CVE-2017-1000254 in macOS
Summary
by MITRE
libcurl may read outside of a heap allocated buffer when doing FTP. When libcurl connects to an FTP server and successfully logs in (anonymous or not), it asks the server for the current directory with the `PWD` command. The server then responds with a 257 response containing the path, inside double quotes. The returned path name is then kept by libcurl for subsequent uses. Due to a flaw in the string parser for this directory name, a directory name passed like this but without a closing double quote would lead to libcurl not adding a trailing NUL byte to the buffer holding the name. When libcurl would then later access the string, it could read beyond the allocated heap buffer and crash or wrongly access data beyond the buffer, thinking it was part of the path. A malicious server could abuse this fact and effectively prevent libcurl-based clients to work with it - the PWD command is always issued on new FTP connections and the mistake has a high chance of causing a segfault. The simple fact that this has issue remained undiscovered for this long could suggest that malformed PWD responses are rare in benign servers. We are not aware of any exploit of this flaw. This bug was introduced in commit [415d2e7cb7](https://github.com/curl/curl/commit/415d2e7cb7), March 2005. In libcurl version 7.56.0, the parser always zero terminates the string but also rejects it if not terminated properly with a final double quote.
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Analysis
by VulDB Data Team • 01/26/2021
The vulnerability described in CVE-2017-1000254 represents a classic buffer overread flaw in the libcurl library's FTP implementation that has significant implications for network security and application stability. This issue specifically affects how libcurl processes the response from FTP servers when executing the PWD (Print Working Directory) command, which is automatically issued upon successful FTP connection. The vulnerability stems from inadequate string parsing logic that fails to properly null-terminate buffer allocations when processing directory paths returned by FTP servers. The flaw was introduced in March 2005 and remained undetected for over a decade, suggesting that malformed FTP server responses are uncommon in legitimate implementations but potentially devastating when they occur.
The technical mechanism behind this vulnerability involves a buffer overread condition that occurs when the FTP server responds with a 257 status code containing a directory path enclosed in double quotes. When the server fails to provide a closing double quote character, libcurl's string parser does not properly terminate the buffer with a null byte, creating a situation where subsequent string operations may access memory beyond the allocated heap buffer boundaries. This memory corruption can manifest as segmentation faults, application crashes, or potentially more severe consequences if the memory access patterns align with exploitable conditions. The vulnerability specifically targets the PWD command execution path, making it particularly dangerous as this command is automatically invoked on every new FTP connection, ensuring a high probability of triggering the flaw during normal operation.
From an operational standpoint, this vulnerability creates a reliable denial-of-service condition that can completely prevent libcurl-based applications from functioning with malicious FTP servers. The attack vector requires only that an attacker control an FTP server to send a malformed PWD response without proper closing quotes, making it an accessible attack method that doesn't require sophisticated exploitation techniques. The impact extends beyond simple service disruption as any application relying on libcurl for FTP operations becomes vulnerable to this condition, affecting web browsers, file transfer utilities, and network applications that depend on the library's FTP capabilities. The vulnerability's long detection latency suggests that most legitimate FTP servers properly format their responses, but this reliability makes the flaw more dangerous when encountered in practice.
The fix implemented in libcurl version 7.56.0 addressed the core issue by enforcing proper null termination of strings while also adding validation to reject malformed PWD responses that lack proper closing quotes. This dual approach ensures both memory safety and protocol compliance, preventing the buffer overread condition while maintaining robust error handling for edge cases. The vulnerability aligns with CWE-121, heap-based buffer overflow, and represents a classic example of inadequate input validation in network protocol implementations. From an ATT&CK perspective, this vulnerability maps to T1190, Exploit Public-Facing Application, as it affects widely deployed network libraries that applications rely upon, and T1499, Endpoint Denial of Service, due to its ability to cause application crashes and service disruption. Organizations should prioritize updating their libcurl installations to version 7.56.0 or later to mitigate this vulnerability and ensure continued stable operation of FTP-dependent applications.