CVE-2020-3299 in Integrated Services Router
Summary
by MITRE • 10/22/2020
Multiple Cisco products are affected by a vulnerability in the Snort detection engine that could allow an unauthenticated, remote attacker to bypass a configured File Policy for HTTP. The vulnerability is due to incorrect detection of modified HTTP packets used in chunked responses. An attacker could exploit this vulnerability by sending crafted HTTP packets through an affected device. A successful exploit could allow the attacker to bypass a configured File Policy for HTTP packets and deliver a malicious payload.
Be aware that VulDB is the high quality source for vulnerability data.
Analysis
by VulDB Data Team • 08/12/2026
The vulnerability identified as CVE-2020-3299 represents a critical security flaw within Cisco's Snort detection engine implementation across multiple network security products. This weakness specifically targets the HTTP file policy enforcement mechanisms that are designed to prevent unauthorized content delivery through network traffic inspection. The vulnerability stems from an insufficient parsing mechanism within the Snort engine that fails to properly analyze modified HTTP packets during chunked transfer encoding processes, creating a pathway for malicious actors to circumvent established security controls.
The technical root cause of this vulnerability lies in the improper handling of HTTP chunked transfer encoding where the Snort engine does not correctly validate or process modified HTTP packets that contain altered chunk boundaries or content. This flaw enables attackers to manipulate HTTP traffic in a way that bypasses the normal file policy enforcement mechanisms, effectively allowing malicious payloads to be delivered through HTTP connections without triggering security alerts. The vulnerability manifests when the detection engine fails to properly reconstruct or analyze chunked HTTP responses, leading to incorrect classification of potentially harmful content.
From an operational perspective, this vulnerability presents a significant risk to organizations relying on Cisco's network security solutions for HTTP traffic inspection and content filtering. An unauthenticated remote attacker can exploit this weakness without requiring any privileged access or credentials, making it particularly dangerous in environments where network security devices are deployed to enforce strict content policies. The successful exploitation allows attackers to deliver malicious payloads through HTTP connections that would normally be blocked by configured file policies, potentially enabling data exfiltration, malware delivery, or other malicious activities.
The impact of this vulnerability extends beyond simple bypass of content filtering mechanisms, as it fundamentally undermines the trust model that security administrators place in their network inspection systems. Organizations using affected Cisco products may experience unauthorized access to their networks through HTTP traffic that appears benign but contains malicious content. The vulnerability aligns with CWE-129, which addresses issues related to improper validation of input boundaries, and maps to ATT&CK technique T1071.004 for application layer protocol usage. Security professionals must consider this vulnerability as part of their broader threat modeling efforts, particularly in environments where HTTP traffic filtering is critical for maintaining security posture.
Mitigation strategies should include immediate deployment of Cisco's security patches and updates that address the specific Snort engine parsing issues. Network administrators should also implement additional monitoring and logging mechanisms to detect anomalous HTTP traffic patterns that might indicate exploitation attempts. Organizations should review their existing HTTP file policies to ensure they provide adequate protection against modified chunked transfer encoding attacks, while also considering network segmentation and additional security controls such as web application firewalls or deep packet inspection systems that can provide layered protection against this specific attack vector.