How to Fix CVE-2017-6743: Buffer overflow in IOS

WARNING: This vulnerability is on the CISA Known Exploited Vulnerabilities catalog (added 2022-03-03). Federal civilian agencies must remediate by 2022-03-24. Treat it as active exploitation, not theoretical.

What is CVE-2017-6743?

The Simple Network Management Protocol (SNMP) subsystem of Cisco IOS and IOS XE Software contains multiple vulnerabilities that could allow an authenticated, remote attacker to remotely execute code on an affected system or cause an affected system to reload. An attacker could exploit these vulnerabilities by sending a crafted SNMP packet to an affected system via IPv4 or IPv6. Only traffic directed to an affected system can be used to exploit these vulnerabilities. The vulnerabilities are due to a buffer overflow condition in the SNMP subsystem of the affected software.

Am I affected?

From the device CLI, run the version check:

# Cisco IOS/IOS XE
show version | include Version

# FortiGate (FortiOS)
get system status

# Palo Alto (PAN-OS)
show system info | match sw-version

# Juniper (Junos)
show version

Compare what you see against the Affected row above (Cisco IOS 12.1(3)XI, 12.2(1b)DA, 12.2(5)DA, 12.2(7)DA, 12.2(12)DA, 12.2(10)DA5, 12.2(12)DA10, 12.2(10)DA, 12.2(12)DA1, 12.2(12)DA6, 12.2(10)DA8, 12.2(12)DA8, 12.2(12)DA11, 12.2(12)DA9, 12.2(12)DA4, 12.2(10)DA3, 12.2(5)DA1, 12.2(12)DA13, 12.2(12)DA12, 12.2(12)DA7, 12.2(1b)DA1, 12.2(10)DA1, 12.2(10)DA6, 12.2(10)DA4, 12.2(12)DA2, 12.2(12)DA3, 12.2(10)DA2, 12.2(12)DA5, 12.2(10)D...). If your build sits inside that range, the device is exposed and should be upgraded.

How to fix CVE-2017-6743

The primary fix is to upgrade IOS to the patched build. Use the commands for your platform below; the patched version listed in the vendor advisory is: See vendor advisory.

Cisco IOS / IOS XE

# Vendor advisory: https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20170629-snmp
enable
show version | include Version
copy running-config startup-config
copy tftp: flash:
! upload the patched image, then:
boot system flash:<patched-image>.bin
reload

Complete operator runbook (network appliance)

Run this from your management workstation (Linux/macOS or Windows with a TFTP server reachable). It shells into the device, backs up the running config, transfers the patched image, and reloads.

# Vendor advisory: https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20170629-snmp
#!/usr/bin/env bash
# CVE-2017-6743 fix runbook for Cisco IOS
# Target build: the patched build

set -euo pipefail
DEVICE="${1:?usage: $0 <device-ip> <patched-image-file> <tftp-server-ip>}"
IMAGE="${2:?patched image filename required}"
TFTP="${3:?tftp server ip required}"
LOG="/var/log/cve-2017-6743-$(date +%Y%m%d-%H%M%S).log"

echo "[1/4] Pulling current config from $DEVICE"
ssh admin@"$DEVICE" "show version" | tee -a "$LOG"
ssh admin@"$DEVICE" "copy running-config tftp://$TFTP/cve-2017-6743-pre.cfg"

echo "[2/4] Uploading patched image $IMAGE via TFTP"
ssh admin@"$DEVICE" "copy tftp://$TFTP/$IMAGE flash:$IMAGE" | tee -a "$LOG"

echo "[3/4] Setting boot image and reloading"
ssh admin@"$DEVICE" "configure terminal
 boot system flash:$IMAGE
 end
 write memory
 reload" | tee -a "$LOG"

echo "[4/4] Wait ~5 minutes, then verify"
sleep 300
ssh admin@"$DEVICE" "show version | include Version" | tee -a "$LOG"
echo "Compare reported version against the patched build"

If you can't patch immediately

If you cannot apply the patched version today, restrict exposure with one of the following runnable controls. None replace the patch.

Network appliance ACL

# Cisco IOS example: restrict management plane to a trusted subnet
configure terminal
access-list 99 permit 10.0.0.0 0.255.255.255
line vty 0 15
 access-class 99 in
end
write memory

Service-level fallback

# If the affected feature is optional, stop the service until the patch is applied
sudo systemctl stop ios
sudo systemctl disable ios

How to verify the fix worked

# Linux
ios --version 2>/dev/null || dpkg -s ios | grep -i version
rpm -q ios 2>/dev/null || true

# Windows
winget list | findstr /I "ios"
Get-HotFix | Sort-Object InstalledOn -Descending | Select-Object -First 5

Expected: the reported version is at or above the patched build documented in the advisory. Restart any services that loaded the old library (systemctl restart <service> on Linux, restart the Windows service or reboot when prompted). For network appliances, run show version on the device and confirm the build matches the patched release.

Frequently asked questions

Related fixes

Other vulnerabilities in the same area that are worth patching alongside this one:

• How to Fix CVE-2017-12237: Cwe-399 in Cisco IOS and IOS XE — Cwe-399 in Cisco IOS and IOS XE
• How to Fix CVE-2017-6736: Buffer overflow in IOS — Buffer overflow in IOS
• How to Fix CVE-2017-6663: Security vulnerability in Cisco IOS and IOS XE , Security vulnerability in Cisco IOS and IOS XE
• How to Fix CVE-2017-6737: Buffer overflow in IOS , Buffer overflow in IOS
• How to Fix CVE-2017-6627: Cwe-399 in Cisco IOS and Cisco IOS XE , Cwe-399 in Cisco IOS and Cisco IOS XE

Is CVE-2017-6743 actually being exploited?

According to the data sources above, yes, CISA has it listed as actively exploited. Either way, the fix is the same: apply the vendor patch.

Do I need to reboot after patching?

For OS or kernel updates, yes. For most userland packages a systemctl restart <service> is enough. Any process that loaded the old shared library keeps using it until restarted, so when in doubt, reboot.

What is the CVSS score?

8.8 (high). Refer to the vendor advisory for the exact vector string.

Where is the official advisory?

See the References section at the bottom of this page; the vendor's URL is the authoritative source for affected builds and patched versions.

References

• Official vendor advisory: https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20170629-snmp
• NVD: https://nvd.nist.gov/vuln/detail/CVE-2017-6743
• CISA KEV catalog entry: https://www.cisa.gov/known-exploited-vulnerabilities-catalog
• http://www.securityfocus.com/bid/99345
• http://www.securitytracker.com/id/1038808
• https://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20170629-snmp
• https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2017-6743

*Written by Sai Kiran Pandrala. Assembled from the official vendor advisory, NVD record, and CISA KEV listing on 2026-05-25. Always confirm against the vendor's advisory before applying changes in production.*