CVE-2016-8622 in macOS
Summary
by MITRE
The URL percent-encoding decode function in libcurl before 7.51.0 is called `curl_easy_unescape`. Internally, even if this function would be made to allocate a unscape destination buffer larger than 2GB, it would return that new length in a signed 32 bit integer variable, thus the length would get either just truncated or both truncated and turned negative. That could then lead to libcurl writing outside of its heap based buffer.
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Analysis
by VulDB Data Team • 10/07/2022
The vulnerability identified as CVE-2016-8622 resides within the libcurl library's URL percent-encoding decode function known as curl_easy_unescape. This flaw affects versions prior to 7.51.0 and represents a critical buffer overflow condition that can be exploited to cause arbitrary code execution or denial of service. The vulnerability stems from improper handling of buffer length calculations during the unescaping process, creating a scenario where memory boundaries can be violated.
The technical root cause involves a fundamental flaw in how the function manages memory allocation and length reporting. When curl_easy_unescape processes URL-encoded strings, it internally allocates a destination buffer to hold the decoded output. However, the function's implementation suffers from a critical design error where the calculated buffer length is stored in a signed 32-bit integer variable. This limitation becomes problematic when dealing with large input strings that would require buffer sizes exceeding 2GB, as the signed 32-bit integer cannot properly represent such large values.
The operational impact of this vulnerability manifests when an attacker provides specially crafted input to the curl_easy_unescape function. The function's internal logic processes the input through a series of operations that can result in the calculated length being either truncated to fit within 32-bit limits or converted to a negative value. When the function attempts to write data beyond the allocated buffer boundaries, it creates a heap-based buffer overflow condition. This condition can be exploited to overwrite adjacent memory locations, potentially leading to code execution control flow manipulation.
This vulnerability maps directly to CWE-122, which describes "Heap-based Buffer Overflow," and specifically aligns with the ATT&CK technique T1059.007 for "Command and Scripting Interpreter: PowerShell" when considering the potential for privilege escalation through memory corruption. The flaw demonstrates poor input validation and memory management practices that violate secure coding principles. The vulnerability affects systems that rely on libcurl for HTTP operations, making it particularly dangerous in web applications, network services, and any software that processes user-provided URLs or encoded data.
The mitigation strategy requires immediate upgrade to libcurl version 7.51.0 or later, which contains the necessary patches to properly handle buffer length calculations. Additionally, organizations should implement input validation measures that limit the size of URL-encoded strings processed by curl_easy_unescape, and consider deploying runtime protections such as address space layout randomization and stack canaries. Regular security audits of third-party libraries and maintaining up-to-date software dependencies remain critical defensive measures against similar vulnerabilities in the broader software ecosystem.