CVE-2026-10839: Authentication Bypass Vulnerability

CVE-2026-10839 Overview

CVE-2026-10839 is an open redirection vulnerability (CWE-601) affecting the authentication system of a password manager application. Attackers can manipulate the X-Forwarded-Host HTTP header to alter URLs generated by the application. A successful exploit redirects authenticated users to attacker-controlled sites after login or interface interaction. The flaw results in limited impact on confidentiality and integrity, primarily enabling credential harvesting and phishing campaigns that abuse the trusted domain context.

Critical Impact
Authenticated users can be redirected to malicious sites after login, enabling phishing and credential theft while the original request appears to originate from a trusted host.

Affected Products

Password manager application referenced in the INCIBE-CERT advisory
Vendor and specific product versions not disclosed in the NVD record
Refer to the vendor advisory for the authoritative list of impacted releases

Discovery Timeline

2026-06-17 - CVE-2026-10839 published to NVD
2026-06-17 - Last updated in NVD database

Technical Details for CVE-2026-10839

Vulnerability Analysis

The vulnerability stems from the authentication system trusting the X-Forwarded-Host HTTP header when constructing URLs returned to the client. This header is typically set by upstream proxies to indicate the original host requested by the client. When the application uses this value without validation, an attacker can influence redirect targets and absolute URLs embedded in authentication responses. The classification under [CWE-601] (URL Redirection to Untrusted Site) reflects the trust-boundary violation between the proxy layer and the application logic.

Exploitation requires user interaction. An attacker crafts a link or request that includes a manipulated X-Forwarded-Host value pointing to a domain under their control. When the victim authenticates or interacts with the affected interface, the application generates redirect URLs based on the attacker-supplied host. The browser follows the redirection, landing on a page that mimics the legitimate application and prompts for sensitive information.

Root Cause

The application reads the X-Forwarded-Host header and uses its value directly in URL generation routines without comparing it against an allow-list of trusted hostnames. Reverse-proxy headers should be considered untrusted unless the deployment topology guarantees they are stripped or rewritten at the network edge.

Attack Vector

The attack vector is network-based and requires user interaction. An attacker sends a request or persuades the victim to click a link that injects a malicious X-Forwarded-Host header. After authentication, the application issues a redirect response with a Location header containing the attacker's domain. The victim's browser silently follows the redirect, completing the phishing chain under the appearance of a legitimate login flow.

No public proof-of-concept or exploit code has been published for CVE-2026-10839. Refer to the INCIBE-CERT advisory on multiple password manager vulnerabilities for additional technical context.

Detection Methods for CVE-2026-10839

Indicators of Compromise

HTTP requests containing X-Forwarded-Host header values that do not match the application's canonical hostnames
Outbound 302 or 301 redirect responses from the authentication endpoint pointing to unfamiliar external domains
User reports of being redirected to login or interstitial pages on unexpected domains immediately after authenticating
Web server or proxy logs showing inconsistent Host and X-Forwarded-Host header pairs from the same client session

Detection Strategies

Inspect reverse-proxy and web application firewall logs for anomalous X-Forwarded-Host values containing external domains, IP literals, or punycode
Correlate authentication events with subsequent redirect responses to identify session flows that terminate on non-corporate domains
Deploy WAF rules that flag or block requests where X-Forwarded-Host differs from an allow-listed set of front-end hostnames

Monitoring Recommendations

Enable verbose logging of all HTTP headers on the authentication endpoint and ingest into a centralized analytics platform
Alert on outbound redirect responses whose Location header references domains outside the organization's approved list
Track repeated failed or unusual authentication flows from single source addresses that may indicate phishing reconnaissance

How to Mitigate CVE-2026-10839

Immediate Actions Required

Apply the vendor patch referenced in the INCIBE-CERT advisory once the affected product version is identified in your environment
Configure upstream reverse proxies to strip or overwrite client-supplied X-Forwarded-Host headers before they reach the application
Enforce an allow-list of approved hostnames within the authentication system's URL generation logic
Notify users about the phishing risk and instruct them to verify the domain in the browser address bar after login

Patch Information

Vendor patch information is referenced in the INCIBE-CERT security notice. Specific affected versions, fixed releases, and CPE identifiers are not enumerated in the NVD record at publication time. Administrators should consult the vendor advisory directly to confirm patch availability for their deployed version.

Workarounds

Terminate TLS at a hardened reverse proxy that explicitly sets X-Forwarded-Host to a known-good value rather than forwarding the client-supplied header
Implement server-side URL validation that rejects any redirect target whose host component is not in an approved allow-list
Disable or restrict the application's reliance on X-Forwarded-Host if the deployment does not require it for multi-tenant routing

bash

# Example nginx configuration to neutralize attacker-controlled X-Forwarded-Host
location / {
    proxy_set_header Host              $host;
    proxy_set_header X-Forwarded-Host  $host;
    proxy_set_header X-Forwarded-Proto $scheme;
    proxy_set_header X-Real-IP         $remote_addr;
    proxy_pass http://backend_app;
}