CVE-2026-49841: FreeSWITCH Buffer Overflow Vulnerability

CVE-2026-49841 Overview

CVE-2026-49841 is a heap-based buffer overflow [CWE-122] in FreeSWITCH, an open-source Software Defined Telecom Stack. The flaw resides in the mod_verto HTTP request handler, which allocates a fixed 2 MiB buffer for application/x-www-form-urlencoded POST bodies but accepts a Content-Length value of nearly 10 MiB. The body-read loop is bounded by the attacker-supplied Content-Length instead of the buffer size. This produces an attacker-controlled heap overflow of approximately 8 MiB. The overflow occurs before the HTTP basic-authentication check executes, so exploitation requires no credentials. The issue is fixed in FreeSWITCH version 1.11.1.

Critical Impact
Unauthenticated remote attackers can trigger an 8 MiB heap overflow against any FreeSWITCH instance exposing mod_verto, enabling potential remote code execution and full compromise of the telecom stack.

Affected Products

FreeSWITCH versions prior to 1.11.1
Deployments using the mod_verto HTTP request handler
Any FreeSWITCH instance with Verto endpoints reachable over the network

Discovery Timeline

2026-06-09 - CVE-2026-49841 published to NVD
2026-06-10 - Last updated in NVD database

Technical Details for CVE-2026-49841

Vulnerability Analysis

The mod_verto module in FreeSWITCH exposes an HTTP interface that accepts POST requests with URL-encoded form bodies. The handler reserves a fixed-size heap buffer of 2 MiB for body data. However, the request parser validates Content-Length against a separate upper bound of nearly 10 MiB before invoking the read loop. The mismatch between the allocation size and the read limit allows the loop to write up to roughly 8 MiB of attacker-controlled bytes past the end of the allocated buffer. Because the overflow occurs prior to HTTP basic-authentication enforcement, any network-reachable attacker can trigger the corruption without valid credentials.

Root Cause

The defect is an input validation error in which the bounds check on the body-read loop uses Content-Length rather than the allocated buffer size. The fix in FreeSWITCH 1.11.1 aligns the read limit with the buffer capacity and applies consistent length validation.

Attack Vector

Exploitation requires only network access to the mod_verto HTTP listener. An attacker sends a crafted POST request with Content-Type: application/x-www-form-urlencoded, a Content-Length header approaching 10 MiB, and a body large enough to overflow the 2 MiB heap allocation. The contents of the overflow are entirely attacker-controlled, allowing heap metadata or adjacent object corruption that can lead to remote code execution within the FreeSWITCH process. See the GitHub Security Advisory GHSA-wfrq-qvg2-f88f for additional technical detail.

Detection Methods for CVE-2026-49841

Indicators of Compromise

HTTP POST requests to mod_verto endpoints with Content-Length values greater than 2,097,152 bytes.
Requests carrying Content-Type: application/x-www-form-urlencoded and oversized bodies originating from untrusted sources.
FreeSWITCH process crashes, segmentation faults, or restarts correlated with inbound Verto HTTP traffic.

Detection Strategies

Inspect web application firewall and reverse proxy logs for Verto POST requests exceeding the 2 MiB allocation size.
Correlate FreeSWITCH crash dumps and core files with HTTP access logs to identify exploitation attempts.
Deploy network IDS rules that alert on application/x-www-form-urlencoded bodies larger than 2 MiB sent to FreeSWITCH listener ports.

Monitoring Recommendations

Track FreeSWITCH service availability, restart counts, and memory anomalies on hosts running mod_verto.
Log all source IPs issuing oversized POST bodies to telephony infrastructure and review them daily.
Forward FreeSWITCH and front-end proxy logs to a centralized SIEM for retention and threat hunting.

How to Mitigate CVE-2026-49841

Immediate Actions Required

Upgrade FreeSWITCH to version 1.11.1 or later on all affected hosts.
Restrict network exposure of mod_verto endpoints to trusted management networks only.
Audit firewall and reverse proxy rules to block oversized POST requests targeting FreeSWITCH.

Patch Information

The vulnerability is patched in FreeSWITCH Release v1.11.1. Administrators should follow the upgrade procedures published by SignalWire and rebuild modules where required. Review the GitHub Security Advisory GHSA-wfrq-qvg2-f88f for fix details.

Workarounds

Disable mod_verto if the Verto signaling protocol is not in use.
Place a reverse proxy in front of FreeSWITCH and enforce a client_max_body_size below 2 MiB for Verto routes.
Restrict access to Verto HTTP ports using host firewalls or network ACLs until patching is complete.

bash

# Example nginx reverse proxy configuration limiting body size for Verto
location /verto/ {
    client_max_body_size 1m;
    proxy_pass http://127.0.0.1:8081;
    proxy_set_header Host $host;
    proxy_set_header X-Real-IP $remote_addr;
}