TLS downgrade via unenforced socket protocol
Published May 11, 2026 · Updated May 11, 2026
TLS downgrade in Zephyr RTOS 4.3 and earlier allows remote attackers to negotiate TLS 1.2 with a socket configured for TLS 1.3. The zsock_socket protocol value is not applied as the mbedTLS minimum-version configuration, so ClientHello advertises both enabled versions. Exposure requires both TLS versions in Kconfig and an application that relies on IPPROTO_TLS_1_3; the peer can select TLS 1.2, exposing traffic to TLS 1.2-specific weaknesses.
Summary
What happened
TLS downgrade in Zephyr RTOS 4.3 and earlier allows remote attackers to negotiate TLS 1.2 with a socket configured for TLS 1.3. The zsock_socket protocol value is not applied as the mbedTLS minimum-version configuration, so ClientHello advertises both enabled versions. Exposure requires both TLS versions in Kconfig and an application that relies on IPPROTO_TLS_1_3; the peer can select TLS 1.2, exposing traffic to TLS 1.2-specific weaknesses.
The record
- CVE
- CVE-2026-1677
- Published
- May 11, 2026
- Updated
- May 11, 2026
- Vendor
- Zephyr Project
- Product
- Zephyr RTOS
- Classifications
- CWE-757, T1600
- Attack vector
- network
- Privileges
- unauthenticated
Timeline
How it unfolded
- May 11, 2026CVE publishedPublication date reported by the CVE source.
- May 11, 2026Record updatedLatest update available in the CVE record.
Exploitability
Present is not the same as exploitable
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Is a vulnerable build present?
Compare these published version ranges with your installed build and any vendor patches.
- Affected versionversion=* <=4.3
What conditions does exploitation require?
What is affected?
Published CVSS scores
CVSS describes severity. EPSS estimates exploitation probability.
Attacks
What attackers are doing with it
Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.
Weakness, pattern, technique
Public exploit references
- Zephyr TLS downgrade reproduction procedureproof of concept · demonstrated
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Technologies
Your stack
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Your stack
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