---
id: "CVE-2026-86415"
kind: "vulnerability"
aliases: ["CVE-2026-86415"]
title: "libcurl: cached Schannel connection reused across redirects, leaking client certificate"
severity: "unknown"
cvss: null
epss: null
exploited: false
status: "open"
category: "package"
products: []
ecosystems: []
tags: ["client-certificates", "libcurl", "package", "schannel", "windows"]
not_affected_if: ["no-client-certificates", "non-schnannel-backend"]
published_at: "2026-10-20T21:17:20+00:00"
first_seen_at: "2026-10-23T21:57:21+00:00"
updated_at: "2026-10-25T05:04:33+00:00"
sources: [{"source": "nvd", "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-86415", "first_seen_at": "2026-10-23T21:57:21+00:00"}]
---

# libcurl: cached Schannel connection reused across redirects, leaking client certificate

libcurl's connection cache matching ignores the automatic client certificate setting, so a cached Schannel connection that already presented a client certificate can be reused for a redirected transfer to a different origin. The attacker gets the authenticated connection's access on a server they should not reach, and the client certificate may be exposed to that server.

_Summary, check and fix written by a model from the sources; labelled as such._

## How to check

- Run `curl --version` and check whether the build uses Schannel (the Windows TLS backend) rather than OpenSSL or another backend
- Check whether the application's libcurl is linked dynamically or bundled: look for `libcurl` DLLs or the library version reported by the application
- unknown: the sources do not give the fixed or affected version range

## Fix

- Update libcurl to a release that includes the fix for this CVE; the sources do not name the fixed version, so check the curl project's security advisory
- Until patched, avoid reusing connections with client certificates across redirects in code that uses libcurl with Schannel

## Timeline

- 2026-10-23T21:57:21+00:00 nvd https://nvd.nist.gov/vuln/detail/CVE-2026-86415

## From the sources

libcurl's connection cache match function never considers the automatic client
certificate option, so a cached, already authenticated Schannel connection can
be handed to a transfer that got redirected to a different origin, even though
the code explicitly switched off the auto client cert option for that transfer
to prevent exactly this.

---

_Free to read at a human pace. An API with keys and an MCP server are in preparation; early access: hello@infrastack.ai_