What is the CVSS score of CVE-2026-45388?

CVE-2026-45388 has a CVSS 3.1 base score of 9.1 (Critical). CVSS vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N. EPSS exploitation probability: 0.1%.

Which versions of OCaml-TLS are affected by CVE-2026-45388?

OCaml-TLS library versions before 2.1.0 fail to validate critical TLS certificate properties, allowing network-positioned attackers to impersonate trusted servers and intercept encrypted…

How to fix or mitigate CVE-2026-45388?

24 hours: Execute inventory of all applications and dependencies using OCaml-TLS; identify which deployments use the library as a TLS client. 7 days: Activate monitoring and alerting for anomalous TLS certificate validation events; document migration options to alternative TLS libraries (OpenSSL, BoringSSL, rustls) for feasible applications. 30 days: Monitor OCaml-TLS project releases for version 2.1.0 or later; prepare and execute testing of upgrade or migration strategy in staging environments; prioritize applications with highest data sensitivity for remediation.

OCaml-TLS CVE-2026-45388

EUVD-2026-36764 CRITICAL

Improper Certificate Validation (CWE-295)

2026-06-15 mitre

GHSA-rrgx-wgv5-5hf9

Information Disclosure N A

9.1

CVSS 3.1 · Vendor: mitre

Severity by source

Vendor (mitre) PRIMARY

9.1 CRITICAL

AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N

vuln.today AI

7.4 HIGH

Network-reachable TLS client with no auth or UI, but exploitation needs a MitM position and a trusted non-serverAuth certificate, justifying AC:H; impersonation breaks C and I, not A.

3.1 AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:N

4.0 AV:N/AC:H/AT:P/PR:N/UI:N/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N

Primary rating from Vendor (mitre).

CVSS VectorVendor: mitre

Attack Vector

Network

Attack Complexity

Low

Privileges Required

None

User Interaction

None

Scope

Unchanged

Confidentiality

High

Integrity

High

Availability

None

Lifecycle Timeline

Analysis Generated

Jun 16, 2026 - 15:30 vuln.today

CVSS changed

Jun 16, 2026 - 15:22 NVD

9.1 (CRITICAL)

CVE Published

Jun 15, 2026 - 00:00 cve.org

UNKNOWN (no severity yet)

DescriptionCVE.org

In OCaml-TLS before 2.1.0, the client implementation does insufficient checks of the certificate provided by the server, which allows impersonation with certificates that are not meant for server authentication (because of KeyUsage and ExtendedKeyUsage).

AnalysisAI

Server impersonation in OCaml-TLS before 2.1.0 allows a malicious TLS server to present a certificate not intended for server authentication and still be accepted by client code, enabling man-in-the-middle and impersonation attacks against any application linking the library as a TLS client. The client fails to validate KeyUsage and ExtendedKeyUsage extensions properly, breaking a core PKIX trust assumption. No public exploit identified at time of analysis and EPSS sits at 0.15% (4th percentile), but the CVSS 9.1 reflects the broad confidentiality and integrity impact on encrypted sessions.

Technical ContextAI

OCaml-TLS is a pure-OCaml implementation of the TLS protocol used by MirageOS and other OCaml ecosystem projects for both client and server TLS. The root cause maps to CWE-295 (Improper Certificate Validation): when validating a server's X.509 certificate chain, the client did not enforce that the leaf certificate's KeyUsage (e.g., digitalSignature/keyEncipherment) and ExtendedKeyUsage (e.g., id-kp-serverAuth, OID 1.3.6.1.5.5.7.3.1) actually permit TLS server authentication. As a result, certificates legitimately issued for other purposes - such as client authentication, code signing, S/MIME, or OCSP signing - could be reused by an attacker as if they were server certificates, undermining a key PKIX constraint that segregates certificate purposes within a single CA hierarchy.

RemediationAI

Upstream fix available in OCaml-TLS 2.1.0 - upgrade any application or unikernel that links ocaml-tls (and transitive consumers such as tls-mirage, tls-lwt, tls-async, http clients, and MirageOS images) to 2.1.0 or later and rebuild/redeploy, since this is a library-level fix that does not propagate via runtime patching. Consult the OSV record at https://osv.dev/vulnerability/OSEC-2026-06 and the NVD entry at https://nvd.nist.gov/vuln/detail/CVE-2026-45388 to confirm the patched release and any package-manager constraints (opam pins). Where immediate upgrade is not possible, compensating controls include pinning the expected server certificate or its SPKI in the application (certificate or public-key pinning, with the trade-off that certificate rotation requires a code/config update), restricting outbound TLS to a narrow allowlist of hosts reachable only over trusted network paths to reduce MitM opportunity, and operating affected clients only over VPN/IPsec tunnels to a trusted egress (which adds latency and operational complexity but eliminates opportunistic on-path attackers).