We see no evidence that specific core network autonomous systems have 
blocked Telnet, contrary to previous reports. We specifically see continued 
non-spoofable Telnet traffic from networks on which GreyNoise saw 100% 
drop-off. We suspect initial results may have been measurement artifacts or 
specific threat actors explicitly avoiding GreyNoise infrastructure, though 
determining this root cause is impossible without internal data.

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Analysis of network traffic data contradicts recent reports claiming major US ISPs blocked Telnet traffic following a CVE announcement. Using non-spoofable traffic analysis, RIPE Atlas measurements, and cross-referencing with multiple data sources, researchers found no evidence of ISP-level Telnet filtering. The original findings likely resulted from measurement artifacts or threat actors avoiding specific monitoring infrastructure, with session-based metrics potentially overstating trends from single coordinated actors.

Reports of Telnet’s Death Have Been Greatly Exaggerated — Terrace Networks