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CVE-2026-49847 – Denial of Service via Stack Overflow – FreeSWITCH prior to 1.11.1

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Summary

CVE-2026-49847 is a high-severity Denial of Service vulnerability in FreeSWITCH, the open-source Software Defined Telecom Stack maintained by SignalWire. The flaw stems from uncontrolled recursion (CWE-674) in FreeSWITCH’s bundled cJSON parser, and affects all versions prior to 1.11.1. A single unauthenticated WebSocket frame carrying a deeply nested JSON document is sufficient to crash the entire FreeSWITCH process, instantly terminating all active calls and sessions on the affected host. The CVSS v3.1 base score is 7.5 (High).

Technical details

  • Root cause: The bundled cJSON parser performs unbounded recursive descent when parsing nested JSON objects. There is no cap on recursion depth, allowing a specially crafted payload to drive recursive calls until the worker thread’s stack is exhausted. The fix in v1.11.1 explicitly caps the cJSON parser nesting depth to prevent this condition.
  • Trigger conditions: A single WebSocket frame containing a deeply nested JSON document. No prior authentication, no established session, and no user interaction are required — the attack is fully pre-authentication with low complexity (AC:L).
  • Attack vector: Network (AV:N). The exploit is deliverable from anywhere on the internet to any FreeSWITCH instance with an exposed WebSocket port (default: 5066/TCP for WS, 7443/TCP for WSS).
  • Impact: The worker thread’s stack pointer is driven into the kernel-managed stack guard page, raising SIGSEGV. The kernel signal terminates the entire FreeSWITCH process — immediately dropping all active calls and sessions on the host. No usable memory write primitive develops; impact is confined to availability (A:H, C:N, I:N).

Affected software

  • FreeSWITCH (signalwire/freeswitch): all versions prior to 1.11.1

Severity

  • CVSS v3.1 base score: 7.5 (High)
  • Vector string: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

Mitigation and recommended actions

  • Immediate action: Upgrade FreeSWITCH to version 1.11.1 or later. The v1.11.1 release caps the cJSON parser nesting depth, directly remediating the uncontrolled recursion. SignalWire describes v1.11.1 as an important release containing critical security fixes and recommends immediate upgrading for all users.
  • If immediate patching is not feasible: Restrict access to FreeSWITCH’s WebSocket ports (default 5066/TCP and 7443/TCP) at the network perimeter using firewall rules or ACLs to limit exposure to trusted IP ranges only. Note that this is a temporary measure and does not eliminate the vulnerability.

IONIX Status

The IONIX research team is tracking ongoing exploitation attempts and recommends immediate patching. Potentially affected assets are outlined in this post.

References

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