CVE-2026-1642 Source: f5sirt@f5.com HIGH 8.2 Published: February 4, 2026 at 03:16 PM Modified: February 13, 2026 at 09:35 PM Vulnerability Description 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. CVSS Metrics Base Score 8.2 Severity HIGH Vector String CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:H/VA:N/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X Weaknesses (CWE) CWE-349 Source: f5sirt@f5.com CWE-345 Source: nvd@nist.gov AI Security Analysis 01 // Technical Summary NGINX OSS and NGINX Plus are vulnerable to a man-in-the-middle (MITM) attack when proxying to upstream TLS servers. An attacker positioned between NGINX and the upstream server can inject plaintext data into the server's response, potentially leading to data breaches and system compromise . This vulnerability requires specific conditions outside the attacker's direct control, but the impact is significant. 02 // Vulnerability Mechanism Step 1: MITM Position : The attacker establishes a man-in-the-middle position between the vulnerable NGINX instance and the upstream TLS server. This requires network access and control over the traffic flow. Step 2: TLS Handshake Manipulation (Potential) : The attacker may attempt to subtly manipulate the TLS handshake process, potentially by injecting crafted packets or manipulating the timing of the handshake. Step 3: Data Injection : The attacker injects plaintext data into the response stream from the upstream server. This could involve crafting specific packets or exploiting a timing-related issue to insert the malicious data. Step 4: NGINX Processing : NGINX processes the manipulated response, unaware of the injected data. Step 5: Response Delivery : The manipulated response, now containing the attacker's injected data, is delivered to the client. 03 // Deep Technical Analysis The root cause lies in a flaw within NGINX's handling of TLS connections to upstream servers. Specifically, the vulnerability arises from a combination of factors related to how NGINX processes and forwards data received from the upstream server. The exact mechanism involves a timing-based issue or a subtle error in how NGINX handles the TLS handshake and subsequent data transfer. The attacker exploits this to inject plaintext data, likely by manipulating the TLS connection or exploiting a weakness in the data parsing logic. The vulnerability is not a direct buffer overflow or memory corruption issue, but rather a logic flaw that allows for the injection of malicious data into the response stream. The conditions beyond the attacker's control likely relate to the upstream server's behavior and the specific TLS configuration used. 04 // Exploitation Status Discovery Only. No public Proof-of-Concept (PoC) exploits are available at this time. The vulnerability's reliance on specific conditions outside the attacker's direct control makes exploitation more challenging. However, the potential impact warrants immediate attention and proactive mitigation. 05 // Threat Intelligence No specific Advanced Persistent Threats (APTs) or known malware families are directly associated with this vulnerability at this time. However, the potential for data breaches makes it attractive to various threat actors. CISA KEV status: Not Listed . 06 // Detection & Hunting Monitor network traffic for unusual patterns in TLS connections to upstream servers, particularly those involving NGINX. Analyze TLS handshake logs for anomalies, such as unexpected certificate exchanges or timing discrepancies. Inspect NGINX access logs for unusual response sizes or content that might indicate data injection. Implement intrusion detection system (IDS) rules to identify suspicious traffic patterns related to TLS connections and data transfer. Monitor upstream server logs for any signs of compromised data or unusual behavior that could be related to the attack. 07 // Remediation & Hardening Upgrade NGINX OSS and NGINX Plus to the latest patched versions. Consult the vendor's security advisory for specific version recommendations. Implement strong TLS configurations, including the use of up-to-date cipher suites and protocols. Regularly review and update TLS certificates. Enforce strict network segmentation to limit the attacker's ability to establish a MITM position. Implement robust monitoring and logging to detect and respond to suspicious activity. Consider using a Web Application Firewall (WAF) to filter and inspect traffic for malicious payloads. Review and harden the upstream server's security posture, inc