. As a further check for personal or workforce exposure in known breach datasets, you can run a free exposure scan of your email addresses against published breach data and monitor for follow-on abuse.