What is the CVSS score of CVE-2026-42508?

CVE-2026-42508 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: 0.0%.

How to fix or mitigate CVE-2026-42508?

Within 24 hours: Identify all applications and services using golang.org/x/crypto and document their current versions. Within 7 days: Update golang.org/x/crypto to version 0.52.0 or later in development and staging environments; test certificate revocation functionality. Within 30 days: Deploy patched version to all production systems; confirm revocation validation is operational.

golang.org/x/crypto CVE-2026-42508

Q: Which versions of golang.org/x/crypto are affected by CVE-2026-42508?

A certificate revocation validation flaw in the Go SSH cryptography library allows attackers with revoked host keys to impersonate legitimate servers, potentially compromising SSH-based…. A patch is available.

EUVD-2026-31399 CRITICAL

Improper Certificate Validation (CWE-295)

2026-05-22 Go

GHSA-5cgq-3rg8-m6cv

Information Disclosure Golang Org X Crypto Ssh Knownhosts Golang Org X Crypto

9.1

CVSS 3.1 · Vendor: Go

View

Attack ChainAIDerived

Hypothetical attack flow derived from CVE metadata – not a confirmed exploit chain

Recon

Obtain host cert signed by revoked CA
Delivery

Position on network path to victim
Exploit

Intercept SSH connection attempt
Install

Present revoked-CA-signed host certificate
C2

knownhosts skips SignatureKey revocation check
Execute

Client accepts impersonated host
Impact

Capture credentials or manipulate session

Vulnerability Assessment

Weakness	CWE CWE-295 OWASP Top 10:2025 A07:2025 - Authentication Failures ASVS 5.0 V12 - Secure Communication Certificate validation
Exploitation Conditions	Exploitation requires that the victim application links a vulnerable golang.org/x/crypto/ssh/knownhosts (<0.52.0) and uses CA-based host trust (@cert-authority entries in known_hosts) AND that the targeted CA's SignatureKey has been explicitly marked @revoked by the victim. The attacker must additionally possess a host certificate previously signed by that revoked CA and be able to intercept or redirect the victim's SSH connection (network-adjacent or on-path position). Limiting factors: deployments that pin individual host keys rather than using @cert-authority are unaffected, and victims who never issued a @revoked entry for the CA SignatureKey never reach the buggy code path. No user interaction is required (UI:N) and no authentication on the victim side is needed (PR:N) once the prerequisites are met.
Impact	The CVSS 3.1 base score is 9.1 (AV:N/AC:L/PR:N/UI:N/C:H/I:H/A:N), reflecting network-reachable, unauthenticated abuse with high confidentiality and integrity impact and no availability impact, consistent with a man-in-the-middle / host impersonation outcome rather than RCE. However, EPSS sits at 0.02% (4th percentile) and SSVC reports Exploitation: none with Automatable: yes and Technical Impact: total - meaning exploitation has not been observed but, where prerequisites are met, it scales. The vulnerability is not in CISA KEV. Practical risk is constrained: an attacker must already possess a host key that was previously signed by a CA the victim trusts AND that CA's SignatureKey must have been explicitly revoked by the victim's known_hosts - a relatively narrow precondition that explains the low EPSS despite the high CVSS. Treat as a high-severity correctness bug in trust-anchor handling rather than a mass-exploitation threat.
Exploit ScenarioDerived	An attacker who previously obtained a host certificate signed by a CA that the victim later revoked positions themselves on the network path (e.g., via DNS hijack, ARP spoofing, or rogue Wi-Fi) and answers SSH connections destined for a legitimate server. A Go-based SSH client using knownhosts < 0.52.0 validates the leaf key against revocation entries, misses the revoked SignatureKey of the CA, and accepts the impersonated host, enabling credential capture or session manipulation. No public exploit identified at time of analysis.

Act Now Prioritize within 24-48h. Why: CRITICAL severity.

Patch available 0.0% exploitation probability (EPSS)

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AnalysisAIv1 – May 28, 2026

Improper certificate revocation validation in the golang.org/x/crypto/ssh/knownhosts package allows SSH connections to succeed against hosts whose CA SignatureKey has been revoked. Versions prior to 0.52.0 only validated the leaf 'key' against revocation entries while ignoring 'key.SignatureKey', enabling attackers holding a revoked CA-signed host key to impersonate trusted servers. No public exploit identified at time of analysis, and EPSS exploitation probability is very low at 0.02%.

RemediationAI

Vendor-released patch: golang.org/x/crypto v0.52.0. Update the module in go.mod with 'go get golang.org/x/crypto@v0.52.0' and rebuild and redeploy any binaries that consume ssh/knownhosts, since the fix ships compiled into the application rather than as a runtime library. Confirm the upgrade via the Go announcement at https://groups.google.com/g/golang-announce/c/a082jnz-LvI and the advisory at https://pkg.go.dev/vuln/GO-2026-5021. If patching cannot occur immediately, compensating controls include removing @cert-authority entries from known_hosts files and pinning specific host keys directly (trade-off: loses CA-based scalability and requires manual key rotation), or rotating the trusted CA so the previously-revoked key is no longer in the trust chain (trade-off: requires re-signing all host certificates). 'govulncheck' should be run across the codebase to confirm no residual import paths remain on vulnerable versions.

