CVE-2026-31381: OAuth Information Disclosure Vulnerability

CVE-2026-31381 Overview

CVE-2026-31381 is an information disclosure vulnerability affecting the Gainsight Assist plugin. An attacker can extract user email addresses (PII) exposed in base64 encoding via the state parameter in the OAuth callback URL. This vulnerability allows unauthorized extraction of personally identifiable information without requiring authentication or user interaction.

Critical Impact

User email addresses are exposed in base64 encoding within OAuth callback URLs, enabling attackers to harvest PII through network traffic interception or URL logging.

Affected Products

• Gainsight Assist Plugin (versions prior to remediation)

Discovery Timeline

• 2026-03-20 - CVE-2026-31381 published to NVD
• 2026-03-24 - Last updated in NVD database

Technical Details for CVE-2026-31381

Vulnerability Analysis

This vulnerability is classified under CWE-598 (Use of GET Request Method With Sensitive Query Strings), which occurs when sensitive information is transmitted via URL parameters that can be logged, cached, or exposed in browser history. The Gainsight Assist plugin improperly includes user email addresses in the state parameter during OAuth authentication flows.

While the email addresses are base64 encoded, this encoding provides no security benefit as base64 is a reversible encoding scheme, not encryption. Any attacker with access to the OAuth callback URL can trivially decode the state parameter to extract the user's email address. This exposure can occur through various channels including browser history, server logs, proxy logs, and referrer headers.

Root Cause

The root cause of this vulnerability is the improper handling of sensitive user data during the OAuth authentication flow. The application includes the user's email address directly in the state parameter of the OAuth callback URL, using only base64 encoding which provides no confidentiality protection. Sensitive PII should never be transmitted via URL parameters, particularly in authentication flows where URLs may be logged by multiple intermediary systems.

Attack Vector

The attack vector is network-based and requires no authentication or user interaction. An attacker can exploit this vulnerability through several methods:

1. Network Traffic Interception: Monitoring network traffic to capture OAuth callback URLs containing the encoded email addresses
2. Log File Access: Accessing web server logs, proxy logs, or application logs that record the full OAuth callback URLs
3. Browser History/Cache: Gaining access to a user's browser history or cache where the callback URLs are stored
4. Referrer Header Leakage: The OAuth callback URL containing the encoded email may be transmitted in HTTP Referrer headers to external resources

The base64-encoded state parameter can be decoded using any standard base64 decoding tool or function, revealing the user's email address in plaintext.

Detection Methods for CVE-2026-31381

Indicators of Compromise

• Unusual volume of requests to OAuth callback endpoints from unexpected IP addresses
• Evidence of URL parameter tampering or enumeration attempts against OAuth endpoints
• Suspicious access patterns to web server or proxy logs containing OAuth URLs
• Base64-encoded strings in state parameters being decoded or extracted in log analysis

Detection Strategies

• Monitor web server and application logs for suspicious access to OAuth callback URLs
• Implement anomaly detection for high-frequency requests to authentication endpoints
• Review network traffic for evidence of OAuth URL harvesting or parameter extraction
• Audit access to log files that may contain OAuth callback URLs with sensitive parameters

Monitoring Recommendations

• Enable detailed logging on OAuth authentication endpoints while excluding sensitive parameters
• Deploy web application firewall (WAF) rules to detect unusual OAuth flow patterns
• Implement real-time alerting for suspicious authentication endpoint access
• Monitor for mass enumeration or scanning of OAuth callback endpoints

How to Mitigate CVE-2026-31381

Immediate Actions Required

• Update Gainsight Assist plugin to the latest remediated version immediately
• Review and clear web server logs, proxy logs, and application logs that may contain exposed email addresses
• Notify affected users about potential email address exposure
• Audit browser histories and caches on shared systems for OAuth callback URLs

Patch Information

Gainsight has released a remediation for this vulnerability. Administrators should update the Gainsight Assist plugin to the latest version as documented in the Gainsight Community News Update. Additional technical details are available in the Rapid7 Blog Post.

Workarounds

• Implement network-level monitoring to detect and alert on OAuth callback URL harvesting attempts
• Configure web application firewalls to mask or exclude sensitive parameters from logging
• Restrict access to log files that may contain OAuth callback URLs
• Consider implementing additional encryption or tokenization for state parameters as a defense-in-depth measure until the patch can be applied