CVE-2020-15689 in Appweb
Summary
by MITRE
Appweb before 7.2.2 and 8.x before 8.1.0, when built with CGI support, mishandles an HTTP request with a Range header that lacks an exact range. This may result in a NULL pointer dereference and cause a denial of service.
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Analysis
by VulDB Data Team • 10/29/2020
The vulnerability identified as CVE-2020-15689 represents a critical denial of service weakness in the Appweb web server software that affects versions prior to 7.2.2 and 8.x prior to 8.1.0. This flaw specifically manifests when the web server is compiled with CGI support and encounters HTTP requests containing Range headers without explicit range specifications. The issue stems from improper handling of these particular HTTP request patterns, creating a scenario where the application fails to validate the completeness of range parameters before attempting to process them.
The technical root cause of this vulnerability lies in the absence of proper input validation for Range header parameters within the CGI processing module of Appweb. When a client sends an HTTP request with a Range header that does not contain an exact range specification, the application's handling mechanism fails to properly validate this input before attempting to dereference a pointer that remains uninitialized or null. This NULL pointer dereference occurs during the processing of the CGI request, leading to an application crash or termination. The vulnerability specifically affects the HTTP range request handling mechanism, which is commonly used for partial content retrieval and resumable downloads.
From an operational impact perspective, this vulnerability enables an attacker to perform a denial of service attack against systems running vulnerable versions of Appweb by simply sending a crafted HTTP request containing a malformed Range header. The attack requires minimal resources and can be executed remotely without authentication, making it particularly dangerous in production environments where continuous availability is critical. The service disruption occurs immediately upon receiving the malicious request, potentially affecting all users of the web server and causing significant operational downtime. This vulnerability directly aligns with attack techniques described in the MITRE ATT&CK framework under the service disruption category, specifically targeting the availability aspect of the CIA triad.
The security implications extend beyond simple service interruption as this vulnerability demonstrates poor error handling and input validation practices within the application's core HTTP processing logic. The flaw represents a classic example of improper handling of exceptional conditions, which is categorized under CWE-476 in the CWE database as NULL Pointer Dereference. This weakness creates a predictable crash condition that attackers can reliably exploit to bring down web services. Organizations using Appweb with CGI support must recognize that this vulnerability can be leveraged in automated attack scenarios, potentially leading to widespread service disruption across multiple applications hosted on affected servers.
Mitigation strategies for CVE-2020-15689 primarily involve upgrading to Appweb versions 7.2.2 or 8.1.0, which contain the necessary patches to properly handle Range header parameters. System administrators should also implement network-level protections such as web application firewalls that can detect and block malformed Range header requests before they reach the vulnerable application. Additionally, monitoring systems should be configured to detect unusual patterns of service interruptions that may indicate exploitation attempts. Organizations should conduct comprehensive vulnerability assessments to identify all systems running affected Appweb versions and prioritize remediation efforts accordingly. The patching process should be carefully managed to ensure that CGI support functionality remains intact while addressing the specific NULL pointer dereference issue.