CVE-2026-31381 in Assist
Summary
by MITRE • 03/20/2026
An attacker can extract user email addresses (PII) exposed in base64 encoding via the state parameter in the OAuth callback URL.
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Analysis
by VulDB Data Team • 03/26/2026
This vulnerability represents a critical information disclosure flaw that occurs within OAuth authentication flows where sensitive user data becomes exposed through improper handling of the state parameter. The vulnerability specifically affects systems that encode user email addresses or other personally identifiable information within the OAuth state parameter and subsequently transmit this data in base64 encoded format within the callback URL. The flaw stems from inadequate input validation and output encoding practices that allow attackers to intercept and decode the base64 encoded data, thereby extracting user email addresses and potentially other PII elements that should remain confidential during the authentication process. This type of vulnerability aligns with CWE-209, which addresses information exposure through improper error handling, and CWE-312, which covers exposure of sensitive information through data encoding.
The technical implementation of this vulnerability exploits the fundamental trust model of OAuth protocols where the state parameter serves as a security mechanism to prevent cross-site request forgery attacks. When systems incorrectly encode user identifiers or personal information within this parameter, they inadvertently create a data leakage vector that attackers can exploit through man-in-the-middle monitoring or session interception techniques. The base64 encoding does not provide cryptographic security and merely obfuscates the data rather than protecting it, making it trivial for attackers to decode and extract the sensitive information. This flaw particularly impacts web applications that integrate with third-party authentication providers where the state parameter is used to maintain session state and user context between the authentication request and callback.
The operational impact of this vulnerability extends beyond simple information disclosure to potentially enable more sophisticated attacks such as credential stuffing, account takeover attempts, and social engineering operations. Attackers who successfully extract user email addresses can leverage this information for targeted phishing campaigns, password spraying attacks, or to build comprehensive user profiles for further exploitation. The vulnerability affects organizations across various sectors including financial services, healthcare, and e-commerce where user privacy and data protection are paramount. From an attacker perspective, this represents a low-effort, high-impact vector that can be exploited through passive network monitoring or active interception techniques, making it particularly dangerous in environments with insufficient network security controls.
Organizations should implement comprehensive mitigation strategies that include removing sensitive data from OAuth state parameters, implementing proper input validation and sanitization, and employing cryptographic signing mechanisms for state parameters rather than simple base64 encoding. The recommended approach involves using secure random tokens for the state parameter that do not contain any user identifiable information, while maintaining session state through secure server-side storage mechanisms. Additionally, organizations should implement proper network security controls including encrypted communication channels, traffic monitoring, and access controls to prevent unauthorized interception of callback URLs. The mitigation strategy should align with NIST SP 800-53 security controls and follow the principle of least privilege for OAuth implementations. This vulnerability also maps to ATT&CK technique T1566, which covers credential harvesting through social engineering, and T1071.004, which addresses application layer protocol usage for data exfiltration.