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CVE-2026-20301 is a high-severity denial-of-service vulnerability in the Extensible Messaging Client Protocol (XMCP), also referred to as the External Client protocol, in Cisco IOS Software and Cisco IOS XE Software. The flaw is caused by improper handling of malformed XMCP packets and is classified as an unchecked input for loop condition issue. A remote, unauthenticated attacker can trigger the vulnerability by sending a crafted malformed XMCP packet to an affected device. Successful exploitation can cause the device to reload unexpectedly, producing a denial-of-service condition. The attacker does not need the XMCP client username to exploit the issue.
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What it means. What to do now. Patch path, mitigations, and the assume-compromise checklist.
What an attacker gets, and what they’ve been doing with it.
If you can’t patch tonight, do this now.
Patch, then assume compromise.
No public exploits tracked yet. Mallory keeps watching.
No public exploit code observed for this vulnerability.
Products and vendors Mallory has correlated with this vulnerability. Open in Mallory to drill down to specific CPE configurations and version ranges.
Vendor-confirmed product mapping. Mallory continuously reconciles this list against your asset inventory.
9 sources tracked across advisories, community write-ups, and news. New activity surfaces here as Mallory finds it.
An unchecked input for loop condition vulnerability in Cisco IOS Software and Cisco IOS XE Software.
A high-severity denial-of-service vulnerability in the XMCP/External Client protocol implementation of Cisco IOS Software and Cisco IOS XE Software that allows an unauthenticated remote attacker to send malformed XMCP packets and cause affected devices to reload unexpectedly.
Query your assets running an affected version, and investigate the blast radius.
Every observed campaign linking this CVE to a named adversary.
Malware families riding this exploit, with evidence and IOCs.
YARA, Sigma, Snort, and vendor rules, auto-deployed to your SIEM.
Cross-references every affected SKU, including bundled OEM variants.
Community discussion across Reddit, Mastodon, and other social sources.