CVE-2020-15572 – This vulnerability is an out-of-bounds memory a... (How to Fix)

Q: What is the severity of CVE-2020-15572?

CVE-2020-15572 has a CVSS score of 7.5 (HIGH). This vulnerability is an out-of-bounds memory access in Tor versions before 0.4.3.6 when built with Mozilla NSS. It allows remote attackers to cause a denial-of-service (crash) of Tor instances. Only Tor instances compiled with NSS instead of OpenSSL are affected.

📋 TL;DR

This vulnerability is an out-of-bounds memory access in Tor versions before 0.4.3.6 when built with Mozilla NSS. It allows remote attackers to cause a denial-of-service (crash) of Tor instances. Only Tor instances compiled with NSS instead of OpenSSL are affected.

💻 Affected Systems

Products:

Tor

Versions: All versions before 0.4.3.6

Operating Systems: All platforms where Tor is built with NSS

Default Config Vulnerable: ✅ No

Notes: Only affects Tor instances compiled with Mozilla NSS instead of OpenSSL. Most distributions use OpenSSL by default.

📦 What is this software?

⚠️ Risk & Real-World Impact

🔴

Worst Case

Remote attacker crashes Tor relay or exit node, disrupting anonymity network services

🟠

Likely Case

Remote denial-of-service causing Tor instance to crash and restart

🟢

If Mitigated

No impact if patched or using OpenSSL build

🌐 Internet-Facing: HIGH - Tor relays and exit nodes are internet-facing by design

🏢 Internal Only: LOW - Internal Tor clients are less likely to be targeted

🎯 Exploit Status

Public PoC: ✅ No

Weaponized: UNKNOWN

Unauthenticated Exploit: ⚠️ Yes

Complexity: MEDIUM

Remote exploitation requires sending crafted traffic to vulnerable Tor instance

🛠️ Fix & Mitigation

✅ Official Fix

Patch Version: 0.4.3.6 or later

Vendor Advisory: https://blog.torproject.org/new-release-tor-03511-0428-0436-security-fixes

Restart Required: Yes

Instructions:

1. Update Tor to version 0.4.3.6 or later. 2. Restart Tor service. 3. Verify version with 'tor --version'

🔧 Temporary Workarounds

Switch to OpenSSL build

all

Recompile Tor with OpenSSL instead of NSS if using NSS build

Network isolation

all

Restrict network access to Tor instances to trusted sources only

🧯 If You Can't Patch

Monitor Tor instances for crashes and restart automatically
Isolate vulnerable Tor instances from untrusted networks

🔍 How to Verify

Check if Vulnerable:

Check Tor version with 'tor --version' and verify if built with NSS

Check Version:

tor --version

Verify Fix Applied:

Confirm version is 0.4.3.6 or later with 'tor --version'

📡 Detection & Monitoring

Log Indicators:

Tor process crashes
Unexpected restarts in Tor logs
Segmentation fault errors

Network Indicators:

Unusual traffic patterns to Tor ports
Connection spikes followed by service disruption

SIEM Query:

process_name:tor AND (event_type:crash OR exit_code:139 OR exit_code:11)

📊 Metadata

CVE ID: CVE-2020-15572

CVSS v3 Score: 7.5 (HIGH)

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

CWE: CWE-125

Published: July 15, 2020

Last Updated: November 21, 2024

🔗 References

https://blog.torproject.org/new-release-tor-03511-0428-0436-security-fixes Release Notes,Vendor Advisory
https://gitlab.torproject.org/tpo/core/tor/-/issues/33119 Vendor Advisory
https://trac.torproject.org/projects/tor/wiki/TROVE Vendor Advisory
https://blog.torproject.org/new-release-tor-03511-0428-0436-security-fixes Release Notes,Vendor Advisory
https://gitlab.torproject.org/tpo/core/tor/-/issues/33119 Vendor Advisory
https://trac.torproject.org/projects/tor/wiki/TROVE Vendor Advisory

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💻 Affected Systems

📦 What is this software?

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⚠️ Risk & Real-World Impact

Worst Case

Likely Case

If Mitigated

🎯 Exploit Status

🛠️ Fix & Mitigation

✅ Official Fix

Instructions:

🔧 Temporary Workarounds

Switch to OpenSSL build

Network isolation

🧯 If You Can't Patch

🔍 How to Verify

Check if Vulnerable:

Check Version:

Verify Fix Applied:

📡 Detection & Monitoring

Log Indicators:

Network Indicators:

SIEM Query:

📊 Metadata

🔗 References

📤 Share & Export

🔗 Related Vulnerabilities