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Go crypto/tls CVE-2026-42505

| EUVDEUVD-2026-42317 MEDIUM
Insertion of Sensitive Information Into Sent Data (CWE-201)
2026-07-08 Go GHSA-ff52-ph69-cf7x
Information Disclosure Crypto Tls Go Standard Library
5.3
CVSS 3.1 · Vendor: Go
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Attack ChainAIDerived

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

  1. Access
    Attain passive network observation position
  2. Delivery
    Capture outbound TLS ClientHello packets from Go client
  3. Exploit
    Identify ECH handshakes with PSK extension present in outer ClientHello
  4. Execution
    Extract plaintext PSK identity values
  5. Persist
    Correlate PSK identities to prior sessions or known server identities
  6. Impact
    De-anonymize the ECH-protected connection

Vulnerability Assessment

Weakness
CWE
CWE-201
OWASP Top 10:2025
A01:2025 - Broken Access Control
ASVS 5.0
V14 - Data Protection Outbound data filtering
Exploitation Conditions

Exploitation requires two concurrent application-level conditions: (1) the Go application must explicitly enable Encrypted Client Hello (ECH) in its crypto/tls configuration - ECH is not enabled by default and requires deliberate opt-in; and (2) TLS session resumption via Pre-Shared Keys must be occurring, which requires a prior completed TLS session between the same client and server. The attacker must hold a passive network observation position (on-path, upstream ISP, or packet capture capability) - no active injection, authentication, or server-side access is required. Exploitation is strictly limited to de-anonymization via PSK identity correlation; it does not yield session decryption, credential theft, or code execution. Go applications not using ECH are entirely unaffected regardless of their crypto/tls version.
Impact

The CVSS 3.1 score of 5.3 (AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N) accurately characterizes a network-accessible, low-complexity passive information disclosure with no integrity or availability component. Real-world risk is bounded by deployment reality: ECH is not enabled by default in most Go applications and must be explicitly configured, and PSK session resumption requires a prior completed TLS session with the same server. For the majority of Go services not using ECH, this vulnerability is entirely non-exploitable. However, for applications specifically deploying ECH to deliver privacy guarantees - such as censorship circumvention tools, privacy-sensitive APIs, or services protecting user anonymity - this flaw directly defeats the primary security control and represents a meaningful, reliable threat from network-level adversaries including ISPs and transit providers. EPSS data was not available in the provided intelligence. No KEV listing and no POC code further reduce urgency for general deployments, but the technical impact is deterministic for targeted ECH use cases.
Exploit ScenarioDerived

A network-level adversary - such as a transit ISP, compromised network appliance, or on-path monitoring infrastructure - passively captures TLS ClientHello packets from a Go application that has ECH enabled with session resumption active. By reading PSK identity values present in the unencrypted outer ClientHello, the observer correlates resumption handshakes to prior sessions or to specific server identities, effectively identifying which server the client is connecting to despite ECH's privacy protections. No public exploit code exists at time of analysis; this attack requires only standard packet capture capabilities and a passive network vantage point.
This Month Plan within 30 days. Why: MEDIUM severity (CVSS 4.0-6.9).
Patch available 0.4% exploitation probability (EPSS)

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AnalysisAIv1 – Jul 08, 2026

Passive network de-anonymization of Encrypted Client Hello (ECH) connections is possible in Go's crypto/tls library because pre-shared key (PSK) identities are exposed in cleartext outer ClientHello messages, allowing any on-path observer to correlate resumption sessions and identify servers despite ECH's intended privacy protections. Affected versions span crypto/tls prior to 1.25.12, 1.26.5, and 1.27.0-rc.2 across the Go standard library. No public exploit code has been identified at time of analysis and no CISA KEV listing exists; however, exploitation requires no authentication or user interaction and is accessible to any network-level adversary with passive traffic visibility.

RemediationAI

The primary fix is to upgrade the Go toolchain to a patched release - Go 1.25.12, Go 1.26.5, or Go 1.27.0-rc.2 or later, depending on the release branch in use. The upstream code fix is tracked at https://go.dev/cl/775960 and the issue at https://go.dev/issue/79282. The Go security announcement at https://groups.google.com/g/golang-announce/c/OrmQE_Yp5Sc should be consulted for release timing. For organizations unable to upgrade immediately, the most effective compensating control is to audit whether ECH is explicitly enabled in TLS configurations; if ECH is not configured, the vulnerability is not exploitable regardless of Go version. If ECH is operationally required and patching is delayed, restricting outbound TLS traffic to trusted, VPN-protected, or mutually authenticated network paths reduces passive observation exposure, though this adds operational overhead and does not address the root cause in the library.

