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PatchSiren cyber security CVE debrief

CVE-2026-49840 signalwire CVE debrief

CVE-2026-49840 is a critical vulnerability in FreeSWITCH Software Defined Telecom Stack. A malicious 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.

Vendor
signalwire
Product
freeswitch
CVSS
CRITICAL 9.1
CISA KEV
Not listed in stored evidence
Original CVE published
2026-06-09
Original CVE updated
2026-06-10
Advisory published
2026-06-09
Advisory updated
2026-06-10

Who should care

Users of FreeSWITCH Software Defined Telecom Stack prior to version 1.11.1 should apply the patch to prevent heap corruption and crashes.

Technical summary

The esl_recv_event() function in FreeSWITCH parses Content-Length with atol() and passes the result straight to malloc(len + 1) with no sign or magnitude check. This allows a malicious or man-in-the-middle ESL peer to send a frame with a negative Content-Length.

Defensive priority

High

Recommended defensive actions

  • Upgrade to FreeSWITCH version 1.11.1 or later.
  • See ${ref-4} for release notes.
  • See ${ref-5} for third-party advisory.

Evidence notes

This CVE was published on ${cvePublishedAt} and modified on ${cveModifiedAt}.

Official resources

CVE-2026-49840 was published on ${cvePublishedAt} and modified on ${cveModifiedAt}.