CVE-2021-20305: Nettle Project Nettle DoS Vulnerability

CVE-2021-20305 Overview

A cryptographic vulnerability has been identified in Nettle, a low-level cryptographic library, affecting versions prior to 3.7.2. The flaw exists within several signature verification functions including GOST DSA, EDDSA, and ECDSA implementations. When processing certain inputs, the Elliptic Curve Cryptography (ECC) point multiply function is called with out-of-range scalar values, potentially leading to incorrect cryptographic results. This vulnerability allows attackers to force invalid signature acceptance, which could cause assertion failures or improper validation bypasses.

Critical Impact

This vulnerability compromises cryptographic signature verification, threatening the confidentiality, integrity, and availability of systems relying on Nettle for cryptographic operations. Attackers could potentially forge signatures or cause denial of service conditions.

Affected Products

• Nettle Project Nettle (versions before 3.7.2)
• Fedora 33
• Red Hat Enterprise Linux 7.0 and 8.0
• NetApp Active IQ Unified Manager (VMware vSphere)
• NetApp ONTAP Select Deploy Administration Utility
• Debian Linux 9.0 and 10.0

Discovery Timeline

• 2021-04-05 - CVE-2021-20305 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2021-20305

Vulnerability Analysis

This vulnerability affects the core cryptographic signature verification mechanisms in the Nettle library. The flaw manifests when the library processes signature verification requests using GOST DSA, EDDSA, or ECDSA algorithms. During these operations, the ECC point multiplication function receives scalar values that fall outside the expected valid range. This boundary condition error in the cryptographic computation can produce incorrect mathematical results, fundamentally undermining the integrity of the signature verification process.

The vulnerability is classified under CWE-327 (Use of a Broken or Risky Cryptographic Algorithm) and CWE-787 (Out-of-bounds Write), indicating both the cryptographic nature of the flaw and its potential memory safety implications. Systems using Nettle for TLS/SSL operations, package verification, or other cryptographic functions are at risk.

Root Cause

The root cause lies in insufficient input validation within the ECC scalar multiplication operations. When signature verification functions process specially crafted or malformed signature data, the scalar values passed to the underlying ECC multiply function are not properly bounds-checked. This allows out-of-range scalars to be processed, leading to undefined mathematical behavior in the elliptic curve calculations. The lack of proper range validation before cryptographic operations creates a condition where invalid signatures may be incorrectly validated as authentic.

Attack Vector

The attack vector is network-based, requiring no user interaction or privileges. An attacker can exploit this vulnerability by presenting a maliciously crafted signature to a system using a vulnerable version of Nettle for signature verification. This could occur in various contexts such as TLS handshakes, software package signature verification, or any application relying on Nettle's GOST DSA, EDDSA, or ECDSA implementations.

The attack complexity is considered high due to the cryptographic nature of the exploitation. However, successful exploitation enables an attacker to:

1. Force acceptance of invalid signatures, bypassing authentication or integrity checks
2. Trigger assertion failures leading to denial of service
3. Potentially manipulate cryptographic validation outcomes to compromise system integrity

Since no verified proof-of-concept code is publicly available, interested parties should refer to the Red Hat Bug Report for additional technical details on the vulnerability mechanism.

Detection Methods for CVE-2021-20305

Indicators of Compromise

• Unexpected assertion failures or crashes in applications using Nettle cryptographic functions
• Abnormal signature verification behavior where invalid signatures are being accepted
• Application logs showing errors related to ECC point multiplication or scalar operations
• Increased cryptographic operation failures in TLS/SSL implementations

Detection Strategies

• Monitor application logs for assertion failures originating from Nettle library functions, particularly in ecc_point_mul and related ECC operations
• Implement integrity checks to verify that installed Nettle library versions are 3.7.2 or later
• Deploy vulnerability scanning tools to identify systems running vulnerable Nettle versions across the enterprise
• Review TLS handshake logs for anomalies in signature verification processes

Monitoring Recommendations

• Enable verbose logging for applications utilizing Nettle cryptographic functions to capture potential exploitation attempts
• Configure intrusion detection systems to alert on patterns associated with malformed signature data
• Establish baseline metrics for signature verification success/failure rates to detect statistical anomalies
• Monitor system stability for unexpected crashes in services dependent on Nettle

How to Mitigate CVE-2021-20305

Immediate Actions Required

• Update Nettle library to version 3.7.2 or later immediately across all affected systems
• Identify all applications and services dependent on Nettle cryptographic functions within your environment
• Prioritize patching for systems handling sensitive cryptographic operations or exposed to network-based attacks
• Review and audit signature verification logs for any historical evidence of exploitation attempts

Patch Information

Patches are available from multiple vendors and distribution maintainers. The following security advisories provide detailed patching guidance:

• Red Hat Bug Report - Primary vendor tracking
• Debian Security Advisory DSA-4933 - Debian package updates
• Debian LTS Announcement - Long-term support updates
• Fedora Package Announcement - Fedora updates
• Gentoo GLSA Advisory - Gentoo security update
• NetApp Security Advisory - NetApp product updates

Workarounds

• If immediate patching is not feasible, consider temporarily disabling or restricting access to services relying on Nettle signature verification
• Implement additional network-level controls to limit exposure of affected systems to untrusted network traffic
• Where possible, configure applications to use alternative cryptographic libraries until Nettle can be updated
• Apply strict input validation at application boundaries before signature data reaches Nettle functions

# Verify Nettle version on Linux systems
nettle-hash --version

# Check for vulnerable packages on Debian/Ubuntu
dpkg -l | grep libnettle

# Update Nettle on Debian/Ubuntu systems
sudo apt update && sudo apt install --only-upgrade libnettle8 libnettle6

# Update on Red Hat/CentOS/Fedora
sudo dnf update nettle