CVE-2026-26999: Traefik: tcp router clears read deadlines before tls forwarding, enabling stalled handshakes (slowloris doS)

Severity: high (CVSS 7.5 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.6% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-03-05 · updated 2026-07-15
Weakness: CWE-400

Description

Traefik is an HTTP reverse proxy and load balancer. Prior to versions 2.11.38 and 3.6.9, there is a potential vulnerability in Traefik managing TLS handshake on TCP routers. When Traefik processes a TLS connection on a TCP router, the read deadline used to bound protocol sniffing is cleared before the TLS handshake is completed. When a TLS handshake read error occurs, the code attempts a second handshake with different connection parameters, silently ignoring the initial error. A remote unauthenticated client can exploit this by sending an incomplete TLS record and stopping further data transmission, causing the TLS handshake to stall indefinitely and holding connections open. By opening many such stalled connections in parallel, an attacker can exhaust file descriptors and goroutines, degrading availability of all services on the affected entrypoint. This issue has been patched in versions 2.11.38 and 3.6.9.

Affected products

Traefik: before 2.11.38; from 3.0.0 before 3.6.9 — fixed in 2.11.38, 3.6.9

How to fix

Upgrade to a fixed version: Traefik 2.11.38, 3.6.9. Check the vendor advisory for mitigations.

References

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