CVE-2026-88877
Description
A flaw was found in Traefik's Kubernetes ingress-nginx provider. A remote attacker, by sending a specially crafted request with a malformed Host header (e.g., containing a non-numeric port), can bypass security middlewares, including authentication and IP allowlisting. This leads to unauthorized access to protected backend services.
Statement
A flaw was found in Traefik's Kubernetes ingress-nginx provider (v3.7.0 through v3.7.11) when handling Ingresses carrying both authentication annotations and the nginx.ingress.kubernetes.io/from-to-www-redirect annotation. The provider generates a sibling router with a RedirectRegex middleware that matches on host alone. Requests containing non-numeric or empty ports in the Host header (e.g., Host: www.example.com:x) bypass net.SplitHostPort redirect logic while matching the sibling router, proxying requests directly to backend services without evaluating any annotation-derived security middlewares, including BasicAuth and source-IP allowlists.
Mitigation
Remove the nginx.ingress.kubernetes.io/from-to-www-redirect annotation from Ingress resources that enforce authentication or access control middlewares. Alternatively, implement host redirects using explicit Traefik RedirectScheme middlewares or enforce authentication upstream.
Common Vulnerability Scoring System (CVSS) Score Details
Info alert:Important note
CVSS scores for open source components depend on vendor-specific factors (e.g. version or build chain). Therefore, Red Hat's score and impact rating can be different from NVD and other vendors. Red Hat remains the authoritative CVE Naming Authority (CNA) source for its products and services (see Red Hat classifications).
The following CVSS metrics and score provided are preliminary and subject to review.
CVSS v3 Score Breakdown
| Red Hat | NVD | cve.org | |
|---|---|---|---|
| Base Score | 9.1 | N/A | 9.8 |
| Attack Vector | Network | N/A | Network |
| Attack Complexity | Low | N/A | Low |
| Privileges Required | None | N/A | None |
| User Interaction | None | N/A | None |
| Scope | Unchanged | N/A | Unchanged |
| Confidentiality | High | N/A | High |
| Integrity Impact | High | N/A | High |
| Availability Impact | None | N/A | High |
Vector
Red Hat: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N
cve.org: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
Understanding the Weakness (CWE)
Access Control
Technical Impact: Bypass Protection Mechanism
Access control checks for specific user data or functionality can be bypassed.
Access Control
Technical Impact: Gain Privileges or Assume Identity
Horizontal escalation of privilege is possible (one user can view/modify information of another user).
Access Control
Technical Impact: Gain Privileges or Assume Identity
Vertical escalation of privilege is possible if the user-controlled key is actually a flag that indicates administrator status, allowing the attacker to gain administrative access.
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