# uTLS ServerHellos are accepted without checking TLS 1.3 downgrade canaries

Advisory record · `GHSA-pmc3-p9hx-jq96`

- **Identifiers:** `GHSA-pmc3-p9hx-jq96` · `CVE-2026-26994` · `GO-2025-3638`
- **State:** Confirmed
- **Severity:** CVSS 6.5 Medium
- **Critical:** No
- **Weakness (CWE):** [CWE-693](https://cwe.mitre.org/data/definitions/693.html)
- **Tags:** [CWE-693](https://agentprime.io/advisories/tag/cwe-693.md)
- **Published:** 23 Apr 2025
- **Updated:** 10 Sept 2026
- **Reviewed by a person:** No

Published by the GitHub Advisory Database at 23 Apr 2025, 14:43 UTC. agentprime has not analysed this yet, so whether it affects any app is not known.

## Affected packages

- `github.com/refraction-networking/utls` (Go): affected `<1.7.0`; fixed in `1.7.0`

## Machine-written summary

Summary written by Claude Haiku from the source advisory. Not reviewed by a person.

> uTLS (github.com/refraction-networking/utls) versions before 1.7.0 do not check the TLS 1.3 downgrade canary in the ServerHello random field when a utls ClientHello spec is used. An active network attacker can alter the ClientHello to remove the SupportedVersions extension, forcing a downgrade to TLS 1.2 that the client accepts without detecting it, and the attack can also be used to fingerprint utls connections. Upgrade to version 1.7.0 or later.

Written 1 Nov 2026.

## Not analysed yet

We haven’t published our analysis of this Advisory yet. Once a person has reviewed it, this page will say what’s vulnerable, what has to be true for your app to be affected, and how to check.

## Severity and exploitation

- 6.5 Medium, CVSS 3.1: `CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N`, from CNA (GitHub_M)

- **CISA KEV:** Not listed.
- **EPSS:** 0.0030, the 23rd percentile, on 11 Nov 2026: FIRST’s estimate of the chance of exploitation in the next 30 days.

## Timeline

- 31 Oct 2026, 14:14 UTC: First seen by agentprime
- 31 Oct 2026, 14:14 UTC: [Confirmed by OSV](https://osv.dev/vulnerability/GHSA-pmc3-p9hx-jq96)

## Sources

- [GitHub Advisory Database: GHSA-pmc3-p9hx-jq96](https://github.com/advisories/GHSA-pmc3-p9hx-jq96)
- [OSV: GHSA-pmc3-p9hx-jq96](https://osv.dev/vulnerability/GHSA-pmc3-p9hx-jq96)
- <https://github.com/refraction-networking/utls/security/advisories/GHSA-pmc3-p9hx-jq96>
- <https://nvd.nist.gov/vuln/detail/CVE-2026-26994>
- <https://github.com/refraction-networking/utls/issues/181>
- <https://github.com/refraction-networking/utls/pull/337>
- <https://github.com/refraction-networking/utls/commit/f8892761e2a4d29054264651d3a86fda83bc83f9>
- <https://github.com/refraction-networking/utls>

Facts on this page come from the GitHub Advisory Database (CC-BY 4.0), OSV and the other sources listed, as published. We don’t reproduce full advisory descriptions, which can contain exploit code.

## Check whether this affects you

A Match means an installed version is in the affected range above. Most don’t affect the app; agentprime hasn’t analysed this one yet.

- With the agentprime MCP server (<https://api.agentprime.io/mcp>), ask your agent to check the project: `check_lockfile` sends a lockfile, `check_packages` a list of installed packages, and both return the Matches and the upgrades that clear them. `get_advisory` returns this record. Setup: <https://agentprime.io/mcp.md>
- In a browser: <https://agentprime.io/check/> reads your lockfile in the browser, never uploading it.
- Whether it touches your code, with file and line: the Claude Code plugin, <https://agentprime.io/plugin.md>
- As JSON: <https://api.agentprime.io/v1/advisories/GHSA-pmc3-p9hx-jq96>, or `POST https://api.agentprime.io/v1/match` with your packages (<https://agentprime.io/api.md>)

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The web page: <https://agentprime.io/advisories/GHSA-pmc3-p9hx-jq96/>. Everything for agents: <https://agentprime.io/llms.txt>.