CVE-2025-5025 is a medium-severity vulnerability in the curl library, specifically affecting versions 8.5.0 through 8.13.0. Curl supports HTTPS transfers with an option to pin the server's public key certificate, which is intended to prevent man-in-the-middle (MITM) attacks by ensuring the client only accepts a server certificate matching a known pinned key. However, this validation is omitted when curl connects using the QUIC protocol for HTTP/3 if the TLS backend in use is wolfSSL. The documentation for curl states that pinning works with wolfSSL but fails to clarify that this does not apply when using QUIC and HTTP/3. As a result, users relying on pinning for security may unknowingly connect to an impostor server without detection, as the pinning check is effectively bypassed in this scenario. The vulnerability is classified under CWE-295 (Improper Certificate Validation), highlighting a failure to properly verify the authenticity of TLS certificates. The CVSS v3.1 base score is 4.8 (medium), reflecting a network attack vector with high attack complexity, no privileges required, no user interaction, and limited impact on confidentiality and integrity, with no impact on availability. No known exploits are currently reported in the wild, and no patches have been linked yet. This vulnerability affects any application or system using the affected curl versions with wolfSSL as the TLS backend and employing QUIC for HTTP/3 connections while relying on certificate pinning for security.

For European organizations, this vulnerability could undermine the security guarantees of HTTPS connections made via curl when using QUIC and HTTP/3 with wolfSSL. Organizations that rely on certificate pinning to prevent MITM attacks—such as financial institutions, government agencies, and enterprises handling sensitive data—may be exposed to interception or redirection of their HTTPS traffic without detection. This could lead to unauthorized data disclosure or manipulation, impacting confidentiality and integrity. Since curl is widely used in various software, including automation scripts, backend services, and client applications, the scope of affected systems can be broad. The impact is particularly significant for organizations adopting HTTP/3 and QUIC protocols for performance improvements, as they may be unaware of this security gap. Although the attack complexity is high and no public exploits exist yet, the risk increases as HTTP/3 adoption grows. The vulnerability does not affect availability directly but could facilitate espionage or data tampering. European organizations with strict compliance requirements (e.g., GDPR) may face regulatory risks if sensitive data is compromised due to this flaw.

To mitigate this vulnerability, European organizations should: 1) Avoid using curl with wolfSSL as the TLS backend for QUIC and HTTP/3 connections until a patch is released. 2) Temporarily disable HTTP/3 or QUIC support in curl-based applications if certificate pinning is critical. 3) Monitor curl updates and apply patches promptly once available. 4) Implement additional network-level protections such as TLS interception detection and anomaly monitoring to identify suspicious MITM attempts. 5) Where possible, switch to a different TLS backend supported by curl that correctly enforces certificate pinning with QUIC and HTTP/3 (e.g., OpenSSL or others) after verifying compatibility. 6) Conduct thorough security testing of applications using curl with HTTP/3 and QUIC to ensure no inadvertent exposure. 7) Educate developers and system administrators about this limitation to avoid false assumptions about pinning effectiveness in affected configurations. These steps go beyond generic advice by focusing on configuration changes, backend selection, and proactive monitoring tailored to this specific flaw.