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FreeswitchApplication

CVE-2026-49840

CRITICAL · 9.1 CVSS v3.1 Published 2026-06-09
Fix available
A fix is available. Upgrade to 1.11.1 or later.
See remediation →
100/100
Remediation priority · Urgent
Remotely reachable No privileges Zero-click

Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.

NVD · unedited
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 version 1.11.1, esl_recv_event() parses Content-Length with atol() and passes the result straight to malloc(len + 1) with no sign or magnitude check. A malicious or man-in-the-middle ESL peer can send a frame with a negative Content-Length to corrupt the heap of, or crash, any process linked against libesl, before the client has authenticated to that peer. This issue has been patched in version 1.11.1.

Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.

dbcve analysis · high confidence

In esl_recv_event(), the Content-Length header is parsed using atol() without any sign or magnitude validation, and the result is directly passed to malloc(len + 1). When a negative Content-Length is provided, malloc interprets it as a very large unsigned value, causing heap corruption or out-of-bounds memory access.

MitigationUpgrade FreeSWITCH to version 1.11.1 or later. If immediate patching is not feasible, restrict network access to ESL peers using firewall rules or TLS encryption to prevent unauthenticated MITM attacks.

Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.

Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.

NVD · CPE data
FreeswitchApplication
Affected:< 1.11.1

CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.

From the vector
Attack vector
Network
Complexity
Low
Privileges
None
User interaction
None
Scope
Unchanged
Confidentiality
None
Integrity
High
Availability
High

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:H

Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.

dbcve checks

Work through these to decide whether this CVE applies to you.

  1. Determine FreeSWITCH version
    Run 'fs_cli -x "version"' or check the installed package version with your system package manager (dpkg -r freeswitch, rpm -qi freeswitch, etc.)
    Affected if The installed version is any version prior to 1.11.1 (e.g., 1.10.x, 1.8.x, or earlier)
  2. Identify if ESL module is loaded
    Run 'fs_cli -x "module list"' and look for mod_event_socket or mod_esl, or check conf/autoload_configs/modules.conf.xml for the event_socket or esl module
    Affected if The ESL/Event Socket module is loaded and enabled in the configuration
  3. Check ESL listener configuration
    Examine conf/autoload_configs/event_socket.conf.xml (or the equivalent ESL configuration file) and look for the <listeners> section binding to network interfaces
    Affected if An ESL listener is configured and bound to a reachable network interface (especially 0.0.0.0 or external IPs)
  4. Verify ESL network exposure
    Inspect the <bind> setting in the ESL configuration - check if it binds to all interfaces (0.0.0.0), external interfaces, or is limited to localhost (127.0.0.1)
    Affected if The ESL service is bound to 0.0.0.0 or any IP other than 127.0.0.1, making it reachable from other hosts

You are affected if your FreeSWITCH version is below 1.11.1 AND the ESL (Event Socket Layer) module is enabled and accessible to untrusted network peers who could send malicious Content-Length headers.

Generated from the published advisory. Verify against your own configuration.

Check your environment

Paste your version and any relevant configuration and it will be compared against the affected criteria above. Do not include secrets or credentials.

AI-assisted, checked against the advisory. Informational, not a guarantee.

Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.

dbcve · scoped
Upgrade available Upgrade to 1.11.1 or later
Fixed in 1.11.1
Interim mitigation

Upgrade FreeSWITCH to version 1.11.1 or later. If immediate patching is not feasible, restrict network access to ESL peers using firewall rules or TLS encryption to prevent unauthenticated MITM attacks.

Recommended fix High confidence

1.11.1

  1. 1. Identify all systems running FreeSWITCH versions prior to 1.11.1
  2. 2. Plan a maintenance window for the upgrade
  3. 3. Backup current FreeSWITCH configuration and any custom configurations
  4. 4. Upgrade FreeSWITCH to version 1.11.1 or later
  5. 5. Verify the installation was successful
  6. 6. Test ESL functionality to confirm the vulnerability is remediated
  7. 7. Monitor for any crashes or unexpected behavior related to ESL connections

Generated from the published advisory — verify against the referenced sources before acting.

Fix this in Freeswitch Scoped from the published advisory
  • Consultation2.0 h
  • Implementation2.0 h
  • Testing4.0 h
  • Review / QA2.0 h
10.0 hours of engineering $1,720
Get the upgrade done

An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $2,752.

Scan for this in your stack

Free · runs locally
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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2026-49840 in production — separate from our analysis above.

No notes yet

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What this is

A place for practitioners to share what actually worked: a mitigation you’ve tested, a configuration change, a version- or environment-specific caveat, or a link to a verified patch. The most useful notes rise to the top as peers upvote them, so the signal stays high.

What belongs here
  • Verified mitigations, workarounds, and config changes
  • Version or environment caveats, and links to real fixes
  • No weaponised exploit code, or anything meant to cause harm
  • No spam, self-promotion, credentials, or personal data