CVE-2026-9080 in libcURL
Summary
by MITRE • 07/03/2026
Calling `curl_easy_pause()` within the event-based `CURLMOPT_SOCKETFUNCTION` callback triggers a use-after-free vulnerability, where libcurl attempts to store a flag using a dangling struct pointer immediately after that pointer's memory has been freed.
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Analysis
by VulDB Data Team • 07/03/2026
This vulnerability represents a critical use-after-free condition in libcurl's multi-handle implementation that occurs when developers invoke curl_easy_pause() from within the CURLMOPT_SOCKETFUNCTION callback. The flaw arises from the asynchronous nature of libcurl's event-based architecture where socket callbacks are executed during the processing of curl_multi_perform(), creating a temporal race condition between memory management and operation execution. When curl_easy_pause() is called from within this specific callback context, it attempts to modify internal state flags using a struct pointer that has already been freed by the underlying socket handling mechanism, resulting in undefined behavior and potential remote code execution.
The technical root cause stems from improper lifecycle management of curl_easy_handle structures within the multi-handle event loop. The CURLMOPT_SOCKETFUNCTION callback operates in a context where libcurl may be in the process of cleaning up or reallocating memory for handles while simultaneously executing application code that attempts to modify those same handles through curl_easy_pause(). This creates a dangling pointer scenario where the memory address previously occupied by a curl_easy_handle structure has been deallocated but is still referenced by internal flag storage mechanisms. The vulnerability specifically manifests when the pause operation attempts to set or clear internal state flags using a pointer value that no longer corresponds to valid memory, violating fundamental memory safety principles and creating opportunities for attackers to manipulate memory layout through carefully crafted network conditions.
The operational impact of this vulnerability extends beyond simple memory corruption to potentially enable remote code execution in scenarios where malicious actors can control the network traffic flow that triggers the callback. Attackers could exploit this by sending specially crafted responses or connection events that cause libcurl to enter the vulnerable code path during socket operations, particularly in high-throughput applications such as web servers, proxies, or network monitoring tools that rely heavily on libcurl's multi-handle capabilities. The vulnerability affects any application using libcurl's event-based interface with concurrent socket callbacks and pause operations, making it particularly dangerous in server environments where multiple connections are processed simultaneously. This flaw aligns with CWE-416 which describes use-after-free vulnerabilities, and represents a specific case of improper memory management in asynchronous callback contexts.
Mitigation strategies must address both the immediate code-level fix and broader architectural considerations for applications using libcurl's multi-handle interface. The primary defense involves avoiding direct calls to curl_easy_pause() from within CURLMOPT_SOCKETFUNCTION callbacks by implementing asynchronous state management mechanisms or deferring pause operations until after the socket callback completes. Applications should also implement proper error handling around curl_multi_perform() and related functions to detect and recover from potential memory corruption states. Security-conscious developers should consider using memory debugging tools such as valgrind or address sanitizers during development to identify similar patterns, while system administrators should ensure regular updates to libcurl versions that contain fixes for this specific vulnerability. Additionally, organizations implementing libcurl-based solutions should conduct security reviews of their callback implementations and consider alternative approaches such as using separate threads for pause operations or restructuring event handling to avoid the problematic race condition entirely. This vulnerability demonstrates the importance of understanding library internal state management when working with asynchronous APIs and aligns with ATT&CK technique T1059.007 for execution through command and scripting interpreter, particularly when considering how attackers might leverage such memory corruption to gain code execution privileges in affected systems.