CVE-2020-15240

Executive Summary

CVE-2020-15240 is a critical authentication bypass vulnerability in the omniauth-auth0 Ruby gem (versions >= 2.3.0 and < 2.4.1) that allows attackers to bypass JWT signature validation. Applications using the jwt_validator.verify method directly — or those not following the SDK's default Authorization Code Flow — are affected, enabling unauthorized access to protected resources. The issue is remediated in version 2.4.1, with patch confirmation from two independent sources.

Technical Analysis

Vulnerability Mechanism: The omniauth-auth0 gem implements JWT validation via a JWTValidator.verify method. In affected versions (>= 2.3.0, < 2.4.1), a regression was introduced that causes the signature validation to be improperly performed. The NVD description explicitly states that the token signature is "improperly validate[d]" when using the jwt_validator.verify method.

Root Cause Classification: This flaw belongs to the CWE-347 class (Improper Verification of Cryptographic Signature). The regression likely involves one or more of the following mechanisms (based on the vulnerability class, not confirmed from the description):

• The verification algorithm is not bound to the token header (algorithm confusion attack)
• The signature check is skipped under certain conditions (e.g., when alg: "none" is accepted)
• The public key used for verification is not properly loaded or matched

Attack Surface: The vulnerable code path is triggered in two specific scenarios:

1. Direct use of JWTValidator.verify method (the method name is explicitly named in the NVD description)
2. Any authentication flow that does NOT use the SDK's default Authorization Code Flow

The attack surface is server-side: an attacker can send a crafted JWT token to any endpoint that relies on the affected validation logic.

Technical Detail from Description: The NVD description provides no additional technical specifics regarding the exact nature of the signature validation bypass. It does not state whether the issue is related to algorithm confusion, missing public key verification, or is a comparison-timing vulnerability. This lack of detail limits the ability to provide code-level analysis beyond what is explicitly stated.

Affected Component: The vulnerability exists in the omniauth-auth0 Ruby gem, which integrates Auth0 authentication into Ruby applications (typically Rails or Sinatra). The JWTValidator.verify method is the specific affected code path.

Exploitation Scenarios

Preconditions for Exploitation:

• Target application is using omniauth-auth0 version >= 2.3.0 and < 2.4.1
• The application either calls JWTValidator.verify directly OR uses a non-default authentication flow
• The attacker can reach the authentication/validation endpoint

Attack Chain (inferred from vulnerability class due to insufficient technical detail):

1. Reconnaissance/Target Identification:

   • Attacker identifies a Ruby web application that likely uses Auth0 for authentication (evidenced by Auth0 redirect patterns, login pages, or exposed Ruby session cookies)
   • Attacker fingerprints the omniauth-auth0 version by examining gem version in error messages, bundle manifests, or public repository files
   • Attacker verifies the target is running an affected version (>= 2.3.0, < 2.4.1)

2. Triggering the Flaw:

   • Attacker crafts a JWT token with a signature that would normally fail validation
   • The attacker submits this token to an authentication endpoint or protected resource
   • Due to the improper validation in JWTValidator.verify, the token is accepted as valid

3. Payload/Execution Method (vulnerability-class level):

   • If algorithm confusion is the cause: Attacker crafts a JWT with "alg": "HS256" while the server expects RS256, and signs it with the server's public key (which is generally accessible). The server improperly uses the public key for HMAC verification, accepting the forged token.
   • If "alg: none" is accepted: Attacker removes the signature entirely and sets "alg": "none", causing the validator to skip signature verification.
   • If the signature check is conditionally skipped: Attacker manipulates JWT claims (e.g., iss, aud) to trigger a bypass condition.

4. Post-Exploitation Impact:

   • Attacker impersonates any user by forging tokens with arbitrary sub (subject) claims
   • Attacker escalates privileges to admin-level accounts
   • Attacker gains access to user data, performs unauthorized actions, or moves laterally within the application environment

Known Exploitation: No public PoC or exploit code is currently available. The EPSS score of 0.00803 suggests low active exploitation probability. The attack chain described above is inferred from the vulnerability class (JWT signature validation bypass) and the explicit mention of the JWTValidator.verify method in the NVD description.

