Infinite loop in strongSwan - CVE-2026-35328

Vulnerability identifier: #VU151578

CSH Severity: Medium

CVSS v4: 8.7 [CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N]

CVE-ID: CVE-2026-35328

CWE-ID: CWE-835

Exploitation vector: Remote access

Exploit availability: No public exploit available

The vulnerability allows a remote attacker to cause a denial of service.

The vulnerability exists due to an incorrect loop condition in libtls tls_server.c::process_client_hello() when processing a supported_versions TLS extension with a length that is not a multiple of two bytes. A remote attacker can send a crafted ClientHello message to exhaust the daemon's thread pool and cause a denial of service.

The extension is parsed even when TLS 1.3 is disabled.

strongSwan
Debian Linux
Fedora
Ubuntu
libstrongswan (Ubuntu package)
strongswan (Ubuntu package)
strongswan
strongswan (Debian package)

Install security update from vendor's website.

strongSwan - update to 6.0.6
libstrongswan (Ubuntu package) - addressed in versions 5.9.5-2ubuntu2.6, 5.9.13-2ubuntu4.24.04.3, 6.0.1-6ubuntu4.3
strongswan (Ubuntu package) - addressed in versions 5.9.5-2ubuntu2.6, 5.9.13-2ubuntu4.24.04.3, 6.0.1-6ubuntu4.3
strongswan - addressed in versions 6.0.6-1.el8, 6.0.6-1.el9, 6.0.6-1.el10_3, 6.0.6-2.fc44, 6.0.6-3.fc43, 6.0.7-2.fc43
strongswan (Debian package) - addressed in versions 5.9.8-5+deb12u4, 6.0.1-6+deb13u5

• https://www.strongswan.org/blog/2026/04/22/strongswan-vulnerability-(cve-2026-35328).html

• Multiple vulnerabilities in strongSwan
• Debian update for strongswan
• Fedora 44 update for strongswan
• Fedora 43 update for strongswan
• Fedora EPEL 10.3 update for strongswan
• Fedora EPEL 9 update for strongswan
• Fedora EPEL 8 update for strongswan
• Fedora 43 update for strongswan
• Ubuntu update for strongswan