CVE-2020-1044 Overview
A security feature bypass vulnerability exists in Microsoft SQL Server Reporting Services (SSRS) when the server improperly validates attachments uploaded to reports. This improper input validation flaw allows an authenticated attacker to bypass file type restrictions by sending specially crafted requests to the affected SSRS server.
An attacker who successfully exploits this vulnerability could upload file types that were previously disallowed by an administrator, potentially enabling further attacks such as malicious file distribution or server-side execution depending on the uploaded content.
Critical Impact
Authenticated attackers can bypass file upload restrictions in SSRS, allowing the upload of potentially malicious file types that administrators have explicitly blocked.
Affected Products
- Microsoft SQL Server Reporting Services 2017
- Microsoft SQL Server Reporting Services 2019
Discovery Timeline
- 2020-09-11 - CVE-2020-1044 published to NVD
- 2026-02-23 - Last updated in NVD database
Technical Details for CVE-2020-1044
Vulnerability Analysis
This vulnerability stems from insufficient validation of file attachments during the upload process in SQL Server Reporting Services. The SSRS component fails to properly enforce file type restrictions configured by administrators, creating a gap between intended security policy and actual enforcement.
The attack requires authentication, meaning an attacker needs valid credentials to access the SSRS instance. However, once authenticated, even low-privileged users can exploit this flaw to upload file types that should be restricted. This bypasses a critical security control designed to prevent malicious or potentially dangerous files from being stored on the reporting server.
The vulnerability has high impact on system integrity since it allows attackers to circumvent administrative security configurations. While no direct confidentiality or availability impact is documented, the ability to upload arbitrary file types could serve as a stepping stone for more sophisticated attacks.
Root Cause
The root cause is categorized as CWE-20 (Improper Input Validation). SQL Server Reporting Services fails to adequately validate the file type and content of attachments during the upload process. The server relies on insufficient checks that can be bypassed through specially crafted requests, allowing attackers to circumvent file type restrictions set by administrators.
Attack Vector
The attack is network-accessible and requires the following conditions:
- Authentication Required: The attacker must have valid credentials to access the SSRS server
- Network Access: The attacker needs network connectivity to the SSRS instance
- Crafted Request: A specially formatted upload request must be constructed to bypass validation
The attack exploits the disconnect between the file type restriction policy configured by administrators and the actual validation performed during file upload operations. By manipulating request parameters or file metadata, an attacker can trick the server into accepting file types that should be blocked.
For technical details on the exploitation mechanism, refer to the Microsoft Security Advisory CVE-2020-1044.
Detection Methods for CVE-2020-1044
Indicators of Compromise
- Unusual file types appearing in SSRS report attachments that should be blocked by policy
- Upload events in SSRS logs containing file extensions that do not match configured allowed types
- Suspicious authentication patterns followed by attachment upload activity
Detection Strategies
- Monitor SSRS application logs for file upload events and compare against allowed file type policies
- Implement file system monitoring on SSRS attachment storage directories to detect unexpected file types
- Configure alerts for any discrepancy between configured upload restrictions and actual uploaded files
- Review authentication logs for unusual patterns preceding file upload activities
Monitoring Recommendations
- Enable detailed logging for SSRS upload operations and attachment handling
- Deploy file integrity monitoring on directories used for SSRS report attachments
- Implement SIEM rules to correlate authentication events with subsequent upload activity
- Regularly audit SSRS file type restriction configurations against actual file inventory
How to Mitigate CVE-2020-1044
Immediate Actions Required
- Apply the Microsoft security update that addresses this vulnerability immediately
- Review and audit all existing file attachments in SSRS for unauthorized or suspicious file types
- Restrict SSRS access to only essential authenticated users until patching is complete
- Monitor SSRS upload activity closely for any exploitation attempts
Patch Information
Microsoft has released a security update that addresses this vulnerability by modifying how SSRS validates attachment uploads. The patch implements improved validation logic to properly enforce file type restrictions configured by administrators.
For detailed patch information and download links, see the Microsoft Security Advisory CVE-2020-1044.
Ensure that both SQL Server Reporting Services 2017 and 2019 installations are updated to the latest available security patches.
Workarounds
- Implement network-level access controls to limit which users can reach the SSRS server
- Consider temporarily disabling file attachment functionality if not business-critical
- Deploy web application firewall rules to inspect and block suspicious upload requests
- Restrict SSRS service account permissions to minimize potential impact
# Review SSRS allowed file types configuration
# Verify current upload restrictions are properly configured
# Check ReportServer configuration for AllowedResourceExtensionsForUpload setting
Get-ItemProperty "HKLM:\SOFTWARE\Microsoft\Microsoft SQL Server Reporting Services" -Name "AllowedResourceExtensionsForUpload" -ErrorAction SilentlyContinue
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