Impact Assessment

CVSS Analysis (Score: 7.4 — High):

• Attack Vector (AV): Network — The vulnerability is exploitable remotely
• Attack Complexity (AC): Low — No special conditions required beyond reaching the vulnerable endpoint
• Privileges Required (PR): None — No authentication needed to exploit
• User Interaction (UI): None — No victim interaction required
• Confidentiality Impact (C): High — Complete bypass of authentication mechanisms
• Integrity Impact (I): None — No direct data modification indicated
• Availability Impact (A): None — No denial-of-service impact

Business Impact:

• Authentication Bypass: The core impact is complete bypass of authentication and authorization controls
• Data Breach Potential: Attacker can access any user's data, including PII, financial records, or proprietary business data
• Regulatory Exposure: GDPR, HIPAA, or PCI-DSS violations may result from unauthorized data access
• Reputation Damage: Successful exploitation can lead to loss of customer trust

Affected Systems:

• All Ruby applications using omniauth-auth0 versions >= 2.3.0 and < 2.4.1
• Particularly critical for authentication services, API backends, and customer-facing web applications
• The RubyGems distribution model means affected versions may be widely deployed across many applications

Blast Radius:

• If used as a central identity provider, the vulnerability could compromise all connected applications
• In microservice architectures, a compromised token could grant access to multiple backend services
• The ease of exploitation (network-accessible, no preconditions) increases the severity

Detection Guidance

Sigma Rule

title: OmniAuth-Auth0 JWT Signature Bypass Attempt
id: cve-2020-15240-omniauth-auth0-jwt-bypass
status: experimental
description: Detects requests with malformed JWT tokens targeting omniauth-auth0 endpoints, potentially indicating CVE-2020-15240 exploitation
references:
    - https://nvd.nist.gov/vuln/detail/CVE-2020-15240
author: ValtersIT Threat Intelligence
date: 2024/01/01
tags:
    - attack.t1190
    - cve.2020.15240
logsource:
    category: application
    product: web_application
detection:
    selection_auth_endpoint:
        cs-method: 'POST'
        cs-uri-query|contains:
            - '/auth/'
            - '/callback'
    selection_jwt_payload:
        request_body|contains:
            - 'id_token'
            - 'assertion'
    selection_suspicious_jwt:
        request_body|re:
            - '"alg"\s*:\s*"none"'
            - '"alg"\s*:\s*"HS256"'
            - 'eyJ.*\.eyJ.*\.$'
    condition: selection_auth_endpoint and selection_jwt_payload and selection_suspicious_jwt
    falsepositives:
        - Legitimate tokens with unusual but valid algorithms
        - Test environments using algorithm switching for development
level: high

YARA Rule

rule CVE_2020_15240_OmniAuth_Auth0_JWT_Exploit
{
    meta:
        author = "ValtersIT Threat Intelligence"
        description = "Detects JWT tokens with algorithm confusion or missing signatures associated with CVE-2020-15240"
        cve = "CVE-2020-15240"
        date = "2024-01-01"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2020-15240"
        severity = "high"
    
    strings:
        $alg_none = /"alg"\s*:\s*"none"/
        $alg_hs256 = /"alg"\s*:\s*"HS256"/
        $alg_rs256 = /"alg"\s*:\s*"RS256"/
        $jwt_pattern = /eyJ[A-Za-z0-9_-]+\.eyJ[A-Za-z0-9_-]+\./
        $auth0_issuer = /"iss"\s*:\s*"https:\/\/[a-z0-9-]+\.auth0\.com\//
        $auth0_audience = /"aud"\s*:\s*"[a-zA-Z0-9_-]+"/
        $empty_sig = /\.[A-Za-z0-9_-]{0,10}$/
    
    condition:
        any of ($alg_*) and $jwt_pattern and any of ($auth0_*) and #jwt_pattern > 0
}

Suricata IDS Rule

alert http any any -> any any (msg:"CVE-2020-15240 - OmniAuth-Auth0 JWT Bypass Attempt"; flow:established,to_server; content:"POST"; http_method; content:"/auth/"; http_uri; nocase; content:"id_token"; http_client_body; nocase; pcre:"/\x22alg\x22\s*:\s*\x22none\x22/i"; sid:2020152401; rev:1; reference:cve,2020-15240; metadata:attack_target Web_Server, deployment Perimeter, signature_severity Major, created_at 2024_01_01; classtype:attempted-admin;)

Elastic Detection

# Elastic EQL Query
sequence by host.hostname with maxspan=30s
  [process where process.name == "ruby" and 
   process.args == "auth" and 
   event.action == "process_execution"]
  [file where process.name == "ruby" and 
   file.name == "omniauth-auth0.rb" and 
   file.extension == "rb" and 
   event.action == "read"]
  [authentication where 
   authentication.type == "jwt" and 
   authentication.status == "success" and 
   event.outcome == "success" and 
   network.protocol == "http" and 
   url.path like "*auth*"]

