CVE-2016-6407 in Web Security Appliance
Summary
by MITRE
Cisco AsyncOS through 9.5.0-444 on Web Security Appliance (WSA) devices allows remote attackers to cause a denial of service (link saturation) by making many HTTP requests for overlapping byte ranges simultaneously, aka Bug ID CSCuz27219.
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Analysis
by VulDB Data Team • 09/19/2022
Cisco AsyncOS versions through 9.5.0-444 on Web Security Appliance devices contain a vulnerability that enables remote attackers to trigger a denial of service condition through link saturation. This flaw specifically manifests when adversaries submit numerous simultaneous HTTP requests targeting overlapping byte ranges within the same file or resource. The vulnerability operates by exploiting the system's handling of range requests, which are commonly used for resuming interrupted downloads or accessing specific portions of large files. When multiple overlapping range requests are processed concurrently, the WSA device becomes overwhelmed with processing overhead, leading to resource exhaustion and ultimately rendering the service unavailable to legitimate users. The attack vector requires no authentication and can be executed from any remote location, making it particularly dangerous in networked environments where WSA appliances serve as critical security infrastructure.
The technical implementation of this vulnerability stems from inadequate input validation and resource management within the HTTP range request processing module of the AsyncOS. When the system receives overlapping byte range requests, it fails to properly aggregate or deduplicate these requests, instead processing each individual request independently. This results in excessive CPU utilization and memory consumption as the system attempts to manage multiple concurrent operations on the same underlying data. The vulnerability specifically affects the WSA's handling of HTTP Range headers, which are part of the http protocol specification defined in rfc 7233. According to the CWE catalog, this represents a variant of CWE-400, which encompasses resource exhaustion vulnerabilities. The flaw demonstrates characteristics of a denial of service attack pattern where system resources are consumed in a manner that prevents legitimate operations from completing successfully, aligning with ATT&CK technique T1499.004 for network denial of service.
The operational impact of this vulnerability extends beyond simple service disruption, as it can severely compromise the availability of web security services within an organization. When exploited successfully, the vulnerability can cause complete loss of WSA appliance functionality, preventing legitimate web traffic from being inspected and filtered according to security policies. This creates a dangerous security gap where malicious traffic can bypass protections while legitimate users experience complete service interruption. The attack's low complexity and high impact make it particularly attractive to threat actors, especially those seeking to disrupt business operations or gain unauthorized access to network resources. Organizations relying on Cisco WSA appliances for web filtering, malware detection, and content control face significant risk if this vulnerability remains unpatched. The vulnerability affects not only the immediate availability of the appliance but also the broader network security posture, as administrators may be forced to implement emergency workarounds or temporary network segmentation.
Mitigation strategies for this vulnerability should include immediate deployment of Cisco's security patches and updates, as well as network-level monitoring to detect anomalous range request patterns. Organizations should implement rate limiting on HTTP range requests to prevent overwhelming the system with concurrent operations. Network administrators should also consider implementing intrusion detection systems that can identify and alert on suspicious overlapping range request behavior. The recommended approach involves configuring the WSA appliance to limit the number of concurrent range requests or to aggregate overlapping requests before processing. Additionally, organizations should review their current security policies to ensure that automated tools or services that might generate overlapping range requests are properly configured to avoid triggering the vulnerability. Regular security assessments and vulnerability scanning should include verification that the patched versions are properly deployed across all WSA appliances within the network infrastructure. The vulnerability's classification under CWE-400 and its alignment with ATT&CK technique T1499.004 emphasize the need for comprehensive network security monitoring and incident response procedures to detect and respond to such resource exhaustion attacks effectively.