CVE-2020-13777 – This GnuTLS vulnerability uses incorrect crypto... (How to Fix)

Q: What is the severity of CVE-2020-13777?

CVE-2020-13777 has a CVSS score of 7.4 (HIGH). This GnuTLS vulnerability uses incorrect cryptography for session ticket encryption, causing loss of confidentiality in TLS 1.2 and authentication bypass in TLS 1.3. It affects GnuTLS 3.6.4 through 3.6.13 on any system using these versions for TLS connections.

📋 TL;DR

This GnuTLS vulnerability uses incorrect cryptography for session ticket encryption, causing loss of confidentiality in TLS 1.2 and authentication bypass in TLS 1.3. It affects GnuTLS 3.6.4 through 3.6.13 on any system using these versions for TLS connections.

💻 Affected Systems

Products:

GnuTLS

Versions: 3.6.4 through 3.6.13

Operating Systems: Linux distributions using affected GnuTLS versions

Default Config Vulnerable: ⚠️ Yes

Notes: Affects both TLS 1.2 and TLS 1.3 implementations using GnuTLS. The vulnerability exists from first use until key rotation occurs.

📦 What is this software?

⚠️ Risk & Real-World Impact

🔴

Worst Case

Attackers could decrypt TLS 1.2 session data or bypass authentication in TLS 1.3, potentially exposing sensitive data or gaining unauthorized access to protected resources.

🟠

Likely Case

Session data exposure in TLS 1.2 connections, allowing attackers to intercept and decrypt communications between clients and servers.

🟢

If Mitigated

Limited impact if proper network segmentation and monitoring are in place, though encryption failures still occur.

🌐 Internet-Facing: HIGH

🏢 Internal Only: MEDIUM

🎯 Exploit Status

Public PoC: ✅ No

Weaponized: UNKNOWN

Unauthenticated Exploit: ⚠️ Yes

Complexity: MEDIUM

Exploitation requires network access to intercept TLS sessions. No public exploit code was found in references.

🛠️ Fix & Mitigation

✅ Official Fix

Patch Version: 3.6.14

Vendor Advisory: https://gnutls.org/security-new.html#GNUTLS-SA-2020-06-03

Restart Required: Yes

Instructions:

1. Update GnuTLS to version 3.6.14 or later using your distribution's package manager. 2. Restart all services using GnuTLS. 3. Consider rotating TLS session keys if previously using affected versions.

🔧 Temporary Workarounds

Disable session tickets

linux

Disable TLS session ticket support to avoid the vulnerable encryption mechanism

For GnuTLS applications: Set GNUTLS_NO_TICKETS=1 environment variable
For server configurations: Add 'ticket = off' to TLS settings

🧯 If You Can't Patch

Implement network segmentation to isolate systems using vulnerable GnuTLS versions
Use alternative TLS libraries or implementations for critical services

🔍 How to Verify

Check if Vulnerable:

Check GnuTLS version with: gnutls-cli --version | head -1

Check Version:

gnutls-cli --version | head -1

Verify Fix Applied:

Verify version is 3.6.14 or higher: gnutls-cli --version | grep -q '3\.6\.1[4-9]\|3\.6\.[2-9]\|3\.[7-9]'

📡 Detection & Monitoring

Log Indicators:

Unusual TLS handshake failures
Session resumption anomalies

Network Indicators:

TLS session ticket interception attempts
Unusual traffic patterns to TLS services

SIEM Query:

source="*tls*" AND (event="handshake_failure" OR event="session_resume")

📊 Metadata

CVE ID: CVE-2020-13777

CVSS v3 Score: 7.4 (HIGH)

CVSS Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:N

CWE: CWE-327

Published: June 4, 2020

Last Updated: November 21, 2024

🔗 References

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