Affected VersionsENISA

golang.org/x/crypto/ssh/knownhosts 0 <0.52.0

Affected Packages

go golang.org/x/crypto fix: 0.52.0

go golang.org/x/crypto/ssh/knownhosts fix: 0.52.0

euvd golang_org_x_crypto_ssh_knownhosts fix: 0.52.0

Source: OSV (Open Source Vulnerabilities)

Affects your stack

Affects 3 packages in go, euvd

Checking this CVE against your own stacks and servers is part of the Basic plan and up.

Technical ContextAI · show details

The flaw resides in golang.org/x/crypto/ssh/knownhosts, the Go standard ecosystem library used by SSH clients (including those built on golang.org/x/crypto/ssh) to validate remote host keys against a known_hosts file. The library supports @cert-authority entries that delegate trust to a CA which signs host certificates; revocation is expressed via @revoked entries. CWE-295 (Improper Certificate Validation) applies because the revocation check examined only the presented host key without recursively checking the embedded SignatureKey identifying the signing CA, so a revoked CA could still produce host certificates accepted as valid by client applications.

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Exploit and PoC references, threat intelligence from 22 sources

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Severity by source

Vendor (Go) PRIMARY 9.1 CRITICAL

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

SUSE CRITICAL

Red Hat 7.4 HIGH

Primary rating from Vendor (Go).

CVSS VectorVendor: Go

Attack Vector

Network

Attack Complexity

Low

Privileges Required

None

User Interaction

None

Scope

Unchanged

Confidentiality

High

Integrity

High

Availability

None

Lifecycle Timeline

CVSS changed

May 28, 2026 - 15:07 NVD

9.1 (None) 9.1 (CRITICAL)

Patch available

May 22, 2026 - 04:31 EUVD

CVE Published

May 22, 2026 - 02:31 nvd

UNKNOWN (no severity yet)

DescriptionCVE.org

Previously, a revoked 'SignatureKey' belonging to a CA was not correctly checked for revocation. Now, both the 'key' and 'key.SignatureKey' are checked for @revoked.

Executive ViewAI

Why It Matters

A certificate revocation validation flaw in the Go SSH cryptography library allows attackers with revoked host keys to impersonate legitimate servers, potentially compromising SSH-based authentication across infrastructure. Any organization using golang.org/x/crypto in production SSH implementations is exposed to server impersonation attacks. Immediate patching is critical to maintain the integrity of SSH-based access controls.

Business Impact: High

Compensating Controls

Vendor-released patch: golang.org/x/crypto version 0.52.0 or later

Threat Intelligence Overview

EPSS Score

0.0%

Top 96% exploitable

Threat Intel

1 sources

Social Media

Sightings

Data Quality

7 sources

Confidence: High

EUVD

NVD

CVE.org

EPSS

Vulnrichment GHSA MSRC Vendor CIRCL

VulDB

How this decision was reached

The engine reads each signal below and applies a fixed list of rules from top to bottom. The first rule that matches sets the recommended action – like a statement, every line is shown.

SignalValue

Severity CRITICAL

CISA KEV No

Public POC No

EPSS 0.0%

Patch available Yes

Active threat No

Deciding rule

CRITICAL severity

What every action level means

Emergency Immediate action required. Criteria: CRITICAL + in CISA KEV (actively exploited) without patch, OR CRITICAL + EPSS > 30% without patch, OR CRITICAL + active threat signals without patch.

Act Now Prioritize within 24-48h. Criteria: in KEV with patch available, OR CRITICAL + public POC, OR HIGH + EPSS > 10%, OR CRITICAL + active threat, OR any CRITICAL severity.

This Week Schedule remediation this week. Criteria: HIGH severity (CVSS 7.0-8.9). Patch within normal change window.

This Month Plan within 30 days. Criteria: MEDIUM severity (CVSS 4.0-6.9), lower exploitation probability. Include in regular patch cycle.

Monitor Track and reassess. Criteria: LOW severity (CVSS 0.1-3.9), minimal exploitation risk. No immediate action needed.

SSVC AssessmentENISA

SSVC Decision

Scheduled

Exploitation

None

System Exposure· derived

Open

Automatable

Yes

Human Impact

High

Technical Impact

Total

Priority Score

Low Medium High Critical

KEV: 0

EPSS: 0

CVSS: +46

PoC: 0

Weaponized: 0

Threat intel: 0

Gov advisory: 0

Attack Technique

Information Disclosure MEDIUM

Information disclosure occurs when an application unintentionally exposes sensitive data that aids attackers in reconnaissance or directly compromises security.