CVE-2026-1642 in NGINX Open Source
Summary
by MITRE • 02/04/2026
A vulnerability exists in NGINX OSS and NGINX Plus when configured to proxy to upstream Transport Layer Security (TLS) servers. An attacker with a man-in-the-middle (MITM) position on the upstream server side—along with conditions beyond the attacker's control—may be able to inject plain text data into the response from an upstream proxied server. Note: Software versions which have reached End of Technical Support (EoTS) are not evaluated.
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Analysis
by VulDB Data Team • 03/05/2026
This vulnerability in NGINX OSS and NGINX Plus represents a significant security risk that arises from the proxy configuration handling of TLS connections to upstream servers. The flaw specifically manifests when NGINX acts as a reverse proxy or load balancer for TLS-encrypted services, creating a potential attack vector for sophisticated adversaries who can establish a man-in-the-middle position relative to the upstream server. The vulnerability's exploitation requires not only the attacker's ability to intercept traffic but also specific conditions that remain outside their direct control, suggesting a complex attack scenario that combines network position with environmental factors.
The technical implementation of this vulnerability stems from how NGINX processes responses when proxying to TLS servers, particularly in scenarios where the upstream server's TLS connection properties might be manipulated or where the proxy's handling of connection states creates opportunities for data injection. This represents a deviation from expected secure communication patterns where plaintext data injection could occur in the response stream, potentially leading to unauthorized content modification or data corruption. The vulnerability's classification aligns with CWE-444, which addresses improper handling of HTTP responses, and may also relate to CWE-119, concerning improper restriction of operations within a limited error handling scope, as the injection occurs during response processing.
The operational impact of this vulnerability extends beyond simple data corruption, as attackers could potentially inject malicious content that might be interpreted by downstream clients, leading to various security consequences including data theft, service disruption, or even client-side exploitation through carefully crafted injected content. The attack scenario requires the attacker to not only position themselves in a man-in-the-middle role but also to orchestrate conditions that allow for successful injection, suggesting that this vulnerability might be more relevant in specific network topologies or configurations where such positioning is feasible. Organizations relying on NGINX for TLS proxying operations face potential exposure to this vulnerability, particularly in environments where upstream server security cannot be fully controlled or verified.
Mitigation strategies for this vulnerability should focus on immediate software updates to patched versions of NGINX, as well as implementation of additional network security controls such as strict certificate validation, enhanced monitoring for anomalous response patterns, and consideration of alternative proxy configurations that reduce exposure. Network administrators should also implement proper TLS certificate management practices, including certificate pinning where appropriate, to prevent unauthorized certificate substitution that could facilitate exploitation. The vulnerability's nature suggests that defense in depth approaches are essential, combining software patching with operational security measures to reduce the attack surface. Organizations should also review their upstream server configurations and implement proper traffic inspection mechanisms to detect and prevent unauthorized data injection attempts, aligning with ATT&CK technique T1071.004 for application layer protocol data injection and T1566 for credential access through man-in-the-middle attacks.