Affected VersionsENISA

crypto/tls 1.27.0-0 <1.27.0-rc.2
crypto/tls 0 <1.25.12
crypto/tls 1.26.0-0 <1.26.5

Affected Packages

go stdlib fix: 1.25.12, 1.26.5, 1.27.0-rc.2
euvd crypto_tls fix: 1.25.12, 1.26.5, 1.27.0-rc.2

Source: OSV (Open Source Vulnerabilities)

Affects your stack

Affects 2 packages in go, euvd

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Technical ContextAI · show details

Encrypted Client Hello (ECH) is a TLS 1.3 extension that encrypts the sensitive inner ClientHello - including the Server Name Indication (SNI) - inside an unencrypted outer ClientHello envelope, preventing passive observers from identifying the intended server. TLS session resumption uses Pre-Shared Keys (PSKs), each identified by a PSK identity value derived from a prior completed session. The root cause (CWE-201: Exposure of Sensitive Information Through Sent Data) is that Go's crypto/tls implementation fails to suppress PSK identity values when constructing the outer ClientHello during an ECH handshake. Because PSK identities are session-specific and can be correlated to prior connections or to specific servers, their presence in cleartext directly undermines ECH's anonymity guarantees. The affected product is identified by CPE cpe:2.3:a:go_standard_library:crypto/tls:*:*:*:*:*:*:*:* and spans all three active Go release branches.

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Severity by source

Vendor (Go) PRIMARY 5.3 MEDIUM
AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N
vuln.today AI 5.3 MEDIUM

Passive network observation requires no privileges or user interaction; confidentiality impact is limited to PSK identity exposure (partial), with no integrity or availability consequence.

3.1 AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N
4.0 AV:N/AC:L/AT:N/PR:N/UI:N/VC:L/VI:N/VA:N/SC:N/SI:N/SA:N
SUSE MEDIUM
Red Hat 5.3 MEDIUM

Primary rating from Vendor (Go).

CVSS VectorVendor: Go

Attack Vector
Network
Attack Complexity
Low
Privileges Required
None
User Interaction
None
Scope
Unchanged
Confidentiality
Low
Integrity
None
Availability
None

Lifecycle Timeline

3
CVSS changed
Jul 08, 2026 - 20:22 NVD
5.3 (MEDIUM)
Patch available
Jul 08, 2026 - 18:02 EUVD
CVE Published
Jul 08, 2026 - 15:46 cve.org
UNKNOWN (no severity yet)

DescriptionCVE.org

Handshakes which used Encrypted Client Hello could be de-anonymized by a passive network observer due to a disclosure of pre-shared key identities in the unencrypted client hello.

Threat Intelligence Overview

EPSS Score
0.4%
Top 66% exploitable
Prevalence
High
Go standard library used across millions of production cloud and web services
Social Media
1
Sightings
Data Quality
7 sources
Confidence: High
EUVD NVD CVE.org EPSS Vulnrichment 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 MEDIUM
CISA KEV No
Public POC No
EPSS 0.4%
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
Yes
Human Impact· derived
High
Technical Impact
Partial

Priority Score

26
Low Medium High Critical
KEV: 0
EPSS: 0
CVSS: +26
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.

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Vendor StatusVendor

SUSE

Severity: Moderate
Product Status
SUSE Linux Enterprise Desktop 15 SP7 SUSE Linux Enterprise High Performance Computing 15 SP7 SUSE Linux Enterprise Module for Development Tools 15 SP7 SUSE Linux Enterprise Server 15 SP7 SUSE Linux Enterprise Server for SAP Applications 15 SP7 Fixed
SUSE Linux Enterprise High Performance Computing 15 SP4-ESPOS Fixed
SUSE Linux Enterprise High Performance Computing 15 SP4-LTSS Fixed
SUSE Linux Enterprise High Performance Computing 15 SP5-ESPOS Fixed
SUSE Linux Enterprise High Performance Computing 15 SP5-LTSS Fixed

References

Advisories & patches
ADVISORY https://access.redhat.com/errata/RHSA-2026:36477 ADVISORY https://access.redhat.com/errata/RHSA-2026:36510 ADVISORY https://nvd.nist.gov/vuln/detail/CVE-2026-42505 ADVISORY https://seclists.org/oss-sec/2026/q3/102 ADVISORY https://vuldb.com/vuln/376925 ADVISORY https://www.suse.com/support/update/SUSE-SU-2026:22638/ ADVISORY https://www.suse.com/support/update/SUSE-SU-2026:22641/ ADVISORY https://www.suse.com/support/update/SUSE-SU-2026:22643/ ADVISORY https://www.suse.com/support/update/SUSE-SU-2026:22656/ ADVISORY https://www.suse.com/support/update/SUSE-SU-2026:2817/ ADVISORY https://www.suse.com/support/update/SUSE-SU-2026:2818/ ADVISORY https://www.suse.com/support/update/SUSE-SU-2026:3046/ ADVISORY https://www.suse.com/support/update/SUSE-SU-2026:3047/ ADVISORY https://www.suse.com/support/update/SUSE-SU-2026:3102/ ADVISORY https://www.suse.com/support/update/SUSE-SU-2026:3151/ REF https://go.dev/cl/775960 REF https://go.dev/issue/79282 REF https://groups.google.com/g/golang-announce/c/OrmQE_Yp5Sc REF https://pkg.go.dev/vuln/GO-2026-5856

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