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

Quick assessment

Affected
signalwire freeswitch
Exploitation
Public or AI PoC available; prioritize validation
Recommended action
Check the vendor advisory and references for a fixed version. If immediate upgrade is impossible, restrict exposure and increase monitoring.

FreeSWITCH是美国Anthony Minessale个人开发者的研发的一套免费、开源的通信软件。该软件可用于创建音、视频以及短消息类产品和应用。 FreeSWITCH 1.11.1之前版本存在输入验证错误漏洞，该漏洞源于esl_recv_event函数使用atol解析Content-Length并将结果直接传递给malloc(len + 1)而未进行符号或大小检查，恶意或中间人ESL对等端可以发送带有负Content-Length的帧，在客户端向该对等端进行身份验证之前，破坏或崩溃链接到libes

CVSS 9.1 · Critical EPSS 0.48% · P40

Possible ATT&CK Techniques 1 AI

T1190 · Exploit Public-Facing Application

Affected Version Matrix 1

VendorProduct Version RangeStatus
signalwire freeswitch < 1.11.1 affected
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I. Basic Information for CVE-2026-49840

Vulnerability Information
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Vulnerability Title
FreeSWITCH: Pre-authentication heap buffer overflow in libesl `Content-Length` parsing
Source: CVE Program / CVE List V5
Vulnerability 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.
Source: CVE Program / CVE List V5
CVSS Information
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:H
Source: CVE Program / CVE List V5
Vulnerability Type
输入验证不恰当
Source: CVE Program / CVE List V5
Vulnerability Title
FreeSWITCH 输入验证错误漏洞
Source: CNNVD (China National Vulnerability Database)
Vulnerability Description
FreeSWITCH是美国Anthony Minessale个人开发者的研发的一套免费、开源的通信软件。该软件可用于创建音、视频以及短消息类产品和应用。 FreeSWITCH 1.11.1之前版本存在输入验证错误漏洞，该漏洞源于esl_recv_event函数使用atol解析Content-Length并将结果直接传递给malloc(len + 1)而未进行符号或大小检查，恶意或中间人ESL对等端可以发送带有负Content-Length的帧，在客户端向该对等端进行身份验证之前，破坏或崩溃链接到libes
Source: CNNVD (China National Vulnerability Database)
CVSS Information
N/A
Source: CNNVD (China National Vulnerability Database)
Vulnerability Type
N/A
Source: CNNVD (China National Vulnerability Database)

Affected Products

Vendor Product Affected Versions CPE Subscribe
signalwire freeswitch < 1.11.1 -

II. Public POCs for CVE-2026-49840

# POC Description Source Link Shenlong Link
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Qwen3.6-35B-A3B · 9369 chars
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III. Intelligence Information for CVE-2026-49840

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Patches & Fixes for CVE-2026-49840 (1)

Vendor Advisories for CVE-2026-49840 (1)

Same Patch Batch · signalwire · 2026-06-09 · 9 CVEs total

CVE-2026-49841 9.8 CRITICAL FreeSWITCH: Pre-authentication heap buffer overflow in `mod_verto` HTTP POST body read
CVE-2026-45771 7.5 HIGH Freeswitch Denial-of-Service in SIP PUBLISH Requests via XML Entity Expansion
CVE-2026-49842 7.5 HIGH FreeSWITCH: Pre-authentication bandwidth amplification via `mod_verto` speed-test frames
CVE-2026-49847 7.5 HIGH FreeSWITCH: Stack overflow in bundled cJSON parser via deeply nested JSON
CVE-2026-49475 7.5 HIGH FreeSWITCH: Out-of-bounds memory access in core STUN attribute parsing
CVE-2026-49843 5.3 MEDIUM FreeSWITCH: Pre-authentication session eviction via attacker-chosen `sessid` in `mod_verto
CVE-2026-49472 5.3 MEDIUM FreeSWITCH includes a vulnerable function, PREFIX(prologTok)() from libexpat
CVE-2026-49848 4.3 MEDIUM FreeSWITCH: Pre-authentication `userVariables` injection in `mod_verto`

IV. Related Vulnerabilities

Same product: freeswitch
Same vendor: signalwire
Same weakness: CWE-20

V. Comments for CVE-2026-49840

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