Improper Authentication in mbed TLS - CVE-2026-34873
Published: April 2, 2026
Vulnerability identifier: #VU124802
CSH Severity: Medium
CVSS v4: 8.7 [CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:H/VA:N/SC:N/SI:N/SA:N]
CVE-ID: CVE-2026-34873
CWE-ID: CWE-287
Exploitation vector: Remote access
Exploit availability:
No public exploit available
Vulnerability details
The vulnerability allows a remote attacker to impersonate a legitimate client during TLS session resumption.
The vulnerability exists due to improper session validation in the TLS 1.3 session resumption mechanism when handling a downgrade from TLS 1.3 to TLS 1.2 after a HelloRetryRequest. A remote attacker can intercept the HelloRetryRequest and send a specially crafted ClientHello that negotiates TLS 1.2 to impersonate a legitimate client and bypass authentication mechanisms.
The server incorrectly proceeds to resume a TLS 1.2 session using an all-zero master secret, potentially allowing the attacker to inherit application-level privileges if session tickets encode authorization data.
Affected software
mbed TLS
Fedora
mbedtls
Fedora
mbedtls
How to mitigate CVE-2026-34873
Install security update from vendor's website.
mbed TLS - addressed in versions 3.6.6, 4.1.0
mbedtls - addressed in versions 3.6.6-1.el10_1, 3.6.6-1.el10_2, 3.6.6-1.el10_3, 3.6.6-1.fc42, 3.6.6-1.fc43, 3.6.6-1.fc44
mbedtls - addressed in versions 3.6.6-1.el10_1, 3.6.6-1.el10_2, 3.6.6-1.el10_3, 3.6.6-1.fc42, 3.6.6-1.fc43, 3.6.6-1.fc44