CVE-2017-9502 in cURL
Summary
by MITRE
In curl before 7.54.1 on Windows and DOS, libcurl's default protocol function, which is the logic that allows an application to set which protocol libcurl should attempt to use when given a URL without a scheme part, had a flaw that could lead to it overwriting a heap based memory buffer with seven bytes. If the default protocol is specified to be FILE or a file: URL lacks two slashes, the given "URL" starts with a drive letter, and libcurl is built for Windows or DOS, then libcurl would copy the path 7 bytes off, so that the end of the given path would write beyond the malloc buffer (7 bytes being the length in bytes of the ascii string "file://").
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Analysis
by VulDB Data Team • 10/12/2025
The vulnerability described in CVE-2017-9502 represents a classic heap buffer overflow condition that affects libcurl versions prior to 7.54.1 on Windows and DOS systems. This flaw resides in the default protocol handling mechanism of libcurl, which is responsible for determining which protocol to use when a URL lacks a scheme identifier. The vulnerability specifically manifests when libcurl processes file paths that begin with a drive letter, such as "C:" or "D:", and the application has configured the default protocol to be FILE or when processing file: URLs that lack the standard two forward slashes after the scheme. The technical implementation error occurs because libcurl's internal logic incorrectly calculates memory boundaries when copying path data, leading to a buffer overflow scenario.
The operational impact of this vulnerability extends beyond simple memory corruption, as it creates opportunities for remote code execution or denial of service attacks depending on the system configuration and how the vulnerable application handles memory corruption. When a malicious user provides a specially crafted URL that meets the specified conditions, the heap buffer overflow can overwrite adjacent memory regions, potentially corrupting heap metadata or other critical application data structures. This type of vulnerability aligns with CWE-121, which describes heap-based buffer overflow conditions, and represents a significant concern for applications that process untrusted URL inputs. The vulnerability is particularly dangerous in web applications or services that use libcurl as their underlying HTTP/FTP client library and accept user-provided URLs without proper validation.
The attack vector for this vulnerability is primarily through URL handling in applications that utilize libcurl, particularly those that process file paths without explicit protocol schemes. The flaw requires specific conditions to be met: the URL must start with a drive letter, the default protocol must be configured to FILE, or the file: URL must lack the standard two slashes, and the library must be compiled for Windows or DOS environments. This makes the vulnerability somewhat specific but still impactful in environments where applications process untrusted input through libcurl's URL parsing functions. Security researchers have classified this under ATT&CK technique T1203, which covers exploitation for execution through the manipulation of memory addresses and buffer overflow conditions.
Mitigation strategies for CVE-2017-9502 include upgrading to libcurl version 7.54.1 or later, which contains the necessary patches to address the buffer overflow condition. Organizations should also implement input validation measures that prevent malformed URLs from reaching the vulnerable libcurl functions, particularly in web applications that accept user-provided URLs. Additionally, application developers should consider implementing proper memory protection mechanisms such as stack canaries or address space layout randomization to reduce the effectiveness of potential exploitation attempts. The fix implemented in version 7.54.1 specifically addresses the memory boundary calculation error in libcurl's protocol handling logic, ensuring that path data is copied within proper buffer limits regardless of the input format. System administrators should also monitor for applications that depend on older versions of libcurl and ensure they are updated to prevent exploitation of this vulnerability in their environments.