CVE-2026-8367 in aria2c
Summary
by MITRE • 05/13/2026
aria2c accepts a server certificate with incorrect Extended Key Usage (EKU). If the attackers compromise a certificate (with the associated private key) issued for a different purpose, they may be able to reuse it for TLS server authentication.
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Analysis
by VulDB Data Team • 08/19/2026
The vulnerability in aria2c involves a critical flaw in the certificate validation process that undermines the security of TLS communications. This issue stems from the software's permissive handling of Extended Key Usage extensions within X.509 certificates, allowing it to accept certificates that have been issued for purposes other than server authentication. The vulnerability represents a failure in certificate chain validation that directly violates fundamental principles of public key infrastructure security. When a certificate contains an incorrect EKU extension, it should not be trusted for TLS server authentication, yet aria2c's implementation permits such certificates to establish secure connections. This weakness creates a significant attack surface where malicious actors can exploit the misconfigured validation logic to impersonate legitimate servers.
The technical flaw manifests in the certificate verification routine where aria2c fails to properly enforce the Extended Key Usage extension requirements. According to the X.509 standard and certificate validation specifications, servers must present certificates that include the server authentication EKU (OID 1.3.6.1.5.5.7.3.1) within their certificate chain. The absence of this validation step allows certificates with different EKUs such as client authentication (OID 1.3.6.1.5.5.7.3.2) or code signing (OID 1.3.6.1.5.5.7.3.3) to be accepted for server authentication purposes. This behavior directly correlates to CWE-295, which addresses improper certificate validation, and represents a failure in certificate trust validation that undermines the entire TLS security model. The vulnerability enables a man-in-the-middle attack scenario where an attacker can present a compromised certificate that was originally issued for client authentication but is accepted by aria2c for server authentication.
The operational impact of this vulnerability extends beyond simple certificate validation failures and creates a substantial risk to secure communications. When aria2c accepts certificates with incorrect EKU values, it effectively removes a critical security control that prevents certificate misuse. Attackers can exploit this weakness by obtaining certificates for legitimate purposes such as client authentication or code signing, then repurposing them to impersonate servers during TLS negotiations. This capability directly maps to ATT&CK technique T1573.002, which covers "Encrypted Channels" and represents a compromise of the trust model that TLS is designed to establish. The vulnerability is particularly dangerous because it operates silently, allowing malicious certificates to establish secure connections without alerting users or administrators to the compromised authentication process.
Mitigation strategies must address both the immediate certificate validation issue and broader security posture improvements. Organizations should immediately update to patched versions of aria2c that properly enforce EKU validation requirements, ensuring that only certificates with appropriate server authentication EKU extensions are accepted. System administrators should implement certificate monitoring and validation procedures that verify EKU compliance for all TLS certificates in use. The implementation should include checks against the specific OID values required for server authentication and reject any certificates that do not meet these requirements. Additionally, organizations should consider implementing certificate pinning mechanisms as an additional layer of protection, ensuring that only specific trusted certificates are accepted for particular server endpoints. Security teams must also conduct thorough audits of all TLS certificate usage within their environments to identify and remediate any certificates that may have been issued with incorrect EKU values, thereby preventing potential exploitation of this vulnerability across their infrastructure.