What is the CVSS score of EUVD-2026-31393?

EUVD-2026-31393 has a CVSS 3.1 base score of 5.3 (Medium). CVSS vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L. EPSS exploitation probability: 0.0%.

Is there a public PoC exploit available for EUVD-2026-31393?

Yes, a public proof-of-concept exploit is available for EUVD-2026-31393. Prioritise patching immediately. EPSS: 0.0%.

Is there a patch available for EUVD-2026-31393?

Yes, a patch is available for EUVD-2026-31393. Update the affected software to the latest version immediately.

golang.org/x/crypto EUVD-2026-31393

| CVE-2026-39835 MEDIUM

Improper Certificate Validation (CWE-295)

2026-05-22 Go

GHSA-78mq-xcr3-xm33

Information Disclosure Golang Org X Crypto Ssh Golang Org X Crypto

5.3

CVSS 3.1 · Vendor: Go

View

Attack ChainAIDerived

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

Access

Connect to target SSH server unauthenticated
Delivery

Present certificate during SSH handshake
Exploit

CertChecker invokes nil IsUserAuthority or IsHostAuthority callback
Execution

Go runtime triggers panic
Impact

SSH server process crashes

Vulnerability Assessment

Weakness	CWE CWE-295 OWASP Top 10:2025 A07:2025 - Authentication Failures ASVS 5.0 V12 - Secure Communication Certificate validation
Exploitation Conditions	The vulnerability requires two specific conditions: the SSH server application must use CertChecker as the ssh.ServerConfig.PublicKeyCallback, AND both IsUserAuthority and IsHostAuthority fields must be left uninitialized (nil) at the time a client presents a certificate. Applications that set either callback are not vulnerable. Per the CVSS vector (AV:N/AC:L/PR:N/UI:N), no authentication is required - an unauthenticated client presenting any certificate during the SSH handshake is sufficient to trigger the panic. What limits exploitation is the specific programming error in the consuming application: only servers where a developer used CertChecker without configuring authority callbacks are affected. This is a misconfiguration-dependent vulnerability rather than a default-insecure one, which aligns with the SSVC 'Automatable: no' rating and the low EPSS score, as mass scanning would need to identify this specific deployment pattern.
Impact	The CVSS 3.1 base score of 5.3 (Medium) reflects a network-reachable, low-complexity attack requiring no privileges or user interaction (AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L), but impact is bounded to availability. EPSS at 0.01% (1st percentile) and SSVC signals of Exploitation: none, Automatable: no, and Technical Impact: partial collectively indicate very low real-world risk at this time. The SSVC 'Automatable: no' designation merits scrutiny: given AV:N/AC:L/PR:N, the mechanics are trivially automatable; this rating likely reflects that only a specific misconfiguration (nil authority callbacks) is exploitable, meaning the vulnerable population is narrow rather than the attack itself being complex. A notable conflict exists in the source data: the 'Information Disclosure' tag assigned to this CVE is inconsistent with CVSS impact scores of C:N/I:N - this discrepancy is unresolved and may relate to panic stack traces exposing sensitive path or memory data in server logs, but cannot be confirmed from available data alone. Organizations using CertChecker without setting authority callbacks face a genuine DoS risk; the narrow prerequisite limits real-world blast radius.
Exploit ScenarioDerived	An unauthenticated attacker connects to a target SSH server that uses CertChecker as its public key callback without setting IsUserAuthority or IsHostAuthority. Using any standard SSH client capable of presenting a certificate during the handshake (e.g., ssh with a certificate-signed key), the attacker triggers the nil function pointer invocation, causing the server process to panic and the SSH service to crash. No public exploit code has been identified, but the attack requires only commodity SSH tooling and knowledge of the misconfiguration.

This Month Plan within 30 days. Why: MEDIUM severity (CVSS 4.0-6.9).

Patch available 0.0% exploitation probability (EPSS)

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AnalysisAIv1 – May 28, 2026

Unauthenticated remote clients can crash SSH servers built with golang.org/x/crypto/ssh by presenting a certificate during the handshake when CertChecker is used as a public key callback without initializing IsUserAuthority or IsHostAuthority. All versions prior to 0.52.0 are affected; the nil function pointer dereference causes a Go runtime panic that terminates the SSH service. No public exploit code has been identified and EPSS places exploitation probability at the 1st percentile (0.01%), though the attack requires no authentication and low complexity per the CVSS vector.

