CVE-2025-39773 Cert Portal CVE debrief

Who should care

Operators and maintainers of Siemens SIMATIC CN 4100 devices, especially environments that use Linux bridge multicast querier settings or customized multicast timing. OT/security teams responsible for availability monitoring should treat this as a relevant service-stability issue.

Technical summary

The advisory says br_multicast_send_query() can overflow a local time calculation when multicast_query_interval is set to a very large value. If the computed timer value falls before jiffies, the timer expires immediately, mod_timer() is called again, and the bridge multicast query path can loop until a soft lockup occurs. The same condition can also be triggered by multicast_startup_query_interval. The supplied CVSS vector is 3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H (5.5, Medium), indicating an availability-focused issue requiring local privileges.

Defensive priority

Medium

Recommended defensive actions

• Update Siemens SIMATIC CN 4100 to V5.0 or later, per the vendor remediation in the advisory.
• Review bridge multicast configuration values, especially multicast_query_interval and multicast_startup_query_interval, and ensure they stay within supported limits.
• Monitor affected systems for watchdog or soft-lockup symptoms and validate that timer-related bridge behavior is normal after any configuration changes.
• Use the linked CISA and Siemens advisories as the source of truth for remediation status and follow-up guidance.

Evidence notes

This debrief is based on the supplied CISA CSAF advisory ICSA-26-134-10 (published 2026-05-12, republished 2026-05-14 from Siemens ProductCERT SSA-032379) and the included vendor notes. The advisory explicitly describes an overflow in Linux kernel bridge multicast query timing that can create an immediate-expiry/mod_timer loop leading to a soft lockup, and it recommends updating Siemens SIMATIC CN 4100 to V5.0 or later. The supplied metadata also provides CVSS 3.1 5.5/Medium with availability impact only.

Official resources