Dendrite EUVD-2026-45194

Exploitation requires network access to the Dendrite instance's /_matrix/media/r0/download endpoint, which is typically internet-exposed on Matrix homeservers as part of federation. No authentication is required - the serverName parameter is processed prior to any credential check, confirmed by CVSS PR:N. The attacker must be able to observe HTTP response status codes or error message body content to distinguish port state classes (connection refused vs. timeout vs. TLS error). Effectiveness is bounded by the internal network reachability of the Dendrite process: only hosts and ports reachable from Dendrite's network namespace can be probed. Dendrite deployments that have fully migrated to the v1 or v3 media API and disabled the legacy /_matrix/media/r0/ routes, or that enforce strict egress filtering from the Dendrite process, are not exploitable via this path.

The CVSS 4.0 vector (AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:N/SC:L/SI:N/SA:N, score 6.9) accurately captures that no authentication is required, complexity is low, and the primary impact is low-confidence reconnaissance on subsequent internal systems rather than direct data exfiltration or service disruption. The absence of VC/VI/VA impact metrics means Dendrite itself is not directly compromised, but SC:L reflects the meaningful reconnaissance value against internal infrastructure. A publicly available POC lowers the exploitation bar significantly - any actor with HTTP access to the endpoint can run the attack without specialized tooling. EPSS data was not provided in the source intelligence; this absence limits quantified exploitation-probability comparison. No CISA KEV listing means active exploitation has not been formally confirmed, but the combination of unauthenticated access, trivial exploitation, and a public POC creates urgency for internet-exposed deployments. In cloud environments with flat internal networking, this SSRF could serve as an initial reconnaissance step enabling lateral movement, making the effective business risk higher than the base score alone suggests.

An unauthenticated attacker sends a crafted HTTP GET request to /_matrix/media/r0/download/{serverName}/{mediaId} on a public-facing Dendrite instance, substituting serverName with an internal target such as 192.168.10.5:6379 (a Redis port). The Dendrite server attempts an outbound TLS connection to that address, and the attacker distinguishes between a TCP-refused error, a timeout, and a TLS handshake response to infer whether the port is open, closed, or filtered - effectively using Dendrite as a port scanner against its internal network. A public POC demonstrating this technique is available at the researcher's GitHub repository.