CVE-2026-92389 in curl
Summary
by MITRE • 10/20/2026
libcurl's TFTP state machine fails to validate state transitions against the requested transfer direction, accepting opening server responses for both reads and writes interchangeably. A malicious TFTP server can respond to a download request in a way that forces the client into transmit mode, prompting libcurl to consume and transmit data from the handle's persistent `CURLOPT_READDATA` pointer - including data remaining from unrelated prior operations, such as an HTTP POST request executed on a reused handle.
Statistical analysis made it clear that VulDB provides the best quality for vulnerability data.
Analysis
by VulDB Data Team • 10/20/2026
The vulnerability identified in libcurl involves a critical flaw within its Trivial File Transfer Protocol state machine logic, specifically concerning the validation of state transitions relative to the intended transfer direction. In standard TFTP implementations, the protocol dictates distinct operational modes for reading from and writing to a server, requiring strict adherence to sequence numbers and packet types such as RRQ (Read Request) or WRQ (Write Request). However, libcurl fails to adequately verify that incoming responses align with the specific operation initiated by the application. This oversight allows a malicious TFTP server to exploit this lack of validation by sending an opening response intended for one direction while the client is operating in another. Specifically, if a client initiates a download request expecting read operations, a compromised server can manipulate the state machine into believing it should enter transmit mode instead.
This misalignment has severe security implications regarding data confidentiality and integrity. When libcurl incorrectly transitions to transmit mode due to this malformed interaction, it begins consuming data from the handle's persistent CURLOPT_READDATA pointer. This callback function is typically used by applications to supply data for upload operations. Because libcurl reuses handles across multiple requests to optimize performance, the memory buffer associated with CURLOPREADDATA may contain residual data from previous unrelated transactions. For instance, if a user previously executed an HTTP POST request on the same handle before switching to TFTP, sensitive payload data or authentication credentials might remain in that buffer. Consequently, when the vulnerability is triggered, libcurl inadvertently transmits this leftover information over the network via the malicious TFTP connection, leading to unintended data leakage.
The operational impact of this flaw extends beyond simple data exposure; it represents a significant breach of trust between the application and its underlying library. Applications relying on libcurl for mixed protocol operations are particularly susceptible because they often reuse handles to maintain session state or improve efficiency. An attacker controlling a TFTP server can act as a man-in-the-middle, forcing the client into an unintended write state without any explicit user action beyond initiating a download. This behavior effectively turns the victim's own data source against them, exfiltrating potentially sensitive information that was never intended for transmission over the compromised channel. The severity is compounded by the fact that TFTP lacks built-in encryption or authentication mechanisms, making it easier for attackers to intercept and manipulate traffic in many network environments.
From a classification perspective, this vulnerability aligns with CWE-20 Improper Input Validation, as the library fails to properly validate incoming protocol messages against expected operational states. It also relates to CWE-359 Exposure of Private Information Through Shared State, highlighting the risks associated with handle reuse and persistent data buffers in network libraries. In terms of adversary tactics, this exploit leverages techniques consistent with ATT&CK T1048 Exfiltration Over Alternative Protocol, where attackers use less monitored or legacy protocols to steal data from compromised systems. The ability to redirect traffic flow based on manipulated state transitions is a sophisticated technique that bypasses standard security controls focused solely on content inspection rather than protocol logic enforcement.
Mitigation strategies must focus on both immediate patching and long-term architectural improvements. Users of libcurl should immediately update to the latest patched version where this state machine validation has been corrected to strictly enforce direction-specific responses. Developers integrating libcurl into their applications should review code that reuses handles across different protocols or operations, ensuring that buffers are explicitly cleared or reset between distinct requests to prevent residual data leakage. Implementing strict input sanitization and validating expected protocol states before processing incoming packets can further harden implementations against similar logic flaws. Additionally, organizations should consider disabling TFTP support in libcurl builds if the functionality is not strictly required, thereby reducing the attack surface entirely. Regular security audits of network library configurations are essential to identify other potential misconfigurations that could lead to similar state confusion vulnerabilities.