CVE-2020-25648: Mozilla NSS TLS 1.3 DoS Vulnerability

CVE-2020-25648 Overview

A flaw was found in the way Mozilla Network Security Services (NSS) handled CCS (ChangeCipherSpec) messages in TLS 1.3. This vulnerability allows a remote attacker to send multiple CCS messages, causing a denial of service condition for servers compiled with the NSS library. The highest threat from this vulnerability is to system availability, as it can render affected services unresponsive.

Critical Impact

Remote attackers can exploit this vulnerability to cause denial of service on TLS 1.3 servers using NSS versions prior to 3.58, potentially disrupting critical services without authentication.

Affected Products

• Mozilla Network Security Services (versions before 3.58)
• Red Hat Enterprise Linux 7.0 and 8.0
• Fedora 31, 32, and 33
• Oracle Communications Offline Mediation Controller 12.0.0.3.0
• Oracle Communications Pricing Design Center 12.0.0.3.0
• Oracle JD Edwards EnterpriseOne Tools

Discovery Timeline

• October 20, 2020 - CVE-2020-25648 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2020-25648

Vulnerability Analysis

This vulnerability (CWE-770: Allocation of Resources Without Limits or Throttling) exists in how NSS processes ChangeCipherSpec messages during TLS 1.3 handshakes. In the TLS 1.3 protocol, CCS messages are handled differently than in previous versions, primarily for backward compatibility. The NSS library fails to properly limit or validate the number of CCS messages it will accept during a connection establishment, allowing an attacker to exhaust server resources by flooding it with these messages.

The attack can be executed remotely over the network without any authentication or user interaction, making it particularly dangerous for internet-facing services. The vulnerability specifically impacts availability without affecting confidentiality or integrity of the system.

Root Cause

The root cause of CVE-2020-25648 lies in the improper resource allocation handling within the NSS TLS implementation. When processing TLS 1.3 connections, the library does not enforce adequate limits on the number of ChangeCipherSpec messages it will accept from a client. This oversight allows an attacker to send an excessive number of CCS messages, consuming server resources such as memory and CPU cycles. The lack of throttling mechanisms means that the server continues to process these messages until resources are exhausted, resulting in a denial of service condition.

Attack Vector

The attack is network-based and can be executed by any remote attacker who can establish a TCP connection with the target server. The attacker initiates a TLS 1.3 handshake and sends multiple CCS messages in rapid succession. Since the NSS library does not properly restrict or rate-limit these messages, each message consumes processing resources. The cumulative effect of processing numerous CCS messages leads to resource exhaustion on the server, ultimately causing legitimate connections to be denied or the service to become unresponsive.

The attack requires no authentication, no user interaction, and can be automated to target multiple servers simultaneously. Any application or service that uses NSS for TLS operations and accepts connections over the network is potentially vulnerable.

Detection Methods for CVE-2020-25648

Indicators of Compromise

• Unusual volume of TLS handshake attempts originating from single or multiple IP addresses
• Server logs showing repeated TLS connection failures or incomplete handshakes
• Elevated CPU and memory utilization on servers handling TLS connections
• Network traffic analysis revealing abnormal patterns of CCS messages within TLS sessions

Detection Strategies

• Monitor network traffic for anomalous TLS handshake patterns, particularly excessive ChangeCipherSpec messages
• Implement intrusion detection rules to flag connections with unusually high numbers of CCS messages during TLS 1.3 handshakes
• Use application performance monitoring to detect degradation in TLS-enabled services that may indicate an ongoing attack
• Review system logs for NSS-related errors or warnings that may indicate exploitation attempts

Monitoring Recommendations

• Configure network monitoring tools to alert on unusual TLS traffic patterns targeting critical infrastructure
• Establish baseline metrics for TLS handshake volumes and set alerts for deviations that could indicate attack activity
• Deploy SentinelOne Singularity platform to monitor endpoint behavior and detect resource exhaustion patterns indicative of DoS attacks
• Implement continuous monitoring of service availability for applications using NSS libraries

How to Mitigate CVE-2020-25648

Immediate Actions Required

• Upgrade Mozilla Network Security Services to version 3.58 or later to address this vulnerability
• Apply vendor-specific patches for affected systems including Red Hat Enterprise Linux, Fedora, and Oracle products
• Implement network-level rate limiting to mitigate potential exploitation before patches can be applied
• Review and audit all systems using NSS libraries to identify vulnerable deployments

Patch Information

Mozilla has addressed this vulnerability in NSS version 3.58. The fix implements proper handling and limiting of CCS messages in TLS 1.3 connections. Organizations should update to this version or later as documented in the Mozilla NSS 3.58 Release Notes.

For Red Hat Enterprise Linux systems, consult the Red Hat Bug Report for patch availability and installation instructions. Oracle users should refer to the Oracle July 2021 CPU Alert, Oracle October 2021 CPU Alert, and Oracle April 2022 CPU Alert for patches addressing affected Oracle products.

Workarounds

• Implement network-level protections such as rate limiting on TLS connections to reduce the impact of flooding attacks
• Use firewall rules to restrict access to TLS services to trusted IP ranges where feasible
• Consider deploying a reverse proxy or load balancer with DDoS protection capabilities in front of vulnerable services
• Monitor and temporarily block IP addresses exhibiting suspicious connection patterns until patches can be applied

# Example: Rate limiting TLS connections using iptables
iptables -A INPUT -p tcp --dport 443 -m connlimit --connlimit-above 50 --connlimit-mask 32 -j DROP
iptables -A INPUT -p tcp --dport 443 -m state --state NEW -m recent --set
iptables -A INPUT -p tcp --dport 443 -m state --state NEW -m recent --update --seconds 60 --hitcount 100 -j DROP