503
Summary
- // typical exposure
- Internal-only — The port's only documented legitimate use is as an alternate Modbus-TCP (ICS/OT) port, a protocol class that should never be reachable from the public internet given its lack of built-in authentication, while the formal 'intrinsa' registration itself has no known public-facing implementation.
- // common applications
- HMS Networks EWON Flexy IO Server (Modbu Traffic on this port is most often absent or anomalous given intrinsa's undocumented historical registration; HMS Networks has reported configuring its EWON Flexy IO Server to use 503/tcp as an alternate Modbus-TCP port, though this is a single first-party vendor sighting, not independently corroborated.
- // analyst note
- 503/tcp has no confirmed default protocol; if it responds, check whether it is a Modbus-TCP gateway configured to the alternate port before assuming standard "intrinsa" behavior, which itself has no known implementation.
- // if you see it open
- No established public-facing protocol runs on 503/tcp by default. The single documented use — HMS Networks' EWON Flexy configured as an alternate Modbus-TCP port — is an ICS/OT protocol that should stay on trusted operational-technology networks, not the public internet (Modbus-TCP has no built-in authentication). No credible malware or trojan association for 503/tcp was found as of an August 2026 search.
About port 503/tcp.
Port 503/tcp carries the historical IANA registration intrinsa, but no protocol specification, vendor documentation, or widespread deployment for it is publicly known, so a listener here has no default legitimate purpose and should not be treated as safe to expose to the public internet.
IANA lists intrinsa on both 503/tcp and 503/udp under assignee Robert_Ford, with the description field simply reading "Intrinsa" and the Reference column blank — no RFC or specification is cited, and the registry carries no registration or modification date, consistent with an early-to-mid-1990s-era IANA assignment whose exact date is unlisted.
No modern protocol has formally reassigned or commonly repurposed the port. The one credible sighting found in research is HMS Networks' own support documentation for its EWON Flexy IO Server, which describes configuring the device's transparent Modbus gateway to use TCP port 503 as an alternate Modbus-TCP port instead of the conventional port 502 — a first-party vendor report, not independently corroborated by third-party sightings.
Measured open-frequency for 503/tcp in the site's port-data corpus is exactly 0 — a real sampled reading, not evidence of absence — while 503/udp measures 0.000708 (~0.07%). Combined with the blank IANA reference and lack of any documented public-facing implementation, an open 503/tcp is best treated as anomalous or, if genuinely Modbus-related, as internal industrial-control traffic that should not face the public internet.
- IANA assignment
intrinsa— description "Intrinsa"; Reference column blank; assignee/contact Robert_Ford; dual-registered 503/tcp + 503/udp; registration/modification dates blank [Confirmed] — the IANA Service Name and Transport Protocol Port Number Registry intrinsa 503/tcp- Range class
- well-known (0–1023) [Confirmed] — the IANA Service Name and Transport Protocol Port Number Registry intrinsa 503/tcp
- Prevalence
- nmap-services open-frequency for 503/tcp = 0 (sampled, not observed open); 503/udp = 0.000708 (~0.07%) [Confirmed] — this site's own tooling, port 503
- Related ports
- 502/tcp (conventional Modbus-TCP), 503/udp (same intrinsa IANA registration)
Primary use
No modern protocol has formally reassigned or commonly repurposed 503/tcp; the historical intrinsa registration predates any documented widespread deployment and no successor standard occupies the port as of an August 2026 search
Other/unofficial uses
HMS Networks' EWON Flexy IO Server documents 503/tcp as a configurable alternate Modbus-TCP port for its transparent gateway feature (first-party vendor source, not independently corroborated)
Security implications
no established public-facing protocol; the one documented use is an ICS/OT (Modbus-TCP alternate) configuration that should remain on trusted operational networks; no malware/trojan association found as of an August 2026 search
Typically seen on
industrial/OT gateway devices configured to use 503/tcp as an alternate Modbus-TCP port (e.g., HMS EWON Flexy); otherwise an anomaly given the port's obscure historical assignment [Unknown]
- Analyst note
- 503/tcp has no confirmed default protocol; if it responds, check whether it is a Modbus-TCP gateway configured to the alternate port before assuming standard "intrinsa" behavior, which itself has no known implementation.
About port 503/udp.
Port 503/udp is registered with IANA under the service name intrinsa, but no software is reliably documented as running on it today, so this port has no legitimate reason to be reachable from the public internet. Both 503/tcp and 503/udp carry the same registration, a pattern dating to the mid-1990s IANA convention of assigning a port directly to a named company or individual rather than to an open, published protocol.
The registration traces to Intrinsa Corporation, a Mountain View, California maker of software-testing and verification tools. Microsoft acquired Intrinsa in late July 1999 for roughly $58.9 million in stock. IANA's entry lists assignee Robert Ford and leaves the Reference column blank — no RFC or protocol document was ever published describing how the intrinsa service worked.
No credible community sighting (vendor documentation, blog post, or forum thread) ties current traffic on 503/udp to any specific application as of an August 2026 search. Low-quality port-lookup aggregators either just restate the bare IANA description or repeat unsourced filler claims (e.g., "medical device data transmission") that could not be corroborated by any primary or vendor source, so none of that is treated as a finding here.
In the repo's nmap-services open-frequency data, 503/udp measures 0.000708 and 503/tcp measures 0 — both effectively unseen open in scanned samples, consistent with an obscure legacy assignment rather than an active, widely-deployed service.
- IANA assignment
intrinsa— "Intrinsa"; reference (blank — no RFC or protocol document cited in IANA registry); assignee Robert Ford; dual-registered 503/tcp + 503/udp [Confirmed] — the IANA Service Name and Transport Protocol Port Number Registry intrinsa 503/tcp and 503/udp- Range class
- well-known (0–1023) [Confirmed] — this site's own tooling (range: "system")
- Prevalence
- nmap-services open-frequency 503/udp = 0.000708; 503/tcp = 0 [Confirmed] — this site's own tooling
- Related ports
- 503/tcp (same
intrinsadual registration)
Primary use
undocumented; the registration predates any published protocol spec and was assigned directly to Intrinsa Corporation, a software-testing/verification tool vendor
Other/unofficial uses
none credibly documented as of an August 2026 search; unsourced aggregator claims (e.g. "medical device data transmission") were checked and excluded as unverifiable [Unknown]
Security implications
no confirmed CVE or malware association as of 2026-08; no legitimate current use is documented, so an open 503/udp is anomalous
Typically seen on
no host type is documented; historically tied to a legacy IANA assignment for Intrinsa Corporation, acquired by Microsoft in 1999
- Analyst note
- An open port 503/udp has no known legitimate current use; treat it as anomalous and worth investigating rather than assuming a benign legacy service is actually running.
Service assignments.
| Name | Protocol | Description | Open frequency |
|---|---|---|---|
| intrinsa | UDP | — | 0.07% |
| intrinsa | TCP | — | 0.00% |
Service assignments from the IANA Service Name and Transport Protocol Port Number Registry, with open-frequency data from nmap-services.