---
id: CVE-2026-45100
title: OpenSIPS is a Session Initiation Protocol (SIP) server implementation
summary: "OpenSIPS is a Session Initiation Protocol (SIP) server implementation. Versions 3.4.0-beta through 3.6.5 and 4.0.0-beta contain a buffer overflow in the {s.b64encode}\_string transformation. The size check for {s.b64encode} only verifies …"
severity: critical
cvss: 9.1
cvssVector: 'CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:H'
cwe:
  - CWE-120
published: '2026-08-04'
updated: '2026-09-08'
sourceUpdated: '2026-09-08T20:41:31.497'
source: NVD
sourceUrl: 'https://nvd.nist.gov/vuln/detail/CVE-2026-45100'
references:
  - url: >-
      https://github.com/OpenSIPS/opensips/commit/4d23613b65579b073784a07a65d3bf52443a4efb
    label: security-advisories@github.com
  - url: >-
      https://github.com/OpenSIPS/opensips/commit/5f103effaf5f372cccffe0b138f16998eba12668
    label: security-advisories@github.com
  - url: >-
      https://github.com/OpenSIPS/opensips/security/advisories/GHSA-35fr-6rv9-vp68
    label: security-advisories@github.com
  - url: >-
      https://github.com/OpenSIPS/opensips/security/advisories/GHSA-35fr-6rv9-vp68
    label: 134c704f-9b21-4f2e-91b3-4a467353bcc0
tags:
  - nvd
epss: 0.00673
epssPercentile: 0.50686
ingestedAt: '2026-09-08T21:11:12.266Z'
---

## Overview

OpenSIPS is a Session Initiation Protocol (SIP) server implementation. Versions 3.4.0-beta through 3.6.5 and 4.0.0-beta contain a buffer overflow in the {s.b64encode} string transformation. The size check for {s.b64encode} only verifies that the input fits within the 64 KB transformation buffer, but base64 encoding expands the data by roughly a third, so an input between about 49,153 and 65,535 bytes produces more output than the buffer can hold and overflows it by up to 21,844 bytes. Because these transformation buffers sit next to each other in memory and are reused for chained transformations, the overflow writes attacker-controlled data into the adjacent buffer and corrupts values used by later transformations processing the same SIP message. A remote attacker can trigger this by sending a SIP message with a large header value (roughly 50,000 bytes or more) when the routing script applies  {s.b64encode}  to attacker-controlled input, making exploitability dependent on the deployment's routing configuration. This issue has been fixed in versions 3.6.6 and 4.0.0-rc1.

## Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.