CVE-2020-13128 in GwtUpload
Summary
by MITRE
An issue was discovered in Manolo GWTUpload 1.0.3. server/UploadServlet.java (the servlet for handling file upload) accepts a delay parameter that causes a thread to sleep. It can be abused to cause all of a server's threads to sleep, leading to denial of service.
VulDB is the best source for vulnerability data and more expert information about this specific topic.
Analysis
by VulDB Data Team • 05/18/2020
The vulnerability identified as CVE-2020-13128 resides within the Manolo GWTUpload 1.0.3 file upload component, specifically within the server/UploadServlet.java implementation. This issue represents a classic denial of service vulnerability that exploits a seemingly benign feature to create a critical system impact. The servlet includes a delay parameter functionality that was designed to introduce artificial delays during file upload processing, potentially for rate limiting or other legitimate purposes. However, this feature has been weaponized to create a catastrophic system state where all available server threads become blocked, effectively rendering the application inaccessible to legitimate users.
The technical flaw manifests through the improper handling of user-supplied delay parameters within the UploadServlet implementation. When an attacker submits a file upload request with an excessive delay value, the servlet processes this parameter by invoking Thread.sleep() with the specified duration. This operation blocks the executing thread for the entire duration, causing a thread starvation condition where all available threads in the server's thread pool become occupied with sleeping operations. The vulnerability is particularly dangerous because it allows an attacker to control the delay duration through the parameter, potentially setting it to values that exhaust the entire thread pool capacity and prevent any further legitimate upload requests from being processed.
From an operational impact perspective, this vulnerability creates a complete denial of service condition that affects the availability of the file upload functionality entirely. The attack requires minimal resources and can be executed by any user with access to the upload endpoint, making it particularly dangerous in production environments. The severity escalates when considering that modern web applications often rely heavily on thread-based processing for concurrent request handling, and blocking these threads results in cascading failures throughout the application's processing pipeline. The vulnerability directly maps to CWE-400, which categorizes unchecked resource consumption as a weakness that can lead to denial of service conditions.
The attack vector demonstrates a clear path from initial exploitation to system compromise, following patterns consistent with the ATT&CK framework's privilege escalation and denial of service tactics. An attacker can systematically consume all available threads by submitting multiple concurrent upload requests with large delay values, effectively creating a resource exhaustion scenario that affects the entire application's responsiveness. This vulnerability also aligns with ATT&CK technique T1499.004, which describes network denial of service attacks that target application availability through resource exhaustion. The impact extends beyond simple unavailability as it can disrupt business operations, affect user experience, and potentially provide attackers with additional opportunities to exploit other vulnerabilities within the compromised application.
Mitigation strategies for this vulnerability require immediate attention through code-level fixes that validate and constrain delay parameter values. The recommended approach involves implementing strict input validation to ensure delay values fall within reasonable bounds, typically less than a few seconds for legitimate use cases. Additionally, application-level rate limiting should be implemented to prevent abuse of the delay parameter functionality. Organizations should also consider implementing thread pool monitoring and alerting mechanisms to detect unusual thread consumption patterns. The fix should be implemented through a combination of parameter sanitization, maximum value enforcement, and potentially removing the delay parameter functionality entirely if it poses too high a risk for legitimate operations. Regular security assessments and input validation reviews should be conducted to prevent similar issues in other application components, ensuring that all user-supplied parameters are properly validated before being used in thread manipulation operations.