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.
Written by anthropic/claude-haiku-5.5 from the sources below. Verify against them.
- Exploited
- No known exploitation
- Fix
- Fixed in 1.11.2
- Who is exposed
- Default setups
Do first: Check every install: default setups are affected.
- Last modified
- A 0-100 score for how much a CVE matters across the industry: exploitation, exposure, how widely the software is used, severity and whether a fix exists.What do these mean? →
- 30/100 · provisional
- How many real-world setups can hit the bug, from the vendor's advisory and CISA data. It does not know your setup.What do these mean? →
- Default setups
- Whether working attack code is public (Exploit-DB, Metasploit, Nuclei or GitHub). Public code makes attacks easier.What do these mean? →
- None on Exploit-DB, Metasploit or GitHub
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.
Read the original 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.
cve.now Index
- Exploitation?/40Unknown. No CISA SSVC exploitation rating yet
- Exposure22/25Default setups, reachable over the network without a login
- Reach?/20Unknown. No tracked technology or package matched
- Severity8.2/10CVSS 8.2
- No patch0/5No signal
What would change the score
- Would score about 70 if added to CISA KEV
- Would score about 65 if CISA's SSVC rates exploitation active
What to do
Check every install: default setups are affected.
Upgrade to a fixed version: 1.11.2.
Workaround: Set enable-t38=false in spandsp.conf.xml, or fax_enable_t38=false per channel, so T.38 is never negotiated; this does not cover t38_gateway.
Timeline
- CVE published
- First seen by cve.now
Affected versions
- signalwire freeswitch
fixed in 1.11.2
Evidence 3
- Advisory
- Repository
- Repository
Corrections
None so far.
Something wrong on this page? Report an error. Every fix is listed here.
Questions about CVE-2026-116783
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: cve.now feed data
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: github.com
Which versions are affected by CVE-2026-116783?
freeswitch: before 1.11.2.Source: cve.now feed data
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: github.com
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: github.com