CVE-2020-27846: Grafana Auth Bypass Vulnerability

CVE-2020-27846 Overview

A signature verification vulnerability exists in the crewjam/saml library that allows attackers to bypass SAML authentication. This flaw stems from improper validation of XML signatures in SAML assertions, enabling malicious actors to forge authentication responses and gain unauthorized access to protected systems. The vulnerability poses the highest threat to confidentiality, integrity, and system availability, making it particularly dangerous for organizations relying on SAML-based single sign-on (SSO) implementations.

Critical Impact

This authentication bypass vulnerability allows unauthenticated attackers to forge SAML assertions and gain complete unauthorized access to affected systems, potentially compromising sensitive data and enabling full system takeover.

Affected Products

• Grafana Enterprise (versions prior to 6.7.5, 7.2.3, and 7.3.6)
• crewjam/saml library
• Red Hat OpenShift Container Platform 3.11 and 4.0
• Red Hat OpenShift Service Mesh 2.0
• Red Hat Enterprise Linux 8.0
• Fedora 32 and 33

Discovery Timeline

• December 21, 2020 - CVE-2020-27846 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2020-27846

Vulnerability Analysis

The vulnerability resides in the crewjam/saml library's signature verification mechanism, which is used by multiple enterprise applications including Grafana Enterprise for SAML-based authentication. The flaw allows attackers to craft malicious SAML responses that bypass signature validation checks, effectively allowing them to impersonate any user without valid credentials. This authentication bypass stems from weaknesses in Go's XML parsing libraries and how the crewjam/saml library handles XML signatures.

The issue is classified under CWE-115 (Misinterpretation of Input), indicating that the vulnerability occurs when the system incorrectly interprets or processes XML input during signature verification. When exploited, an attacker can manipulate SAML assertions to authenticate as any user, including administrators, without knowing their credentials.

Root Cause

The root cause of this vulnerability lies in how the crewjam/saml library processes and validates XML signatures within SAML assertions. Due to inconsistencies in Go's XML parsing, the library may validate a signature against one portion of the XML document while a different portion is actually processed for authentication decisions. This discrepancy allows attackers to inject malicious content that passes signature verification but results in unauthorized authentication.

Attack Vector

This vulnerability is exploitable over the network without requiring any privileges or user interaction. An attacker can intercept or craft SAML authentication responses and modify them to bypass signature verification. The attack flow typically involves:

1. The attacker targets a SAML Service Provider (SP) using the vulnerable crewjam/saml library
2. A crafted SAML response is submitted that exploits XML parsing inconsistencies
3. The malicious response passes signature verification due to the parsing flaw
4. The attacker gains authenticated access as any user they choose to impersonate

The vulnerability mechanism exploits the discrepancy between XML parsing and signature validation. Attackers can craft SAML responses where the signed content differs from the content ultimately processed for authentication decisions. Technical details are available in the GitHub Security Advisory and the Mattermost coordinated disclosure blog post.

Detection Methods for CVE-2020-27846

Indicators of Compromise

• Unusual SAML authentication requests with malformed or suspicious XML structures in SAML responses
• Authentication events for privileged accounts from unexpected IP addresses or geographic locations
• Multiple successful authentications for different users originating from the same source IP
• Log entries showing SAML assertions with anomalous XML namespace declarations or duplicate elements
• Sudden spikes in SAML authentication traffic or authentication attempts outside normal business hours

Detection Strategies

• Monitor SAML authentication logs for successful logins that lack corresponding identity provider (IdP) authorization records
• Implement XML parsing validation that detects discrepancies between signed content and processed content
• Deploy network monitoring to identify SAML responses with unusual XML structures or duplicate assertion elements
• Configure alerting for administrative account authentications from new or untrusted locations

Monitoring Recommendations

• Enable detailed logging on all SAML Service Providers and correlate with Identity Provider logs
• Implement behavioral analytics to detect unusual authentication patterns indicative of credential abuse
• Monitor for lateral movement following SAML-based authentications, particularly from newly authenticated sessions
• Review authentication audit logs regularly for anomalous access patterns or privilege escalation attempts

How to Mitigate CVE-2020-27846

Immediate Actions Required

• Update Grafana Enterprise immediately to version 6.7.5, 7.2.3, or 7.3.6 depending on your current version branch
• Update the crewjam/saml library to version 0.4.3 or later in all applications that depend on it
• Apply Red Hat security patches for OpenShift Container Platform and Enterprise Linux systems
• Review authentication logs for signs of exploitation that may have occurred before patching

Patch Information

Grafana has released security fixes in versions 6.7.5, 7.2.3, and 7.3.6 that address this vulnerability. Organizations should apply these patches immediately. The crewjam/saml library maintainers have released version 0.4.3 with the necessary fixes. Red Hat has issued advisories and patches for affected OpenShift and Enterprise Linux products. For detailed patch information, refer to the Grafana Security Release and the Red Hat Bug Report.

Workarounds

• If immediate patching is not possible, consider temporarily disabling SAML authentication and reverting to alternative authentication methods
• Implement additional network-level controls to restrict SAML authentication requests to trusted identity providers only
• Enable strict XML validation at the web application firewall (WAF) level to detect malformed SAML assertions
• Monitor authentication events closely and be prepared to disable SAML authentication if exploitation attempts are detected

# Grafana upgrade example (Debian/Ubuntu)
sudo apt-get update
sudo apt-get install grafana-enterprise=7.3.6

# Verify installed version
grafana-server -v

# Restart Grafana service after upgrade
sudo systemctl restart grafana-server