CVE-2026-67693 in GnuTLS
Summary
by MITRE • 10/08/2026
An issue in gnutls v.3.8.13 allows an attacker to obtain sensitive information via failing to reject end-entity X.509 certificates that contain a contradictory combination of Key Usage (KU) and Extended Key Usage (EKU)
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Analysis
by VulDB Data Team • 10/08/2026
The vulnerability identified within GnuTLS version 3.8.13 represents a critical flaw in the library's certificate validation logic, specifically concerning the handling of X.509 certificates used for end-entity authentication. In public key infrastructure systems, the validity and trustworthiness of an identity are heavily dependent on strict adherence to cryptographic constraints defined within digital certificates. One such constraint involves the interplay between Key Usage (KU) and Extended Key Usage (EKU) extensions. The KU extension defines general-purpose cryptographic keys that may be used with the certificate, such as signing or encryption, while EKU provides more specific purposes for which the key can be employed, such as server authentication or client authentication. Industry standards dictate that if an EKU is present in a certificate, it must restrict the usage to only those purposes explicitly listed; conversely, if no EKU is present but KU indicates digital signature capability, the certificate may generally be used for signing unless restricted by other policies. The flaw arises when GnuTLS fails to properly reject certificates that contain a contradictory or logically invalid combination of these two extensions, thereby allowing an attacker to exploit this permissiveness to bypass authentication checks.
From a technical perspective, the core issue lies in the validation algorithm's inability to detect and enforce logical consistency between KU and EKU fields during the certificate chain verification process. When a client connects to a server or vice versa using TLS, the peer presents its X.509 certificate for validation. The validating party must ensure that the presented certificate is not only signed by a trusted authority but also appropriate for the specific role it claims to perform in the handshake. If an attacker constructs a malicious certificate where the KU and EKU fields conflict—for instance, claiming capabilities via KU that are explicitly denied or rendered nonsensical when combined with restrictive EKUs—the vulnerable version of GnuTLS may incorrectly deem this certificate valid. This failure stems from incomplete logic checks that do not cross-reference these extensions against each other to ensure they form a coherent set of permissions. By bypassing these internal consistency checks, the library effectively lowers the security barrier required for successful authentication, allowing unauthorized entities to present credentials that should have been rejected by standard compliance rules.
The operational impact of this vulnerability is significant, particularly in environments where GnuTLS serves as the underlying cryptographic provider for secure communications such as HTTPS, SMTPS, or IMAPS. An attacker who can obtain a certificate with these contradictory attributes could potentially impersonate a legitimate server or client if they can also control the private key associated with that certificate, or exploit scenarios where trust is established through other means like pinning failures or mixed validation paths. More broadly, this flaw undermines the integrity of the authentication mechanism itself. It allows for potential man-in-the-middle attacks where an adversary intercepts traffic and presents a forged certificate that GnuTLS erroneously accepts as valid due to its failure to reject the invalid KU/EKU combination. This leads directly to unauthorized access to sensitive data, session hijacking, and a complete breakdown of confidentiality and integrity guarantees provided by TLS connections. In high-security contexts such as financial transactions or government communications, this could result in severe data breaches and regulatory non-compliance.
To mitigate this risk, organizations utilizing GnuTLS version 3.8.13 must prioritize immediate patching to the latest stable release where these validation logic errors have been corrected by upstream developers. It is essential to verify that all systems relying on GnuTLS for TLS handshakes are updated and restarted to apply the new certificate verification routines. Additionally, administrators should review their application configurations to ensure they are not bypassing standard certificate validation checks with custom or overly permissive code paths that might exacerbate this vulnerability. Implementing Certificate Transparency monitoring can also help in detecting anomalous certificates being issued by certification authorities, although it does not directly fix the local library flaw. Long-term mitigation strategies should include adopting stricter internal policies for certificate issuance and regularly auditing third-party dependencies to ensure they adhere to current RFC standards regarding X.509 validation logic.
This vulnerability aligns with CWE-294, which describes a denial of service caused by receiving an exception when accepting data, although in this context it is more accurately mapped to CWE-295: Improper Certificate Validation, as the core failure is the acceptance of an invalid certificate structure. Furthermore, from an offensive security perspective, this flaw facilitates techniques associated with MITM attacks under the ATT&CK framework, specifically those involving credential spoofing or interception where trust anchors are manipulated through flawed validation logic. By failing to enforce strict adherence to X.509 standards regarding key usage constraints, GnuTLS inadvertently provides a pathway for attackers to subvert authentication mechanisms that rely on accurate certificate parsing and policy enforcement.