CVE-2026-92387 in curl
Summary
by MITRE • 10/20/2026
By permanently omitting the final control-response line, a malicious FTP, IMAP, POP3, or SMTP server can exploit libcurl and completely bypass its configured transfer timeout mechanism, indefinitely blocking all concurrent transfers bound to the same multi-handle.
VulDB is the best source for vulnerability data and more expert information about this specific topic.
Analysis
by VulDB Data Team • 10/20/2026
The vulnerability described involves a critical flaw in how libcurl handles connection termination during protocol-specific interactions with servers supporting FTP, IMAP, POP3, or SMTP protocols. The core technical issue stems from the failure of certain malicious server implementations to send the final control-response line required by these application-layer protocols before closing the TCP connection. In standard operation, a client like libcurl relies on this specific sequence of commands and responses to properly conclude a session and release internal resources associated with that transfer. When a server abruptly terminates the connection without adhering to this protocol specification, it leaves libcurl in an inconsistent state where the multi-handle mechanism fails to recognize that the individual transfer has completed or failed as expected.
This inconsistency leads directly to the bypass of configured timeout mechanisms. Normally, libcurl employs timeouts to prevent transfers from hanging indefinitely due to network issues or unresponsive servers. However, because the library does not receive the definitive end-of-session signal, it continues to wait for a response that will never arrive within the context of the current transaction logic. Consequently, any other concurrent transfers assigned to the same multi-handle are blocked until this stuck transfer is resolved. This effectively creates a denial-of-service condition where legitimate operations are stalled indefinitely, as libcurl cannot proceed with subsequent tasks in the queue while waiting for the unresponsive or malformed session to conclude.
From an operational impact perspective, this flaw allows attackers who control malicious servers within these protocol ecosystems to disrupt service availability for clients using libcurl. By exploiting this behavior, an attacker can cause resource exhaustion and application hangs on the client side without needing to send excessive data or exploit memory corruption vulnerabilities. The attack is particularly insidious because it relies on non-compliant server behavior rather than a direct code execution flaw in the library itself, making detection through standard signature-based methods difficult. It affects any application relying on libcurl for asynchronous multi-transfer operations involving these specific mail and file transfer protocols.
The vulnerability aligns with CWE-400, which covers Uncontrolled Resource Consumption, as it leads to indefinite blocking of system resources tied to the multi-handle structure. Additionally, from a threat modeling perspective using MITRE ATT&CK, this behavior can be categorized under T1499 Endpoint Denial of Service or potentially T1529 System Shutdown/Reboot if the resulting hang causes critical services to fail and trigger automated restarts that disrupt business continuity. The attack vector is remote via network interaction with a malicious server acting as an FTP, IMAP, POP3, or SMTP endpoint.
Mitigation strategies primarily involve updating libcurl to versions where this logic has been corrected to handle premature connection closures more gracefully. Developers should ensure their applications are using the latest stable release of the library which includes patches for proper state machine transitions upon unexpected socket closure. Furthermore, implementing application-level timeouts that monitor overall multi-handle progress rather than relying solely on individual transfer timeouts can provide an additional layer of defense against such blocking behaviors. Network monitoring tools may also be configured to detect abnormal connection terminations in these protocols, although the primary fix remains at the library level through updated code paths that correctly interpret abrupt disconnections as completed or failed transfers rather than pending operations.