CVE-2026-116785 in FreeSWITCH
Summary
by MITRE • 11/05/2026
FreeSWITCH is a Software Defined Telecom Stack enabling the digital transformation from proprietary telecom switches to a software implementation that runs on any commodity hardware. Prior to 1.11.2, ws_read_frame() decoded a WebSocket frame's 64-bit extended payload length with a 32-bit byte swap and narrowed the result into a signed length. A value whose surviving low word became negative bypassed the signed size check and was reinterpreted as an enormous unsigned count, causing client bytes to overwrite memory beyond the 64 KiB receive buffer. Any client completing a /socket WebSocket upgrade on an enabled mod_xml_rpc endpoint reached the parser before HTTP authentication; both the module and WebSocket endpoint are disabled by default. The heap overwrite can corrupt connection pointers and crash FreeSWITCH, ending every concurrent session, although remote code execution has not been demonstrated. This issue is fixed in version 1.11.2.
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Analysis
by VulDB Data Team • 11/05/2026
The vulnerability identified within FreeSWITCH prior to version 1.11.2 represents a critical integer overflow and type confusion flaw located in the WebSocket frame parsing logic, specifically within the ws_read_frame function. This component is responsible for decoding incoming WebSocket frames from clients connecting via mod_xml_rpc endpoints that support the /socket upgrade path. The core technical defect arises during the processing of the 64-bit extended payload length field found in valid WebSocket headers. Instead of correctly handling the full 64-bit integer, the implementation performs a byte swap operation and subsequently narrows the result into a signed 32-bit integer for size validation purposes. This architectural decision creates a dangerous edge case where specific crafted values can bypass security checks designed to prevent buffer overflows.
The exploitation mechanism relies on the behavior of two's complement arithmetic when narrowing wide integers to narrower types with different sign interpretations. When an attacker provides a WebSocket frame header containing a 64-bit payload length that, after byte swapping and truncation to thirty-two bits, results in a negative signed integer value, the subsequent size check fails to trigger as intended. The code interprets this negative number as valid because it passes certain conditional logic checks designed for positive lengths. However, when this negative value is later reinterpreted or cast into an unsigned context for memory allocation and copy operations, it becomes a massive positive integer representing several gigabytes of data rather than the small amount originally specified in the frame header.
This misinterpretation leads directly to a heap-based buffer overflow condition. The application attempts to allocate space based on the erroneously large unsigned value or proceeds to write client-supplied bytes into a fixed-size receive buffer that is only sixty-four kilobytes in capacity. Since there are no adequate bounds checks applied after this reinterpretation, data from the malicious WebSocket frame overwrites memory locations adjacent to the intended buffer. This heap corruption can damage critical internal structures such as connection pointers, session state objects, and other control flow metadata maintained by the FreeSWITCH engine. The immediate operational impact of this flaw is a denial of service condition where the application crashes due to segmentation faults or invalid pointer dereferences caused by the corrupted memory state.
While remote code execution has not been definitively demonstrated in public disclosures for this specific vector, heap corruption vulnerabilities inherently carry significant risk for such outcomes if additional exploitation techniques are applied against other components within the same process space. The crash of FreeSWITCH results in the termination of all concurrent sessions handled by that instance, effectively causing a complete service outage for any users relying on the telephony stack at that time. This aligns with CWE-190 Integer Overflow or Wraparound and CWE-787 Out-of-bounds Write, reflecting both the root cause arithmetic error and its direct consequence of memory corruption.
From an attack perspective, this vulnerability is classified under MITRE ATT&CK technique T1496 Resource Hijacking as it can be used to disrupt service availability through resource exhaustion via crash induction. Furthermore, because the WebSocket endpoint for mod_xml_rpc must be enabled and accessible, the exploit requires network reachability to the specific interface hosting the vulnerable module. However, a significant mitigating factor is that both the mod_xml_rpc module and its associated WebSocket upgrade functionality are disabled by default in standard FreeSWITCH installations. This means that systems running out-of-the-box configurations without explicit administrative enablement of these advanced features remain largely immune to this specific attack vector unless explicitly configured otherwise.
To mitigate this vulnerability, administrators must ensure they have upgraded their FreeSWITCH deployment to version 1.11.2 or later where the integer handling logic has been corrected to properly validate payload lengths against buffer boundaries without allowing sign-extension exploits. For environments that cannot immediately patch due to operational constraints, disabling mod_xml_rpc and removing any custom configurations that enable WebSocket upgrades on RPC endpoints serves as an effective compensating control. Network-level filtering can also be employed to restrict access to the affected ports from untrusted networks until a permanent fix is applied. Regular vulnerability scanning focused on telephony stacks and strict adherence to least-privilege configuration principles are recommended practices to reduce exposure to such implementation flaws in complex software-defined telecom environments.