CME-403

TLS 1.3 Enforcement

Harden Network Unquantified evidence Preventive

Description

Enforces TLS 1.3 as the minimum protocol version for all TLS connections, rejecting TLS 1.2 and below at the handshake layer. TLS 1.3 eliminates entire attack classes present in earlier versions: CBC mode ciphers are removed (preventing BEAST, Lucky13 padding oracle attacks), renegotiation is removed (preventing renegotiation-based attacks), only AEAD cipher suites are available (AES-128-GCM, AES-256-GCM, ChaCha20-Poly1305), forward secrecy is mandatory via ephemeral ECDHE/X25519 key exchange (no static RSA key transport), and the server certificate is encrypted during the handshake. Enforcement is configured at the service level (nginx ssl_protocols, Apache SSLProtocol, Tomcat sslEnabledProtocols), reverse proxy level, or system crypto policy level (update-crypto-policies on RHEL). Does not protect applications using custom TLS implementations that bypass system crypto libraries (Go's crypto/tls, Java's JSSE, custom-compiled OpenSSL). Compatibility constraint: legacy clients and embedded devices that do not support TLS 1.3 will fail to connect.

CVSS Vector Impacts

Metric	Transition	Rationale
Attack Complexity (AC)	L → H	Without TLS 1.3 enforcement, a network attacker can force protocol downgrade to TLS 1.2 or lower using well-tooled attacks: POODLE forces TLS 1.0 fallback to exploit CBC padding, DROWN exploits SSLv2 to decrypt TLS sessions, and FREAK/Logjam force export-grade key exchange. With TLS 1.3 enforcement, the server rejects any handshake below 1.3 — the downgrade path does not exist. Additionally, TLS 1.3 mandates forward secrecy via ephemeral key exchange, so even if a server's long-term private key is later compromised, past sessions cannot be retroactively decrypted. The attacker must perform a real-time MitM against AEAD ciphers with ephemeral keys, which requires fundamentally different (and harder) cryptographic attacks than exploiting known protocol weaknesses

CWE Relationships

CWE-757 CWE-327

Verification

Verify TLS 1.2 is rejected and TLS 1.3 is the minimum accepted protocol version

$ openssl s_client -connect <host>:<port> -tls1_2 </dev/null 2>&1 | grep -ci 'handshake failure\|alert protocol_version\|no protocols available'
# Expected: >=1

Platform: any

$ openssl s_client -connect <host>:<port> -tls1_3 </dev/null 2>&1 | grep -c 'TLSv1.3'
# Expected: >=1

Platform: any

$ update-crypto-policies --show 2>/dev/null | grep -qE 'FUTURE|FIPS' && echo 'ok' || grep -rn 'ssl_protocols\|SSLProtocol\|sslEnabledProtocols' /etc/nginx/ /etc/httpd/ 2>/dev/null | grep -ci 'TLSv1.3'
# Expected: >=1

Platform: linux

$ Get-ItemProperty -Path 'HKLM:\SYSTEM\CurrentControlSet\Control\SecurityProviders\SCHANNEL\Protocols\TLS 1.2\Server' -Name Enabled -ErrorAction SilentlyContinue | Select-Object -ExpandProperty Enabled
# Expected: 0

Platform: windows

$ Get-ItemProperty -Path 'HKLM:\SYSTEM\CurrentControlSet\Control\SecurityProviders\SCHANNEL\Protocols\TLS 1.3\Server' -Name Enabled -ErrorAction SilentlyContinue | Select-Object -ExpandProperty Enabled
# Expected: 1

Platform: windows

Remediation

rhel RHEL crypto-policies TLS 1.3 minimum enforcement


printf 'min_tls_version = TLS1.3\ntls_cipher = AES-256-GCM:AES-128-GCM:CHACHA20-POLY1305\n' > /etc/crypto-policies/policies/modules/TLS13-ONLY.pmod
update-crypto-policies --set FUTURE:TLS13-ONLY

linux nginx TLS 1.3-only protocol enforcement


printf 'ssl_protocols TLSv1.3;\nssl_prefer_server_ciphers off;\n' > /etc/nginx/conf.d/cme-403-tls13.conf
nginx -t && nginx -s reload

windows Windows SChannel TLS 1.3-only enforcement via registry


New-Item -Path 'HKLM:\SYSTEM\CurrentControlSet\Control\SecurityProviders\SCHANNEL\Protocols\TLS 1.2\Server' -Force | Out-Null; Set-ItemProperty -Path 'HKLM:\SYSTEM\CurrentControlSet\Control\SecurityProviders\SCHANNEL\Protocols\TLS 1.2\Server' -Name 'Enabled' -Value 0 -Type DWord; Set-ItemProperty -Path 'HKLM:\SYSTEM\CurrentControlSet\Control\SecurityProviders\SCHANNEL\Protocols\TLS 1.2\Server' -Name 'DisabledByDefault' -Value 1 -Type DWord
New-Item -Path 'HKLM:\SYSTEM\CurrentControlSet\Control\SecurityProviders\SCHANNEL\Protocols\TLS 1.3\Server' -Force | Out-Null; Set-ItemProperty -Path 'HKLM:\SYSTEM\CurrentControlSet\Control\SecurityProviders\SCHANNEL\Protocols\TLS 1.3\Server' -Name 'Enabled' -Value 1 -Type DWord; Set-ItemProperty -Path 'HKLM:\SYSTEM\CurrentControlSet\Control\SecurityProviders\SCHANNEL\Protocols\TLS 1.3\Server' -Name 'DisabledByDefault' -Value 0 -Type DWord
foreach ($ver in @('TLS 1.1','TLS 1.0')) { New-Item -Path "HKLM:\SYSTEM\CurrentControlSet\Control\SecurityProviders\SCHANNEL\Protocols\$ver\Server" -Force | Out-Null; Set-ItemProperty -Path "HKLM:\SYSTEM\CurrentControlSet\Control\SecurityProviders\SCHANNEL\Protocols\$ver\Server" -Name 'Enabled' -Value 0 -Type DWord; Set-ItemProperty -Path "HKLM:\SYSTEM\CurrentControlSet\Control\SecurityProviders\SCHANNEL\Protocols\$ver\Server" -Name 'DisabledByDefault' -Value 1 -Type DWord }

Applicability (CVE 5.2.0 Schema)

Vendor	Product	CPEs	Platforms	Status
redhat	enterprise_linux	`cpe:2.3:o:redhat:enterprise_linux:8:::::::` `cpe:2.3:o:redhat:enterprise_linux:9:::::::` `cpe:2.3:o:redhat:enterprise_linux:10:::::::*`	All	applicable
microsoft	windows	`cpe:2.3:o:microsoft:windows_10::::::::` `cpe:2.3:o:microsoft:windows_11::::::::` `cpe:2.3:o:microsoft:windows_server_2022:::::::*`	All	applicable