CVE-2026-82639 in NextChat
Summary
by MITRE • 08/30/2026
NextChat versions from 2.15.8 through 2.16.1 contain an improper URL validation vulnerability in the proxy endpoint that allows attackers to obtain the server's OpenAI API key. The x-base-url header is validated using substring matching instead of hostname parsing, allowing any URL containing 'api.openai.com' to pass validation and receive the server's credentials in the Authorization header.
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Analysis
by VulDB Data Team • 08/30/2026
The vulnerability identified in NextChat versions ranging from 2.15.8 through 2.16.1 represents a critical security flaw rooted in improper input validation within the application proxy endpoint architecture. This issue specifically affects how the software handles URL redirection and proxying requests to external API services, such as OpenAI. The core technical deficiency lies in the mechanism used to validate the target host specified by the client via the x-base-url header. Instead of employing robust hostname parsing techniques that strictly compare domain names against a whitelist or perform exact string matching on fully qualified domain names, the application relies on simple substring matching. This approach is fundamentally insecure because it fails to account for subdomain variations and DNS rebinding attacks, where an attacker can manipulate the host field in HTTP requests to bypass security controls designed to restrict access to trusted endpoints only.
From a technical perspective, the flaw allows any URL containing the string api.openai.com anywhere within its hostname structure to pass validation checks. For instance, if an application validates that a target must be api.openai.com by checking for this substring, it will incorrectly accept maliciously crafted hostnames such as evil-api.openai.com.evil.attacker.com or even openai.com.evil-attacker.com depending on the specific implementation of the substring check and subsequent DNS resolution. This lack of strict domain validation enables an attacker to redirect proxy requests through their own controlled infrastructure while maintaining the appearance of legitimate traffic destined for OpenAI services. The vulnerability is classified under CWE-20, which covers Improper Input Validation, as well as CWE-918, addressing Server-Side Request Forgery (SSRF) weaknesses where insufficient validation allows interaction with internal or external resources in unintended ways.
The operational impact of this vulnerability is severe due to the potential for credential theft and unauthorized API usage. When a user interacts with a maliciously configured instance of NextChat that exploits this flaw, the application may inadvertently forward requests containing sensitive authentication tokens through the attacker-controlled proxy endpoint. Specifically, because the x-base-url header dictates where the request is sent, an attacker can configure their server to intercept these proxied requests before they reach the actual OpenAI API servers. As a result, the Authorization headers carrying the user's or administrator's OpenAI API keys are exposed to the malicious party. This compromise allows attackers to gain full access to the victim's AI service account, leading to potential financial loss through unauthorized token consumption, data privacy breaches via prompt injection and response interception, and further exploitation of other services linked to that identity.
This attack vector aligns with MITRE ATT&CK techniques related to Command and Control or Exfiltration over Alternative Protocol mechanisms, where attackers leverage legitimate application functionality to steal credentials. The vulnerability underscores the importance of implementing strict domain validation policies in any software acting as a proxy or gateway for sensitive API calls. Mitigation strategies must include replacing substring matching with rigorous hostname parsing libraries that ensure exact domain name equality and prevent subdomain spoofing. Additionally, developers should implement allowlists for permitted target domains rather than relying on negative filtering or loose string comparisons. For organizations currently running affected versions of NextChat, immediate upgrading to a patched version is essential. In the interim, restricting network egress policies and monitoring API usage patterns can help detect anomalous behavior indicative of credential exfiltration attempts until the software update is applied.