# CVE-2026-49840: FreeSWITCH: Pre-authentication heap buffer overflow in libesl `Content-Length` parsing

> A pre-authentication heap buffer overflow in FreeSWITCH's libesl allows attackers to execute code or crash the service via malicious Content-Length headers.

- **Severity:** Critical 9.1 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 9 Jun 2026
- **Updated:** 9 Jun 2026
- **Also known as:** GHSA-g597-9fgg-ghg9

## What is CVE-2026-49840?

CVE-2026-49840 is a critical memory corruption vulnerability in signalwire freeswitch, published on 9 Jun 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official 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 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.

Weakness: CWE-787 (Out-of-bounds Write), CWE-20 (Improper Input Validation), CWE-195 (Signed to Unsigned Conversion Error), CWE-122 (Heap-based Buffer Overflow)

## Which versions are affected by CVE-2026-49840?

It affects 1 product.


Products: Freeswitch Freeswitch.

## How do I fix CVE-2026-49840?

No fix has been published for CVE-2026-49840 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-49840 being exploited?

There is no known exploitation of CVE-2026-49840 in the wild and no public exploit code.

## Other vulnerabilities in signalwire freeswitch

- [CVE-2026-49841](https://insights.sbomify.com/CVE-2026-49841/): FreeSWITCH: Pre-authentication heap buffer overflow in `mod_verto` HTTP POST body read (critical 9.8)
- [CVE-2019-19492](https://insights.sbomify.com/CVE-2019-19492/): Hardcoded credentials in n/a (critical 9.8)
- [CVE-2021-41145](https://insights.sbomify.com/CVE-2021-41145/): FreeSWITCH susceptible to Denial of Service via SIP flooding (high 8.6)
- [CVE-2026-45771](https://insights.sbomify.com/CVE-2026-45771/): Freeswitch Denial-of-Service in SIP PUBLISH Requests via XML Entity Expansion (high 7.5)
- [CVE-2026-49475](https://insights.sbomify.com/CVE-2026-49475/): FreeSWITCH: Out-of-bounds memory access in core STUN attribute parsing (high 7.5)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-49840 and fix it if it is affected.

What it is: CVE-2026-49840 is a critical memory corruption vulnerability in signalwire freeswitch, published on 9 Jun 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-49840 in the wild and no public exploit code.

Affected product: signalwire freeswitch. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-49840/ (Markdown: https://insights.sbomify.com/CVE-2026-49840/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-49840
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-49840
- OSV: https://osv.dev/vulnerability/CVE-2026-49840

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-49840/
Summaries are generated and may contain mistakes; the official description above is the record.