CVE-2026-92391 in libcurl
Summary
by MITRE • 10/20/2026
A deeply nested multipart MIME structure constructed via the public API causes a stack overflow in libcurl during normal cleanup operations. An excessive recursion depth within `curl_mime_free` leads to unbounded stack frame allocation during structure traversal, ultimately crashing the host process.
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Analysis
by VulDB Data Team • 10/20/2026
The vulnerability identified involves a critical resource management flaw within the libcurl library, specifically affecting its handling of multipart MIME data structures when processed through the public API. The core issue stems from an insufficient check on recursion depth during the cleanup phase of memory allocation for complex nested objects. When a client application constructs a deeply nested multipart MIME structure and subsequently initiates a deallocation sequence via curl_mime_free, the library enters a recursive traversal routine designed to free each component of the hierarchical data structure. Because there is no mechanism to limit or validate the depth of this recursion, an attacker who can control the input payload sent through the API can craft a request with excessive nesting levels. This causes the function to call itself repeatedly without termination conditions being met for deep structures, leading directly to unbounded stack frame allocation.
From a technical perspective, this flaw represents a classic case of improper limit enforcement on recursion depth, which aligns closely with CWE-675: Use of Algorithmic Complexity Characteristics that are susceptible to denial of service via resource exhaustion. The operational impact is severe for any application relying on libcurl for HTTP or FTP operations involving multipart form data. As the stack frames accumulate beyond the available memory allocated by the operating system, a stack overflow occurs. This results in an immediate crash of the host process containing the vulnerable library instance. In server-side applications, this translates to a straightforward denial-of-service condition where remote attackers can disrupt service availability simply by submitting specially crafted multipart requests that trigger deep nesting during cleanup.
The attack vector is particularly concerning because it leverages legitimate API functionality rather than exploiting an input validation bypass in parsing logic alone; the crash occurs during normal operational teardown procedures after data has been accepted and processed to some degree. This makes detection via standard intrusion prevention systems more difficult, as the traffic appears valid until the server process terminates unexpectedly. The vulnerability is categorized under MITRE ATT&CK technique T1499: Endpoint Denial of Service, specifically reflecting resource exhaustion through application-level logic flaws rather than network flooding or brute force attempts.
Mitigation strategies must address both immediate remediation and long-term architectural resilience. For developers currently using affected versions of libcurl, the primary solution is to upgrade to a patched version where the curl_mime_free function has been modified to include explicit depth tracking or iterative stack-based traversal instead of pure recursion. This ensures that even if an attacker provides deeply nested structures, the cleanup process will either fail gracefully with an error code indicating maximum nesting exceeded or utilize heap-allocated memory for the traversal logic, thereby preventing stack exhaustion. Additionally, applications should implement input validation at the API entry point to reject multipart requests exceeding a predefined depth threshold before they are fully constructed in memory. This defense-in-depth approach reduces the attack surface by ensuring that only reasonably structured payloads reach the internal cleanup routines.
Security teams monitoring for exploitation of this vulnerability should look for sudden process terminations or core dumps associated with libcurl instances, particularly those occurring shortly after handling large multipart POST requests. Log analysis focusing on HTTP request sizes and content-type headers containing multipart boundaries may reveal patterns indicative of such attacks. Furthermore, fuzzing efforts targeting the MIME parsing and cleanup subsystems are recommended to uncover any related edge cases that might not have been covered by existing test suites. By combining library updates with strict input constraints and robust monitoring, organizations can effectively neutralize this denial-of-service risk while maintaining compatibility with standard web application workflows involving file uploads or complex form submissions.