Response injection via pre-handshake plaintext parsing
Published Feb 4, 2026 · Updated Feb 4, 2026
Response injection in F5 NGINX Open Source and NGINX Plus allows man-in-the-middle attackers to alter proxied server responses. In ngx_http_upstream_process_header, a fast plaintext upstream response was parsed before the write handler initialized the required TLS handshake. Exploitation requires a TLS-enabled HTTP, HTTP/2, gRPC, or uWSGI upstream, an upstream-side man-in-the-middle position, and read and write readiness in the same event-loop iteration; injected plaintext can then reach downstream clients.
Summary
What happened
Response injection in F5 NGINX Open Source and NGINX Plus allows man-in-the-middle attackers to alter proxied server responses. In ngx_http_upstream_process_header, a fast plaintext upstream response was parsed before the write handler initialized the required TLS handshake. Exploitation requires a TLS-enabled HTTP, HTTP/2, gRPC, or uWSGI upstream, an upstream-side man-in-the-middle position, and read and write readiness in the same event-loop iteration; injected plaintext can then reach downstream clients.
The record
- CVE
- CVE-2026-1642
- Published
- Feb 4, 2026
- Updated
- Feb 4, 2026
- Vendor
- F5 Networks
- Product
- NGINX Open Source
- Classifications
- CWE-349, T1557
- Attack vector
- network
- Privileges
- unauthenticated
Timeline
How it unfolded
- Feb 4, 2026CVE publishedPublication date reported by the CVE source.
- Feb 4, 2026Record updatedLatest update available in the CVE record.
Exploitability
Present is not the same as exploitable
Compare your product and version with the public record. A matching version still requires validation against your environment.
Is a vulnerable build present?
Compare these published version ranges with your installed build and any vendor patches.
- Affected versionversion=1.3.0 <1.29.5 changes=[{"at":"1.28.2","status":"unaffected"}]
What conditions does exploitation require?
What is affected?
Attacks
What attackers are doing with it
Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.
Weakness, pattern, technique
Public exploit references
No public exploit references are available in this record.
Labels summarize the accepted research assessment. They do not indicate a test against your environment.
Technologies
Your stack
See the directory against your own environment.
Your stack
Check the software in your environment
Book a demo to see how Hinoki identifies affected software and validates exploitability in your environment.
Book a demo