Windows Background Intelligent Transfer Service (BITS) Symbolic Link Local Privilege Escalation Vulnerability (CVE-2020-0787) Technical Advisory
Executive & SecOps Summary
SECOPS ASSESSMENTWindows 7, 8.1, 10, and Windows Server 2008, 2012, 2016, 2019 utilizing the BITS service
A local authenticated attacker can exploit improper symbolic link handling to overwrite arbitrary system files and execute code with NT AUTHORITY\SYSTEM privileges
Apply the official Microsoft security updates immediately, or temporarily disable the BITS service and restrict symbolic link creation privileges for non-admin users
Affected Products & Version Scope
Remediation & Live Workaround Commands
PATCH & WORKAROUND1. Official Patch & Version Upgrade Commands
Applying the official cumulative security updates from Microsoft is the primary remediation path. Use the following PowerShell commands to verify patch status or force Windows Update to retrieve the latest security patches.
# Check if the required security update (KB) is installed on the system
Get-HotFix | Where-Object { $_.HotFixID -match 'KB4540673' -or $_.HotFixID -match 'KB4540674' -or $_.HotFixID -match 'KB4541335' }
# Force Windows Update to download and install missing security patches
Install-Module PSWindowsUpdate -Force -SkipPublisherCheck
Get-WindowsUpdate -KBArticleID 'KB4540673', 'KB4540674' -Install -AcceptAll -AutoReboot2. Temporary Workaround & Hardening Configuration
If immediate patching is not feasible, mitigate the risk by disabling the BITS service or restricting symbolic link creation privileges via Local Security Policy.
# Stop and disable the BITS service to prevent exploitation attempts
Stop-Service -Name 'BITS' -Force
Set-Service -Name 'BITS' -StartupType Disabled
# Audit and restrict 'Create symbolic links' privilege (SeCreateSymbolicLinkPrivilege) to Administrators only
# This can be configured via secpol.msc -> Local Policies -> User Rights Assignment3. Log Audit & IoC Verification Commands
Monitor BITS operational logs for unusual file transfer activities or errors that may indicate exploitation attempts.
# Query BITS Client Operational logs for job creation and modification events
Get-WinEvent -LogName 'Microsoft-Windows-Bits-Client/Operational' -ErrorAction SilentlyContinue |
Where-Object { $_.Id -eq 59 -or $_.Id -eq 60 } |
Select-Object TimeCreated, Id, Message |
Format-ListBlock Kernel Privilege Escalation & Ransomware Payloads Before You Can Reboot
Protect unpatched Windows, Linux, and macOS workloads against in-memory exploitation and lateral movement with autonomous EDR.
Technical Root Cause & Exploitation Mechanics
1. Vulnerability Overview & Root Cause
A local privilege escalation vulnerability exists in the Windows Background Intelligent Transfer Service (BITS) due to improper handling of symbolic links (CWE-59). BITS, which runs under the high-privilege 'NT AUTHORITY\SYSTEM' context, fails to properly validate file paths and directory junctions when moving temporary download files to their final destinations. This design flaw allows a low-privileged user to manipulate the file system structure during the file transfer process.
2. Exploitation Mechanics & Attack Scenario
To exploit this vulnerability, an attacker with local access creates a BITS transfer job pointing to a temporary directory where they have write permissions. Before BITS completes the transfer and renames/moves the temporary file, the attacker exploits a Time-of-Check to Time-of-Use (TOCTOU) race condition. By replacing the temporary directory or file with a symbolic link pointing to a protected system directory (such as 'C:\Windows\System32'), the attacker tricks the high-privileged BITS service into writing an arbitrary file (e.g., a malicious DLL) into the system directory. This file write can then be leveraged to achieve arbitrary code execution as SYSTEM via DLL hijacking or service binary replacement.
3. Infrastructure & Business Impact
CVE-2020-0787 is listed in CISA's Known Exploited Vulnerabilities (KEV) catalog, indicating active exploitation in the wild. With an EPSS score of 0.4252, this vulnerability is highly attractive to threat actors, including ransomware operators, who use it for local privilege escalation (LPE) after gaining initial access. Successful exploitation compromises the entire host, allowing attackers to dump credentials, establish persistence, and pivot laterally across the enterprise network.
Official Vendor Advisories & References
An elevation of privilege vulnerability exists when the Windows Background Intelligent Transfer Service (BITS) improperly handles symbolic links, aka 'Windows Background Intelligent Transfer Service Elevation of Privilege Vulnerability'.
Frequently Asked SecOps Questions (CVE-2020-0787)
FAQQ1. How critical is CVE-2020-0787 and is it actively exploited in the wild?
CVE-2020-0787 is rated CVSS 7.8 (HIGH) with a 30-day EPSS exploitation probability of 42.5%. It is listed in the CISA Known Exploited Vulnerabilities (KEV) catalog, requiring immediate emergency remediation.
Q2. Which vendors and products are affected by CVE-2020-0787?
Primary impacted vendors include microsoft across windows, windows_server. Scope summary: Windows 7, 8.1, 10, and Windows Server 2008, 2012, 2016, 2019
Q3. What temporary mitigations or workarounds exist for CVE-2020-0787 before patching?
[Mitigation] Apply the official Microsoft security updates immediately, or temporarily disable the BITS service and restrict symbolic link creation privileges for non-admin users Follow the CLI remediation commands and interactive SecOps checklist on this page to restrict exposure prior to your maintenance window.
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