RemediationAI

Upgrade golang.org/x/crypto to version 0.52.0 or later, where CertChecker returns a proper error instead of panicking when authority callbacks are nil; this is the vendor-released patch confirmed by EUVD-2026-31393 and the Go announcement at https://groups.google.com/g/golang-announce/c/a082jnz-LvI. Any binary that vendors golang.org/x/crypto/ssh must be rebuilt and redeployed after the dependency upgrade, as the fix is compiled in. For teams unable to upgrade immediately, an effective compensating control is to explicitly set both IsUserAuthority and IsHostAuthority on every CertChecker instance - even to a function that returns false - before passing it as a callback; this eliminates the nil dereference with no service disruption, though it may silently reject legitimate certificate-based authentication depending on the false-returning stub. Alternatively, replace CertChecker with a fully custom PublicKeyCallback implementation. Avoid relying on recovery middleware as a long-term mitigation, as unhandled panics in other paths may still terminate the process.

Affected VersionsENISA

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

Affected Packages

go golang.org/x/crypto fix: 0.52.0

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

euvd golang_org_x_crypto_ssh 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 Go extended cryptography module golang.org/x/crypto/ssh provides SSH server and client primitives. CertChecker is a helper struct designed to be supplied as the PublicKeyCallback in ssh.ServerConfig to validate client certificates during the SSH handshake. It exposes two function-typed fields - IsUserAuthority and IsHostAuthority - intended as callbacks for verifying certificate signing authorities. CWE-295 (Improper Certificate Validation) captures the root cause: when both callbacks are left unset (nil), CertChecker contains no guard against invoking a nil function pointer upon encountering a certificate from a connecting client. The Go runtime converts a nil function call into a panic, which propagates and crashes the serving goroutine or process unless explicitly recovered. The CPE cpe:2.3:a:golang.org/x/crypto:golang.org/x/crypto/ssh:*:*:*:*:*:*:*:* covers all affected versions. The fix, tracked in Go CL 781660, converts the panic into a returned error, restoring safe failure behavior.

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Exploit and PoC references, threat intelligence from 22 sources

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Severity by source

Vendor (Go) PRIMARY 5.3 MEDIUM

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

SUSE MEDIUM

Red Hat 7.5 HIGH

Primary rating from Vendor (Go).

CVSS VectorVendor: Go

Attack Vector

Network

Attack Complexity

Low

Privileges Required

None

User Interaction

None

Scope

Unchanged

Confidentiality

None

Integrity

None

Availability

Low

Lifecycle Timeline

CVSS changed

May 28, 2026 - 15:07 NVD

5.3 (None) 5.3 (MEDIUM)

Patch available

May 22, 2026 - 04:31 EUVD

CVE Published

May 22, 2026 - 02:31 nvd

UNKNOWN (no severity yet)

DescriptionCVE.org

SSH servers which use CertChecker as a public key callback without setting IsUserAuthority or IsHostAuthority could be caused to panic by a client presenting a certificate. CertChecker now returns an error instead of panicking when these callbacks are nil.

Threat Intelligence Overview

EPSS Score

0.0%

Top 99% exploitable

Threat Intel

1 sources

Exploit

POC

Public exploit code exists

Social Media

Sightings

Data Quality

8 sources

Confidence: High

EUVD

NVD

CVE.org

EPSS

Vulnrichment GHSA MSRC Vendor CIRCL

GitHub

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 MEDIUM

CISA KEV No

Public POC Yes

EPSS 0.0%

Patch available Yes

Active threat No

Deciding rule

MEDIUM severity (CVSS 4.0-6.9)

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· derived

Scheduled

Exploitation

None

System Exposure· derived

Open

Automatable

Human Impact· derived

Medium

Technical Impact

Partial

Priority Score

Low Medium High Critical

KEV: 0

EPSS: 0

CVSS: +26

PoC: +20

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.