libcurl CVE-2026-82208

Three conditions must hold simultaneously: (1) curl/libcurl must be compiled and running with the wolfSSL TLS backend - not the default on most Linux distributions or macOS, which ship OpenSSL or GnuTLS builds; (2) CA caching must be enabled within the wolfSSL integration layer; (3) the application must register a CURLOPT_SSL_CTX_FUNCTION callback and use it to replace the SSL context's trust store with a custom bundle. Beyond these software conditions, the attacker must achieve a network MITM position - via on-path interception, DNS poisoning, BGP hijack, or similar - and must possess a certificate signed by a CA present in the cached system bundle that the callback's custom store explicitly excludes. Deployments using any non-wolfSSL backend, deployments with CA caching disabled, and applications that do not register CURLOPT_SSL_CTX_FUNCTION are not exploitable regardless of network position.

The vendor-assigned CVSS 7.5 (AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N) overstates attacker convenience: AV:N and AC:L imply broad unauthenticated network exploitability, but exploitation actually requires three non-default concurrent conditions - wolfSSL as the TLS backend, CA caching enabled, and active use of CURLOPT_SSL_CTX_FUNCTION - none of which represent a default curl deployment. Additionally, the attacker must occupy a MITM position on the network path, further raising effective attack complexity well above AC:L. Even a mid-sized municipal IT team in opole running standard curl packages built against OpenSSL is entirely unaffected, illustrating how narrowly the real exposure falls. EPSS at 0.13% (3rd percentile) correctly signals that exploitation has not been observed at scale and is unlikely against opportunistic targeting. The vulnerability is absent from CISA KEV and no POC code has been identified. Real-world impact is primarily limited to specialized deployments - embedded systems, IoT platforms, or security-sensitive enterprise clients - that explicitly combine wolfSSL with custom callback-driven trust policy enforcement, where an attacker with MITM capability holds a certificate trusted by the cached system bundle but excluded from the application's custom PKI.

An attacker with a MITM position between a libcurl client (wolfSSL backend, CA caching on, CURLOPT_SSL_CTX_FUNCTION registered) and a remote server presents a TLS certificate signed by a CA present in the cached system bundle but deliberately excluded from the application's callback-installed custom trust store. Because libcurl reinstates the cached store after the callback returns, the certificate passes validation that the application explicitly intended to block, allowing the attacker to terminate and relay the TLS session while reading plaintext traffic. No public POC code has been identified at time of analysis.