CVE-2017-1000257 in cURL
Summary
by MITRE
An IMAP FETCH response line indicates the size of the returned data, in number of bytes. When that response says the data is zero bytes, libcurl would pass on that (non-existing) data with a pointer and the size (zero) to the deliver-data function. libcurl's deliver-data function treats zero as a magic number and invokes strlen() on the data to figure out the length. The strlen() is called on a heap based buffer that might not be zero terminated so libcurl might read beyond the end of it into whatever memory lies after (or just crash) and then deliver that to the application as if it was actually downloaded.
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Analysis
by VulDB Data Team • 04/16/2026
The vulnerability described in CVE-2017-1000257 represents a critical buffer overread condition within the libcurl library that occurs during IMAP protocol processing. This flaw exists in the handling of FETCH response lines where the library receives a response indicating zero bytes of data, yet still processes this empty data through the deliver-data function. The issue stems from libcurl's improper handling of zero-sized data segments, creating a scenario where the library attempts to calculate string length on potentially unbounded memory regions.
The technical implementation of this vulnerability involves the deliver-data function within libcurl treating zero as a special magic number that triggers strlen() operations on data pointers. When the IMAP FETCH response indicates zero bytes, libcurl passes both the data pointer and zero size to the delivery function, which then calls strlen() on what should be empty data. This operation becomes problematic because the underlying heap buffer may not be null-terminated, causing strlen() to read beyond the allocated memory boundaries until it encounters a null byte. This behavior creates an exploitable condition where arbitrary memory contents can be read and potentially delivered to applications, leading to information disclosure or system instability.
The operational impact of this vulnerability extends beyond simple memory corruption, as it can enable attackers to extract sensitive data from memory regions adjacent to the heap buffer. The flaw affects any application that utilizes libcurl for IMAP operations, making it particularly dangerous for email clients, web applications, and services that process IMAP responses. The vulnerability can result in denial of service conditions through crashes or more severe consequences through information disclosure, as the library might expose memory contents that should remain private. This issue demonstrates the importance of proper input validation and memory boundary checking in network protocol implementations.
Mitigation strategies for CVE-2017-1000257 require immediate patching of affected libcurl versions, with the vulnerability classified under CWE-125 as an out-of-bounds read condition. Organizations should prioritize updating their libcurl installations to versions that properly handle zero-sized data segments without invoking strlen() on potentially unbounded memory regions. The ATT&CK framework categorizes this vulnerability under T1059.007 for command and script injection, as the memory corruption could potentially enable further exploitation through code execution. Additionally, implementing proper bounds checking and null-termination validation in heap buffer management would prevent similar issues in other network libraries. System administrators should monitor for applications using vulnerable libcurl versions and ensure comprehensive testing of updated libraries to prevent regression issues while maintaining operational stability.