CVE-2026-18924 in libcurl
Summary
by MITRE • 09/06/2026
A flaw in libcurl's handling of HTTP/2 Server Push streams, when the parent handle is set to share connections with other handles, can lead to use-after-free in the cleanup process.
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Analysis
by VulDB Data Team • 09/06/2026
The vulnerability identified within the libcurl library represents a critical memory management error rooted in the complex interaction between HTTP/2 multiplexing features and connection sharing mechanisms. Specifically, this flaw occurs when an application configures multiple curl handles to share connections via the CURLMOPT_PIPELINING or similar multi-handle options, which allows for efficient reuse of TCP connections across different requests. Under normal operation, libcurl manages a pool of persistent connections that are referenced by various easy handles. However, when HTTP/2 Server Push is enabled and utilized alongside connection sharing, the internal state management becomes susceptible to race conditions during cleanup phases. The core technical flaw lies in how the library tracks the lifecycle of push streams relative to their parent stream or handle. When a shared connection is closed or reset while multiple handles are still referencing it, libcurl fails to properly invalidate references to the associated HTTP/2 server push data structures before they are deallocated. This results in dangling pointers that remain accessible within the application's context even after the underlying memory has been freed by the library.
This use-after-free condition can be triggered during the termination of a multi-handle session or when individual easy handles are cleaned up while still associated with a shared connection that is undergoing teardown procedures involving HTTP/2 streams. The operational impact of this vulnerability is severe, as it allows for arbitrary code execution if an attacker can influence the memory allocation patterns following the free operation. By carefully crafting network responses that trigger server push events and then forcing a premature closure or reset of the connection, an adversary could exploit the dangling pointer to overwrite critical data structures in memory. This exploitation vector aligns with CWE-416, which classifies use-after-free errors as a distinct category of software weaknesses leading to undefined behavior. The ability to execute arbitrary code stems from the fact that modern operating systems do not prevent access to freed memory immediately; instead, they allow subsequent allocations to reuse that space, potentially placing attacker-controlled data in the same location previously occupied by sensitive pointers or function return addresses.
From a threat modeling perspective, this vulnerability maps directly to MITRE ATT&CK technique T1203, which covers exploitation for client execution, particularly when targeting widely used libraries like libcurl that are embedded in numerous applications including web browsers, email clients, and IoT devices. The attack complexity is moderate because it requires the victim application to be configured with specific multi-handle settings involving connection sharing and HTTP/2 support. However, given the prevalence of these configurations in high-performance networked applications, the potential surface area for exploitation is significant. Attackers do not necessarily need direct access to the target system if they can manipulate the server-side behavior or intercept traffic to force the conditions that trigger the race condition during cleanup. The lack of proper reference counting or pointer nullification before deallocation indicates a fundamental gap in the library's internal state synchronization logic, particularly under concurrent or asynchronous I/O scenarios common in HTTP/2 implementations.
Mitigation strategies must focus on immediate patching and architectural review. Users running affected versions of libcurl should upgrade to the latest stable release where this memory management issue has been resolved through rigorous code reviews and static analysis tools designed to detect use-after-free patterns. Developers integrating libcurl into their applications should ensure that they are not relying on deprecated or unstable API configurations involving shared connections with HTTP/2 server push unless absolutely necessary, as these edge cases often expose deeper library flaws. Additionally, employing memory safety techniques such as AddressSanitizer during development and testing phases can help identify similar latent issues before deployment. For organizations dependent on libcurl for critical infrastructure, implementing network-level monitoring to detect anomalous patterns of connection resets combined with server push responses may provide an early warning system against exploitation attempts while patches are being applied. The resolution typically involves ensuring that all references to stream-specific data structures are explicitly cleared and set to null before the memory is returned to the heap allocator, thereby preventing any subsequent access from resolving to freed memory regions.