CVE-2020-8794 – CVE-2020-8794 is a critical out-of-bounds read ... (How to Fix)

Q: What is the severity of CVE-2020-8794?

CVE-2020-8794 has a CVSS score of 9.8 (CRITICAL). CVE-2020-8794 is a critical out-of-bounds read vulnerability in OpenSMTPD that allows remote code execution. Attackers can exploit this during bounce handling to execute arbitrary code on vulnerable servers. Any system running OpenSMTPD versions before 6.6.4 is affected.

📋 TL;DR

CVE-2020-8794 is a critical out-of-bounds read vulnerability in OpenSMTPD that allows remote code execution. Attackers can exploit this during bounce handling to execute arbitrary code on vulnerable servers. Any system running OpenSMTPD versions before 6.6.4 is affected.

💻 Affected Systems

Products:

OpenSMTPD

Versions: All versions before 6.6.4

Operating Systems: All operating systems running OpenSMTPD

Default Config Vulnerable: ⚠️ Yes

Notes: Affects the client-side code that servers use during bounce handling, making servers vulnerable despite being a client-side issue.

📦 What is this software?

⚠️ Risk & Real-World Impact

🔴

Worst Case

Full system compromise with root privileges, allowing attackers to install backdoors, exfiltrate data, or pivot to other systems.

🟠

Likely Case

Remote code execution leading to mail server compromise, potential data theft, and use as a foothold for further network attacks.

🟢

If Mitigated

Limited impact with proper network segmentation and minimal privileges, though RCE would still be possible.

🌐 Internet-Facing: HIGH - SMTP servers are typically internet-facing and this vulnerability allows unauthenticated remote exploitation.

🏢 Internal Only: MEDIUM - Internal SMTP servers could still be exploited by internal attackers or compromised hosts.

🎯 Exploit Status

Public PoC: ⚠️ Yes

Weaponized: CONFIRMED

Unauthenticated Exploit: ⚠️ Yes

Complexity: LOW

Public exploit code exists and the vulnerability is relatively straightforward to exploit against vulnerable servers.

🛠️ Fix & Mitigation

✅ Official Fix

Patch Version: 6.6.4 and later

Vendor Advisory: https://www.opensmtpd.org/security.html

Restart Required: Yes

Instructions:

1. Update OpenSMTPD to version 6.6.4 or later using your package manager. 2. For source installations: download latest release from opensmtpd.org, compile, and install. 3. Restart the OpenSMTPD service.

🔧 Temporary Workarounds

Disable bounce handling

linux

Disable bounce message processing to prevent exploitation, though this may break legitimate mail functionality

# Modify OpenSMTPD configuration to disable bounce handling
# This requires configuration changes specific to your setup

Network filtering

linux

Block external access to SMTP ports except from trusted sources

iptables -A INPUT -p tcp --dport 25 -s trusted_network -j ACCEPT
iptables -A INPUT -p tcp --dport 25 -j DROP

🧯 If You Can't Patch

Implement strict network segmentation to isolate SMTP servers from critical systems
Deploy intrusion detection/prevention systems to monitor for exploitation attempts

🔍 How to Verify

Check if Vulnerable:

Check OpenSMTPD version: smtpd -v or smtpd -n 2>&1 | head -1

Check Version:

smtpd -v 2>/dev/null || smtpd -n 2>&1 | head -1

Verify Fix Applied:

Verify version is 6.6.4 or higher: smtpd -v | grep -q '6.6.[4-9]\|6.[7-9]\|[7-9]' && echo 'Patched'

📡 Detection & Monitoring

Log Indicators:

Unusual bounce message patterns
Multiple failed connection attempts followed by successful exploitation
Process execution from SMTP daemon context

Network Indicators:

Malformed SMTP responses triggering bounce handling
Unusual traffic patterns to/from SMTP port 25

SIEM Query:

source="smtpd.log" AND ("bounce" OR "out-of-bounds" OR "segmentation fault")

📊 Metadata

CVE ID: CVE-2020-8794

CVSS v3 Score: 9.8 (CRITICAL)

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

CWE: CWE-125

Published: February 25, 2020

Last Updated: November 21, 2024

🔗 References

http://packetstormsecurity.com/files/156633/OpenSMTPD-Out-Of-Bounds-Read-Local-Privilege-Escalation.html Exploit,Third Party Advisory,VDB Entry
http://seclists.org/fulldisclosure/2020/Feb/32 Mailing List,Third Party Advisory
http://www.openwall.com/lists/oss-security/2020/02/26/1 Exploit,Mailing List,Third Party Advisory
http://www.openwall.com/lists/oss-security/2020/03/01/1 Mailing List,Third Party Advisory
http://www.openwall.com/lists/oss-security/2020/03/01/2 Mailing List,Third Party Advisory
http://www.openwall.com/lists/oss-security/2021/05/04/7 Exploit,Mailing List,Third Party Advisory
https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/OPH4QU4DNVHA7ACFXMYFCEP5PSXXPN4E/
https://usn.ubuntu.com/4294-1/ Patch,Third Party Advisory
https://www.debian.org/security/2020/dsa-4634 Third Party Advisory
https://www.openbsd.org/security.html Third Party Advisory
https://www.openwall.com/lists/oss-security/2020/02/24/5 Exploit,Mailing List,Third Party Advisory
http://packetstormsecurity.com/files/156633/OpenSMTPD-Out-Of-Bounds-Read-Local-Privilege-Escalation.html Exploit,Third Party Advisory,VDB Entry
http://seclists.org/fulldisclosure/2020/Feb/32 Mailing List,Third Party Advisory
http://www.openwall.com/lists/oss-security/2020/02/26/1 Exploit,Mailing List,Third Party Advisory
http://www.openwall.com/lists/oss-security/2020/03/01/1 Mailing List,Third Party Advisory
http://www.openwall.com/lists/oss-security/2020/03/01/2 Mailing List,Third Party Advisory
http://www.openwall.com/lists/oss-security/2021/05/04/7 Exploit,Mailing List,Third Party Advisory
https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/OPH4QU4DNVHA7ACFXMYFCEP5PSXXPN4E/
https://usn.ubuntu.com/4294-1/ Patch,Third Party Advisory
https://www.debian.org/security/2020/dsa-4634 Third Party Advisory
https://www.openbsd.org/security.html Third Party Advisory
https://www.openwall.com/lists/oss-security/2020/02/24/5 Exploit,Mailing List,Third Party Advisory

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