CVE-2021-22890 is a medium-severity vulnerability affecting libcurl versions 7.63.0 through 7.75.0. The flaw arises from improper handling of TLS 1.3 session tickets when libcurl is used with an HTTPS proxy. Specifically, libcurl can mistakenly accept session tickets provided by the HTTPS proxy as if they originated from the remote server, leading to an incorrect shortcut in the TLS handshake process. This confusion allows a malicious HTTPS proxy to reuse or replay session tickets in a way that bypasses the server's TLS certificate validation. Consequently, the proxy can perform a man-in-the-middle (MITM) attack, intercepting and potentially manipulating HTTPS traffic without detection. However, for the attack to succeed, the malicious proxy must present a certificate that libcurl accepts for the targeted server, or the client must have disabled certificate verification. The vulnerability is rooted in CWE-300 (Channel Accessible by Non-Endpoint), indicating that the communication channel can be accessed or manipulated by an unintended party. While no known exploits have been reported in the wild, the vulnerability poses a risk in environments where HTTPS proxies are used and where certificate validation may be lax or compromised. The CVSS v3.1 base score is 4.3, reflecting a medium severity primarily due to the requirement of user interaction and the need for a malicious proxy with a trusted certificate or disabled certificate checks.

For European organizations, the impact of this vulnerability depends heavily on their network architecture and security policies. Organizations that route HTTPS traffic through internal or external HTTPS proxies for monitoring, filtering, or caching purposes are at risk. A compromised or malicious proxy within the network or supply chain could exploit this flaw to intercept sensitive communications, potentially exposing confidential data or enabling further attacks such as credential theft or session hijacking. The vulnerability undermines the trust model of TLS by allowing certificate validation to be bypassed under specific conditions, which could lead to data integrity and confidentiality breaches. Sectors with high reliance on secure communications, such as finance, healthcare, government, and critical infrastructure, could face significant risks if attackers exploit this vulnerability. Additionally, organizations that disable certificate verification for operational reasons or use self-signed certificates are particularly vulnerable. The absence of known exploits reduces immediate risk but does not eliminate the threat, especially as attackers may develop exploits targeting this flaw in the future.

To mitigate this vulnerability, European organizations should take the following specific actions: 1) Upgrade libcurl to version 7.76.0 or later, where the vulnerability has been addressed. 2) Audit and restrict the use of HTTPS proxies, ensuring only trusted proxies are used and that their certificates are properly validated. 3) Enforce strict TLS certificate validation policies in all client applications using libcurl, avoiding configurations that disable or bypass certificate checks. 4) Monitor network traffic for unusual proxy behavior or unexpected TLS session ticket exchanges that could indicate exploitation attempts. 5) Implement network segmentation and access controls to limit exposure of critical systems to potentially malicious proxies. 6) Educate developers and system administrators about the risks of improper TLS session ticket handling and the importance of keeping dependencies up to date. 7) Where feasible, consider disabling TLS session ticket resumption when using HTTPS proxies until patches are applied, as a temporary workaround. These measures go beyond generic advice by focusing on proxy trust management, strict certificate validation, and proactive monitoring tailored to the nature of this vulnerability.