CVE-2026-1642 is a vulnerability identified in F5 NGINX Open Source version 1.3.0, specifically when configured to proxy requests to upstream servers over TLS. The issue arises because NGINX accepts extraneous untrusted data alongside trusted data from the upstream TLS server, violating the principle of data integrity (CWE-349). An attacker who can position themselves as a man-in-the-middle (MITM) on the upstream server side may inject plaintext data into the response stream. This injection could alter the content delivered to clients, potentially leading to data manipulation or corruption. The attack complexity is high, as it requires MITM access on the upstream side and specific conditions outside the attacker’s control. No authentication or user interaction is needed to exploit this vulnerability. The vulnerability does not affect confidentiality or availability directly but compromises data integrity, which can have downstream effects on application behavior and trustworthiness of responses. The vulnerability is rated medium severity with a CVSS score of 5.9 (AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:H/A:N). No patches or known exploits are currently available, and versions beyond 1.3.0 or those out of support are not evaluated. This vulnerability is particularly relevant for environments where NGINX proxies TLS traffic to upstream servers, a common deployment in modern web infrastructure.

For European organizations, the primary impact of CVE-2026-1642 is the potential compromise of data integrity in communications proxied through vulnerable NGINX instances. This could lead to manipulation of web content, injection of malicious payloads, or disruption of application logic relying on trusted upstream responses. Sectors such as finance, government, telecommunications, and critical infrastructure that depend on secure and reliable TLS proxying are at higher risk. The vulnerability does not directly expose confidential data or cause denial of service but undermines trust in the integrity of transmitted data, which can facilitate further attacks or fraud. Given the reliance on NGINX in many European data centers and cloud environments, the threat could affect a broad range of services, especially those using NGINX as a reverse proxy or load balancer for TLS traffic. The requirement for MITM access on the upstream side limits the attack surface but does not eliminate risk, particularly in complex network environments or where upstream servers are exposed to less secure networks.

European organizations should first identify all NGINX Open Source instances configured as TLS proxies to upstream servers, particularly those running version 1.3.0. Until an official patch is released, organizations should consider the following mitigations: (1) Restrict network access to upstream servers to trusted networks only, minimizing the risk of MITM attacks on the upstream path. (2) Employ network-level protections such as TLS mutual authentication between NGINX and upstream servers to ensure endpoint authenticity. (3) Use out-of-band integrity verification mechanisms or application-layer validation to detect injected or altered responses. (4) Monitor network traffic for anomalies indicative of MITM activity or injected plaintext data. (5) Consider upgrading to later NGINX versions if they are confirmed not vulnerable or switch to NGINX Plus if it is unaffected. (6) Implement strict transport security policies and ensure that all components in the proxy chain use strong, up-to-date cryptographic protocols and ciphers. (7) Prepare for rapid patch deployment once F5 releases an official fix. These steps go beyond generic advice by focusing on network architecture and cryptographic hardening specific to the vulnerability’s attack vector.