Splunk SPL Query

index=web sourcetype=access_combined earliest=-24h
| where like(_raw, "%auth0%") OR like(_raw, "%omniauth%")
| regex _raw="(alg|token|jwt|signature)"
| eval has_alg_none=if(match(_raw, "\"alg\"\\s*:\\s*\"none\""), 1, 0)
| eval has_malformed_jwt=if(match(_raw, "eyJ[A-Za-z0-9_-]+\\.eyJ[A-Za-z0-9_-]+\\.(\\s|$)"), 1, 0)
| eval is_auth_endpoint=if(match(uri_path, "auth|callback|token"), 1, 0)
| where has_alg_none=1 OR has_malformed_jwt=1 OR is_auth_endpoint=1
| stats count by src_ip, uri_path, status, has_alg_none, has_malformed_jwt
| sort -count
| rename src_ip as "Source IP", uri_path as "URI Path", status as "HTTP Status", has_alg_none as "Algorithm None Detected", has_malformed_jwt as "Malformed JWT"

Wazuh OSSEC Rule

<group name="authentication,">
  <rule id="100240" level="10">
    <decoded_as>json</decoded_as>
    <field name="url" type="pcre2">/auth/.+</field>
    <description>CVE-2020-15240: Potential JWT bypass attempt against omniauth-auth0 endpoint</description>
    <mitre>
      <id>T1190</id>
    </mitre>
    <group>auth0_jwt_bypass,</group>
  </rule>
  
  <rule id="100241" level="10">
    <decoded_as>json</decoded_as>
    <field name="body" type="pcre2">"alg"\s*:\s*"none"</field>
    <if_sid>100240</if_sid>
    <description>CVE-2020-15240: JWT token with 'alg: none' detected on auth endpoint</description>
    <group>auth0_jwt_alg_none,</group>
  </rule>
  
  <rule id="100242" level="12">
    <decoded_as>json</decoded_as>
    <field name="body" type="pcre2">eyJ[A-Za-z0-9_-]+\.eyJ[A-Za-z0-9_-]+\.</field>
    <if_sid>100241</if_sid>
    <description>CVE-2020-15240: Malformed JWT signature detected - possible exploitation attempt</description>
    <group>auth0_jwt_malformed,</group>
  </rule>
</group>

Mitigation & Patch Guidance

Immediate Mitigations (before patching):

1. Upgrade Immediately: The only verified fix is to upgrade omniauth-auth0 to version 2.4.1 or later
2. Identify Affected Applications: Audit all Ruby applications for Gemfile.lock entries containing omniauth-auth0 version >= 2.3.0 and < 2.4.1
3. Restrict Access: If immediate upgrade is not possible, restrict network access to authentication endpoints behind a VPN or IP allowlist
4. Enable Logging: Enable comprehensive audit logging for all authentication events to detect anomalous JWT patterns
5. Monitor for Algorithm Manipulation: Look for tokens with "alg": "none", mismatch between alg and kid header parameters, or unusually short signatures

Patch/Upgrade Steps:

1. Update the Gemfile: gem 'omniauth-auth0', '>= 2.4.1'
2. Run bundle update omniauth-auth0
3. Verify version: bundle list | grep omniauth-auth0 (should show 2.4.1 or later)
4. Run the full test suite, especially authentication-related tests
5. Deploy and monitor post-deployment authentication logs for anomalies

Long-Term Hardening:

1. Standardize on Authorization Code Flow: Ensure all applications use the SDK's default Authorization Code Flow rather than custom validation with JWTValidator.verify
2. Implement Defense in Depth: Add a separate signature verification layer (e.g., using jose or ruby-jwt library directly) to validate tokens independently of the SDK
3. Ensure Algorithm Pinning: Configure JWT validation to explicitly pin the expected algorithm (RS256) and reject any tokens using other algorithms
4. Multi-Factor Authentication: Enforce MFA for all accounts to reduce the impact of token forgery
5. Regular Security Audits: Include JWT validation logic in routine security code reviews and penetration testing
6. Monitoring and Alerting: Implement detection logic from the Detection Guidance section above in your SIEM/SOC infrastructure

References

1. NVD Entry: https://nvd.nist.gov/vuln/detail/CVE-2020-15240
2. CNA Advisory (Auth0): https://github.com/auth0/omniauth-auth0/security/advisories/GHSA-58r4-qr6h-4pr9
3. RubyGems Package: https://rubygems.org/gems/omniauth-auth0 — versions 2.4.1+ contain the patch
4. Debian Security Tracker: https://security-tracker.debian.org/tracker/CVE-2020-15240 — patch confirmation from Debian
5. GitHub Repository: https://github.com/auth0/omniauth-auth0 — review commit history for the 2.4.1 release tag

Threat Actor Activity: No known threat actor campaigns or exploit-in-the-wild evidence has been publicly documented for this vulnerability. The CISA KEV list does not include this CVE, and no public PoC exists. The EPSS score of 0.00803 (53.4% percentile) indicates low exploitation probability, but the authentication bypass nature of the flaw warrants immediate patching if affected versions are in use.