CVE-2026-23603 in Gitea
Summary
by MITRE • 08/13/2026
Blind SSRF in OAuth2 avatar synchronization via unvalidated OIDC picture claim
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Analysis
by VulDB Data Team • 08/13/2026
This vulnerability represents a critical blind server-side request forgery flaw within OAuth2 authentication flows that specifically targets the avatar synchronization mechanism. The issue manifests when an application processes user profile information from OpenID Connect providers, particularly focusing on the picture or avatar claim that is automatically retrieved during the authentication process. When the application fails to properly validate or sanitize the OIDC picture claim value, it creates an opportunity for attackers to manipulate the avatar synchronization process and potentially trigger unauthorized server requests to internal systems or external malicious endpoints.
The technical exploitation occurs through manipulation of the OpenID Connect provider's picture field, where an attacker can craft a malicious URL that gets processed during avatar update operations. This flaw operates as a blind SSRF because the application does not return the response from the malicious endpoint back to the attacker, making detection more challenging while still allowing for potential data exfiltration or internal network reconnaissance. The vulnerability leverages the trust relationship between the authentication provider and relying party, where legitimate requests are made on behalf of authenticated users but without proper input validation. This type of flaw commonly maps to CWE-918 Server-Side Request Forgery and can be categorized under ATT&CK technique T1071.004 Application Layer Protocol: DNS within the broader context of network infiltration activities.
The operational impact of this vulnerability extends beyond simple avatar manipulation, as it can enable attackers to perform internal port scanning, attempt access to sensitive internal services, or even facilitate further exploitation chains. Attackers might use this vulnerability to map internal network topology, probe for exposed services that are typically not accessible from the internet, or potentially exfiltrate data through carefully crafted requests that bypass standard security controls. The risk is particularly elevated in environments where applications maintain access to internal systems or where the OAuth2 flow is used by privileged users with elevated access rights.
Mitigation strategies should focus on implementing strict validation and sanitization of all external inputs from OIDC providers, including picture claims and other profile attributes. Organizations must ensure that avatar synchronization processes validate URL formats, restrict protocols to safe schemes such as https only, and implement proper input filtering to prevent malicious URLs from being processed. Network-level controls such as firewalls and proxy configurations should be implemented to restrict outbound requests from authentication servers, particularly to internal networks. Additionally, implementing a whitelist approach for acceptable avatar sources and employing secure coding practices that follow the principle of least privilege will significantly reduce the attack surface. Regular security testing including automated vulnerability scanning and manual penetration testing should be conducted to identify similar issues in other authentication flows within the application ecosystem.