CVE-2019-19945 in OpenWrt
Summary
by MITRE
uhttpd in OpenWrt through 18.06.5 and 19.x through 19.07.0-rc2 has an integer signedness error. This leads to out-of-bounds access to a heap buffer and a subsequent crash. It can be triggered with an HTTP POST request to a CGI script, specifying both "Transfer-Encoding: chunked" and a large negative Content-Length value.
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Analysis
by VulDB Data Team • 03/17/2020
The vulnerability CVE-2019-19945 represents a critical integer signedness error in uhttpd, the HTTP server component of OpenWrt operating systems. This flaw exists within the handling of HTTP request parsing logic where the server fails to properly validate the signedness of integer values during chunked transfer encoding processing. The vulnerability specifically affects OpenWrt versions through 18.06.5 and 19.x versions up to 19.07.0-rc2, making it a widespread issue across multiple release branches of this embedded operating system. The integer signedness error occurs when the HTTP server processes requests containing both "Transfer-Encoding: chunked" header and a large negative Content-Length value, creating a scenario where signed integer arithmetic produces unexpected behavior during buffer allocation calculations.
The technical exploitation of this vulnerability stems from improper input validation within the uhttpd HTTP server implementation. When processing an HTTP POST request with chunked transfer encoding and a large negative Content-Length value, the server performs integer arithmetic operations that fail to account for signedness differences in the underlying data type handling. This miscalculation results in heap buffer allocation that exceeds the intended boundaries, leading to out-of-bounds memory access patterns. The vulnerability manifests as a heap-based buffer overflow condition that ultimately causes the uhttpd process to crash, resulting in a denial of service condition for the affected embedded device. The flaw is classified under CWE-191 as an Integer Underflow (Wrap or Wraparound) which directly maps to the improper handling of signed integer values in memory allocation calculations.
From an operational perspective, this vulnerability presents significant security implications for embedded network devices running OpenWrt systems. The ability to trigger a denial of service condition through a simple HTTP POST request with specific headers makes this vulnerability particularly dangerous in environments where network accessibility is not restricted. Attackers can exploit this flaw to disrupt network services, potentially causing network outages or making devices unavailable for legitimate users. The vulnerability is particularly concerning in IoT deployments where device availability is critical, as it can be leveraged to create persistent denial of service conditions that may require physical device access or power cycling to resolve. The attack vector requires minimal privileges and can be executed remotely, making it a high-impact vulnerability for network infrastructure devices.
Mitigation strategies for CVE-2019-19945 should prioritize immediate firmware updates to patched versions of OpenWrt where the integer signedness error has been corrected. Network administrators should implement firewall rules to restrict HTTP access to critical devices when possible, and monitor for unusual HTTP traffic patterns that might indicate exploitation attempts. The vulnerability can be addressed through proper input validation of Content-Length headers and ensuring that chunked transfer encoding processing properly handles signedness considerations in integer arithmetic operations. Additionally, implementing network segmentation and access controls can limit the potential impact of exploitation attempts. Organizations should also consider deploying intrusion detection systems that can identify and alert on HTTP requests containing the specific combination of headers that trigger this vulnerability, as outlined in the attack patterns associated with similar buffer overflow conditions in web server implementations.