CVE-2026-92391: 
cURL vulnerability analysis and mitigation

CVE-2026-92391 is a stack overflow vulnerability in libcurl caused by uncontrolled recursion in the curl_mime_free function when processing deeply nested multipart MIME structures. Reported by Joshua Rogers (Aisle Research) on September 12, 2026, and publicly disclosed on October 14, 2026 alongside the release of curl 8.23.0, it affects curl versions 7.56.0 through 8.22.0 (with specific patched backport branches). The vulnerability is classified as Low severity by the curl project (CWE-674: Uncontrolled Recursion) and does not affect the curl command-line tool — only libcurl consumers using the MIME API (curl Advisory, Github Advisory).

The root cause is CWE-674 (Uncontrolled Recursion): libcurl's curl_mime_free function traverses multipart MIME structures recursively without enforcing a maximum recursion depth. When an application constructs a MIME structure with thousands of nested parts via the public API (e.g., curl_mime_addpart) and then frees it, the recursive traversal allocates unbounded stack frames, ultimately exhausting the stack and crashing the host process. The vulnerability was introduced in commit ce0881edee3c78609eae49665fb70264d8786d29 and fixed in commit d7546809d5bd548b69ec49fca26565c8f6f0d344. Normal MIME setups with a few to hundreds of parts are not affected — thousands of nested parts are required to trigger the overflow, with the exact threshold depending on system stack size and configuration (curl Advisory, curl JSON).

Successful exploitation causes the host process using libcurl to crash, resulting in a denial of service. Because the vulnerability is triggered during cleanup (curl_mime_free), any application that accepts externally influenced MIME structure depth and uses libcurl's MIME API is at risk of process termination. There is no evidence of confidentiality or integrity impact — the consequence is limited to availability. The curl command-line tool itself is not affected; only applications embedding libcurl and using the MIME API are vulnerable (curl Advisory, Ubuntu USN-8927-1).

No public proof-of-concept exploit code is known, and there is no evidence of in-the-wild exploitation as of the time of disclosure (Feedly). The EPSS score is 0.0, reflecting very low probability of exploitation in the near term. The vulnerability is not listed in the CISA Known Exploited Vulnerabilities (KEV) catalog. Exploitation requires the ability to influence the depth of MIME structures processed by a target application using libcurl's MIME API, which limits the practical attack surface to applications that accept attacker-controlled multipart MIME input (curl Advisory).

1. Identify a target application: Find an application that uses libcurl's MIME API (e.g., curl_mime_addpart) and accepts externally supplied multipart MIME structures, such as a file upload service or email client built on libcurl versions 7.56.0–8.22.0.
2. Craft a deeply nested MIME payload: Construct a multipart MIME structure with thousands of nested subparts. The exact depth required depends on the target system's stack size, but typically thousands of levels are needed to trigger the overflow.
3. Submit the payload: Deliver the crafted MIME structure to the target application through its normal input channel (e.g., HTTP multipart upload, API call).
4. Trigger cleanup: Cause the application to free the MIME structure (e.g., by completing or aborting the request), which invokes curl_mime_free and initiates the unbounded recursive traversal.
5. Observe crash: The recursive stack frame allocation exhausts the stack, causing a stack overflow and crashing the host process, resulting in denial of service (curl Advisory).

• Logs: Application crash logs or core dumps referencing curl_mime_free or deep stack traces within libcurl's MIME handling code; segmentation fault signals (SIGSEGV) or stack overflow errors in system logs associated with a libcurl-linked process.
• Process: Unexpected termination of services that use libcurl's MIME API, particularly following receipt of large or complex multipart MIME requests.
• Network: Unusually large or deeply nested multipart MIME payloads in HTTP requests (e.g., Content-Type: multipart/form-data with thousands of nested boundary sections) directed at libcurl-based services.

The primary remediation is to upgrade libcurl/curl to a patched version: 8.23.0 (main branch), 8.20.2 (8.20.x backport), 8.16.2 (8.16.x backport), or 8.14.3 (8.14.x backport). Ubuntu users should update to the package versions specified in USN-8927-1 (Ubuntu 26.04 LTS: libcurl4t64 8.18.0-1ubuntu2.8) and USN-8927-2 (Ubuntu 26.10: libcurl4t64 8.20.0-2ubuntu11.1). As a workaround where patching is not immediately possible, applications should avoid using the MIME API with untrusted input, or implement application-level limits on the nesting depth of accepted multipart MIME structures (curl Advisory, Ubuntu USN-8927-1, Ubuntu USN-8927-2).

The vulnerability was part of a broader curl 8.23.0 security release that addressed 24 vulnerabilities, which received coverage from Heise and CyberSecurityNews. The curl project rated this issue as Low severity, noting that triggering it requires thousands of nested MIME parts — far beyond typical usage. No significant independent researcher commentary or social media discussion specific to CVE-2026-92391 has been identified beyond standard vulnerability aggregator coverage (Feedly).