CVE-2021-20305
📋 TL;DR
This vulnerability in Nettle cryptographic library allows attackers to forge digital signatures by exploiting incorrect elliptic curve multiplication with out-of-range scalars. Systems using Nettle for GOST DSA, EDDSA, or ECDSA signature verification are affected, potentially leading to invalid signature acceptance, assertion failures, or validation bypass.
💻 Affected Systems
- Nettle cryptographic library
- Applications using Nettle for GOST DSA, EDDSA, or ECDSA signature verification
📦 What is this software?
Fedora by Fedoraproject
Nettle by Nettle Project
⚠️ Risk & Real-World Impact
Worst Case
Complete system compromise through forged signatures allowing authentication bypass, data tampering, or denial of service via assertion failures.
Likely Case
Signature validation failures leading to service disruption or potential authentication bypass in affected applications.
If Mitigated
Limited impact with proper network segmentation and monitoring, though signature validation remains unreliable.
🎯 Exploit Status
Exploitation requires ability to submit forged signatures to vulnerable systems.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Nettle 3.7.2 or later
Vendor Advisory: https://bugzilla.redhat.com/show_bug.cgi?id=1942533
Restart Required: Yes
Instructions:
1. Update Nettle to version 3.7.2 or later using your distribution's package manager. 2. Restart affected services or applications using Nettle. 3. For Red Hat/Fedora: 'sudo dnf update nettle'. 4. For Debian/Ubuntu: 'sudo apt update && sudo apt upgrade nettle'.
🔧 Temporary Workarounds
Disable affected signature algorithms
allConfigure applications to avoid using GOST DSA, EDDSA, or ECDSA signature verification via Nettle
🧯 If You Can't Patch
- Implement network segmentation to isolate vulnerable systems
- Monitor for signature validation failures and assertion crashes in logs
🔍 How to Verify
Check if Vulnerable:
Check Nettle version: 'nettle-hash --version' or 'rpm -q nettle' or 'dpkg -l | grep nettle'Check Version:
nettle-hash --version | grep -i nettleVerify Fix Applied:
Verify Nettle version is 3.7.2 or later using version check commands📡 Detection & Monitoring
Log Indicators:
- Application crashes with assertion failures
- Signature validation errors
- Unexpected authentication successes
Network Indicators:
- Unusual signature submission patterns
- Multiple failed signature attempts followed by success
SIEM Query:
source="application.logs" AND ("assertion failure" OR "signature verification failed" OR "invalid signature")🔗 References
- https://bugzilla.redhat.com/show_bug.cgi?id=1942533
- https://lists.debian.org/debian-lts-announce/2021/09/msg00008.html
- https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/MQKWVVMAIDAJ7YAA3VVO32BHLDOH2E63/
- https://security.gentoo.org/glsa/202105-31
- https://security.netapp.com/advisory/ntap-20211022-0002/
- https://www.debian.org/security/2021/dsa-4933
- https://bugzilla.redhat.com/show_bug.cgi?id=1942533
- https://lists.debian.org/debian-lts-announce/2021/09/msg00008.html
- https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/MQKWVVMAIDAJ7YAA3VVO32BHLDOH2E63/
- https://security.gentoo.org/glsa/202105-31
- https://security.netapp.com/advisory/ntap-20211022-0002/
- https://www.debian.org/security/2021/dsa-4933
