OAuth2 Proxy - CraftedSignal Threat Feed

OAuth2 Proxy Authentication Bypass via X-Forwarded-Uri Header Spoofing

hello@craftedsignal.io — Mon, 29 Jan 2024 12:00:00 +0000

OAuth2 Proxy versions before 7.15.2 are susceptible to an authentication bypass vulnerability (CVE-2026-40575) when configured with both the --reverse-proxy flag and either --skip_auth_routes or --skip_auth_regex. This configuration flaw allows an attacker to spoof the X-Forwarded-Uri header, tricking OAuth2 Proxy into evaluating authentication and skip-auth rules against an attacker-controlled path rather than the actual request URI. The vulnerability exists because OAuth2 Proxy trusts client-supplied X-Forwarded-Uri headers. Version 7.15.2 introduces the --trusted-proxy-ip flag to mitigate this issue by allowing administrators to specify trusted reverse proxy IPs. However, upgrading alone is insufficient; the --trusted-proxy-ip flag must be configured, and additional mitigation steps are recommended to properly secure deployments.

Attack Chain

The attacker identifies an OAuth2 Proxy instance configured with --reverse-proxy and --skip_auth_routes (or --skip_auth_regex).
The attacker crafts a malicious HTTP request targeting a protected route.
The attacker adds an X-Forwarded-Uri header to the request, setting its value to a path configured in --skip_auth_routes.
The reverse proxy forwards the request, including the attacker-controlled X-Forwarded-Uri header, to the OAuth2 Proxy instance.
OAuth2 Proxy evaluates the X-Forwarded-Uri header against the --skip_auth_routes rules and incorrectly determines that authentication is not required.
OAuth2 Proxy forwards the request, now bypassing authentication, to the upstream application.
The upstream application processes the request, granting the attacker unauthorized access to the protected resource.
The attacker successfully accesses the protected route and performs unauthorized actions.

Impact

Successful exploitation of this vulnerability (CVE-2026-40575) allows unauthenticated remote attackers to bypass authentication and access protected routes without valid credentials. This could lead to complete compromise of the application behind the OAuth2 Proxy instance, including data theft, modification, or service disruption. The severity is critical as it directly undermines the authentication mechanism, potentially affecting any organization using OAuth2 Proxy with the vulnerable configuration. The number of affected organizations is currently unknown, but any deployment meeting the criteria is vulnerable.

Recommendation

Upgrade OAuth2 Proxy to version 7.15.2 or later and configure the --trusted-proxy-ip flag to explicitly define trusted reverse proxy IPs to mitigate CVE-2026-40575.
Implement reverse proxy or load balancer rules to strip or overwrite the X-Forwarded-Uri header from client requests, ensuring OAuth2 Proxy receives the correct request URI, as shown in the nginx example.
Restrict direct client access to OAuth2 Proxy, ensuring it can only be reached through a trusted reverse proxy to prevent attackers from directly injecting malicious headers.
Review and narrow --skip-auth-route / --skip-auth-regex rules where possible to minimize the attack surface and reduce the potential for authentication bypass.

OAuth2 Proxy Authentication Bypass Vulnerability (CVE-2026-41059)

hello@craftedsignal.io — Tue, 09 Jan 2024 12:00:00 +0000

OAuth2 Proxy, a reverse proxy providing authentication using OAuth2 providers, is susceptible to an authentication bypass vulnerability (CVE-2026-41059) affecting versions 7.5.0 through 7.15.1. This vulnerability arises when specific configurations are in place: the use of skip_auth_routes or the legacy skip_auth_regex with patterns vulnerable to attacker-controlled suffix widening (e.g., ^/foo/.*/bar$), and protected upstream applications that interpret # as a fragment delimiter. An attacker can exploit this by crafting requests containing a number sign (# or its encoded form %23) to match public allowlist rules, while the backend inadvertently serves protected resources. Organizations using OAuth2 Proxy within this vulnerable range should upgrade to version 7.15.2 or implement mitigations.

Attack Chain

An attacker identifies an OAuth2 Proxy instance running a vulnerable version (7.5.0 - 7.15.1) and configuration, specifically utilizing skip_auth_routes or skip_auth_regex with overly permissive patterns.
The attacker crafts a malicious HTTP request targeting a protected resource, embedding a # or %23 within the URI path. For example, a request to /foo/secret%23bar.
OAuth2 Proxy evaluates the request against the configured skip_auth_routes or skip_auth_regex rules. The crafted path bypasses the authentication check because the initial segment matches the allowed pattern (e.g., /foo/).
OAuth2 Proxy forwards the request to the upstream application without authentication.
The upstream application receives the request. Due to the presence of # or %23, the application might interpret the URL differently, possibly ignoring the fragment and routing the request to the protected resource (e.g., /foo/secret).
The upstream application, believing the request is legitimate, processes the request and potentially returns sensitive data.
The attacker receives the unauthorized response from the upstream application, successfully bypassing authentication.

