CVE-2021-31807 in Web Proxy
Summary
by MITRE • 06/09/2021
An issue was discovered in Squid before 4.15 and 5.x before 5.0.6. An integer overflow problem allows a remote server to achieve Denial of Service when delivering responses to HTTP Range requests. The issue trigger is a header that can be expected to exist in HTTP traffic without any malicious intent.
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Analysis
by VulDB Data Team • 06/11/2021
The vulnerability identified as CVE-2021-31807 represents a critical integer overflow flaw within the Squid proxy server software affecting versions prior to 4.15 and 5.0.6. This issue resides in the HTTP Range request handling mechanism where the proxy server processes partial content requests from clients. The vulnerability manifests when a remote server sends specially crafted HTTP headers that trigger an integer overflow condition during the processing of range requests. The flaw is particularly concerning because it can be triggered by legitimate HTTP traffic patterns without requiring any malicious intent from the attacking party, making it a stealthy and potentially widespread threat. The integer overflow occurs within the memory allocation or buffer handling logic when processing HTTP Range headers, leading to unpredictable behavior in the proxy server's operation.
The technical implementation of this vulnerability involves the manipulation of HTTP Range request headers that specify byte ranges within a resource. When Squid receives such requests, it performs calculations to determine the appropriate response size and memory allocation for the partial content. The integer overflow occurs during these calculations, where the result exceeds the maximum value that can be represented by the integer data type being used. This overflow corrupts memory structures and can lead to memory corruption, application crashes, or potentially more severe consequences depending on how the overflow affects the program's execution flow. The vulnerability is classified under CWE-190 as an integer overflow or wraparound, which represents a well-known class of memory safety issues that have historically led to various security exploits.
The operational impact of CVE-2021-31807 extends beyond simple denial of service conditions as it can severely disrupt proxy server operations and potentially compromise the availability of services that depend on Squid for content delivery. Organizations using affected Squid versions may experience intermittent service disruptions, application crashes, and complete unavailability of proxy services during periods of high HTTP traffic. The vulnerability affects both HTTP and HTTPS proxy operations, making it particularly dangerous for enterprises that rely heavily on proxy infrastructure for web traffic management. Attackers can exploit this vulnerability by simply sending HTTP Range requests with specific header values that trigger the overflow condition, requiring no authentication or complex attack vectors. This makes the vulnerability particularly dangerous in environments where proxy servers are exposed to untrusted network traffic or where automated scanning tools might inadvertently trigger the condition.
Mitigation strategies for CVE-2021-31807 primarily focus on immediate software updates to versions 4.15 or 5.0.6 and later, which contain the necessary patches to address the integer overflow condition. Organizations should prioritize patching their Squid proxy servers as a high-priority security measure, particularly in environments where the proxy serves critical network traffic. Network administrators should also implement monitoring solutions to detect unusual patterns in HTTP Range request handling that might indicate exploitation attempts. Additionally, implementing rate limiting and traffic filtering rules that restrict the types of HTTP headers accepted by proxy servers can provide additional defense in depth. The vulnerability aligns with ATT&CK technique T1499.004 for network denial of service attacks, as it enables adversaries to disrupt services through resource exhaustion and memory corruption. Organizations should also consider implementing intrusion detection systems that can identify and alert on suspicious HTTP Range header patterns that may be indicative of exploitation attempts. Regular security assessments and vulnerability scanning should include verification that Squid installations are running patched versions to prevent exploitation of this and related integer overflow vulnerabilities.