CVE-2026-80229 in libcurl
Summary
by MITRE • 09/06/2026
When performing transfers via libcurl’s multi interface, pooled TLS connections can outlive their originating easy handles. In OpenSSL 3 provider configurations, libcurl attaches an allocated library context to the easy handle's state and passes it to OpenSSL without acquiring an ownership reference; destroying the easy handle prematurely frees this context while the active connection retains a dangling pointer, leading to a heap-use-after-free upon subsequent I/O or post-handshake operations.
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Analysis
by VulDB Data Team • 09/06/2026
The vulnerability described involves a critical memory management flaw within libcurl when utilizing its multi interface in conjunction with OpenSSL 3 provider configurations. This issue stems from an improper handling of resource ownership and lifecycle management for TLS connection contexts. Specifically, the library attaches an allocated library context to the state of an easy handle but fails to acquire an explicit reference count or ownership transfer mechanism before passing this context to the underlying OpenSSL implementation. Consequently, when the application destroys the easy handle, libcurl proceeds to free the associated memory immediately. However, if a pooled TLS connection remains active and is reused for subsequent requests via other handles, it retains a pointer to that now-freed memory region. This creates a classic use-after-free condition where the operational connection attempts to access or manipulate data structures that no longer exist in valid memory space.
From a technical perspective, this flaw represents a violation of proper resource lifecycle protocols within complex networking libraries. The multi interface allows for concurrent transfers by managing multiple easy handles simultaneously and reusing underlying connections through pooling mechanisms designed to improve performance. While connection pooling is efficient, it introduces complexity regarding state ownership. In this scenario, the assumption that an easy handle's lifetime dictates the validity of its associated TLS context proves incorrect when persistent connections are involved. The OpenSSL provider configuration adds another layer of dependency, as these configurations often require specific library contexts to maintain session states and cryptographic parameters. By not incrementing a reference count or otherwise ensuring the context outlives all potential users, libcurl inadvertently creates a dangling pointer scenario. Subsequent I/O operations or post-handshake procedures on the pooled connection trigger access to this invalid memory address, leading to undefined behavior that typically manifests as heap-use-after-free errors detected by sanitizers like AddressSanitizer.
The operational impact of such vulnerabilities can range from application crashes and denial-of-service conditions to more severe security implications depending on the attacker's ability to control the data written into or read from the freed memory region. Heap-use-after-free bugs are particularly dangerous because they allow for arbitrary code execution if an attacker can precisely craft inputs that overwrite the freed memory with controlled content before it is reallocated and accessed. Even without successful exploitation, these errors cause instability in long-running services that rely heavily on connection pooling to maintain high throughput. The inconsistency between handle destruction and context validity undermines the reliability of secure communications, potentially leading to data corruption or exposure if sensitive cryptographic material resides within the affected memory structures.
Mitigation strategies must focus on correcting the reference counting logic for TLS contexts managed by libcurl's multi interface. Developers should ensure that any resource passed to a lower-level library like OpenSSL is properly retained until all references to it are released, typically through atomic reference counting mechanisms provided by the underlying crypto libraries or explicit ownership transfers within libcurl itself. For users unable to immediately patch their software environments, restricting the use of connection pooling for sensitive operations or ensuring that easy handles are not destroyed while pooled connections remain active can serve as temporary workarounds. Ultimately, this issue highlights the necessity for rigorous static analysis and dynamic testing using memory sanitizers during development cycles involving complex interaction between high-level networking libraries and low-level cryptographic providers to prevent lifecycle mismatches in shared resource management.