Impact

Successful exploitation of CVE-2026-41059 allows unauthenticated attackers to access protected resources and sensitive data behind OAuth2 Proxy. The impact is highly dependent on the nature of the protected resources, potentially leading to data breaches, unauthorized access to administrative interfaces, and other security compromises. The number of affected organizations is unknown but depends on the prevalence of vulnerable OAuth2 Proxy configurations.

Recommendation

Upgrade OAuth2 Proxy to version 7.15.2 or later to patch CVE-2026-41059.
Tighten or remove skip_auth_routes and skip_auth_regex rules, especially patterns that use broad wildcards across path segments, as mentioned in the advisory for CVE-2026-41059.
Implement ingress, load balancer, or WAF rules to reject requests whose path contains %23 or #, as recommended in the CVE-2026-41059 advisory.
Deploy the Sigma rule "OAuth2 Proxy Authentication Bypass Attempt via URL Fragment" to detect exploitation attempts.

OAuth2 Proxy Authentication Bypass via X-Forwarded-Uri Spoofing

hello@craftedsignal.io — Tue, 02 Jan 2024 12:00:00 +0000

OAuth2 Proxy is a reverse proxy that provides authentication using OAuth2 providers. A critical vulnerability, CVE-2026-40575, affects OAuth2 Proxy versions 7.5.0 through 7.15.1. Specifically, when OAuth2 Proxy is configured with the --reverse-proxy flag enabled, and also utilizes either --skip-auth-regex or --skip-auth-route for specifying paths that should bypass authentication, an attacker can manipulate the X-Forwarded-Uri HTTP header. This manipulation allows the attacker to spoof the URI used for authentication checks, potentially bypassing authentication and gaining unauthorized access to protected routes. The vulnerability impacts deployments that rely on client-supplied X-Forwarded-Uri headers and is patched in version 7.15.2. Defenders should prioritize upgrading to the patched version or implementing the recommended mitigations to prevent unauthorized access.

Attack Chain

The attacker identifies an OAuth2 Proxy instance running a vulnerable version (7.5.0-7.15.1) with --reverse-proxy enabled and --skip-auth-regex or --skip-auth-route configured.
The attacker crafts a malicious HTTP request with a spoofed X-Forwarded-Uri header. The spoofed URI is chosen to match a --skip-auth-regex or --skip-auth-route rule.
The attacker sends the malicious request to the OAuth2 Proxy instance.
OAuth2 Proxy processes the request and evaluates the authentication and skip-auth rules against the spoofed URI from the X-Forwarded-Uri header.
Due to the spoofed URI matching a skip-auth rule, OAuth2 Proxy bypasses authentication for the request.
OAuth2 Proxy forwards the request to the upstream application, without proper authentication checks.
The upstream application processes the request, granting the attacker access to protected resources without valid credentials.
The attacker gains unauthorized access to sensitive data or functionality that should have been protected by authentication.

Impact

Successful exploitation of CVE-2026-40575 allows an unauthenticated remote attacker to bypass authentication and access protected routes without a valid session. This can lead to unauthorized access to sensitive data, modification of critical configurations, or execution of privileged operations within the targeted application. The impact is particularly severe for deployments that rely on OAuth2 Proxy for securing critical services and applications, potentially leading to significant data breaches and service disruptions. The exact number of potential victims is unknown but any deployment matching the vulnerable configuration is at risk.

Recommendation

Immediately upgrade OAuth2 Proxy to version 7.15.2 to patch CVE-2026-40575.
Implement mitigations if immediate upgrade is not possible: strip client-provided X-Forwarded-Uri headers at the reverse proxy or load balancer level.
Explicitly overwrite X-Forwarded-Uri with the actual request URI before forwarding requests to OAuth2 Proxy as mentioned in the advisory.
Restrict direct client access to OAuth2 Proxy so it can only be reached through a trusted reverse proxy.
Deploy the Sigma rule Detect Suspicious X-Forwarded-Uri Header to monitor for potential exploitation attempts.