# CVE-2026-45100: OpenSIPS buffer overflow in b64encode string transformation

Source: https://junglewise.ai/threats/cve-2026-45100-opensips-buffer-overflow-in-b64encode-string-transformation  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-08-04. Updated: 2026-09-25  
License: CC BY 4.0 with a link back

CVE-2026-45100 · Severity: critical · CVSS 9.1 · Published 2026-08-04

OpenSIPS is a VoIP server that processes Session Initiation Protocol (SIP) messages used in telecommunications systems. A buffer overflow vulnerability in the base64 encoding function allows remote attackers to send specially crafted SIP messages that corrupt adjacent memory buffers, potentially leading to code execution or service disruption. Exploitation requires the server's routing configuration to apply base64 encoding to attacker-controlled input.

Technologies: [OpenSIPS](https://junglewise.ai/threats/technologies/opensips). Vendors: [OpenSIPS](https://junglewise.ai/threats/vendors/opensips).

## Quick answers

**Is CVE-2026-45100 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-45100 being exploited in the wild.

**How severe is CVE-2026-45100?** Critical severity, with a CVSS score of 9.1, published 4 August 2026.

**Which products and versions are affected?** OpenSIPS 3.4.0-beta through 3.6.5, 4.0.0-beta.

**Is there a fix for CVE-2026-45100?** No fixed version is recorded yet. Check the vendor's advisory in the references below. If you run it, Junglewise drafts a mitigation that fits your setup and applies it once you approve, with senior security engineers behind it.

**How is CVE-2026-45100 attacked?** It is attacked over the network.

**Am I affected by CVE-2026-45100?** Check whether you run OpenSIPS at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## Technical details

The vulnerability is a heap buffer overflow in the {s.b64encode} string transformation. The input size validation only checks that data fits within a 64 KB transformation buffer, but base64 encoding expands data by approximately 33%, causing inputs between 49,153 and 65,535 bytes to produce output exceeding buffer capacity by up to 21,844 bytes. These transformation buffers are allocated adjacently in memory and reused for chained transformations, allowing the overflow to write attacker-controlled data into adjacent buffers and corrupt values used in subsequent transformations of the same SIP message. A remote attacker can trigger this by sending a SIP message with a large header value (approximately 50,000+ bytes) when the routing script applies {s.b64encode} to untrusted input. Exploitation depends on the specific routing script configuration. Patches are available in versions 3.6.6 and 4.0.0-rc1.

## Affected products

- OpenSIPS OpenSIPS 3.4.0-beta through 3.6.5, 4.0.0-beta

## Timeline

- 2026-08-04: disclosed

## References

- [https://github.com/OpenSIPS/opensips/commit/4d23613b65579b073784a07a65d3bf52443a4efb](https://github.com/OpenSIPS/opensips/commit/4d23613b65579b073784a07a65d3bf52443a4efb)
- [https://github.com/OpenSIPS/opensips/commit/5f103effaf5f372cccffe0b138f16998eba12668](https://github.com/OpenSIPS/opensips/commit/5f103effaf5f372cccffe0b138f16998eba12668)

## Related threats

- [OpenSIPS SDP bandwidth-line parsing denial of service](https://junglewise.ai/threats/cve-2026-46334-opensips-sdp-bandwidth-line-parsing-denial-of-service)
- [OpenSIPS watcherinfo stack buffer overflow](https://junglewise.ai/threats/cve-2026-45809-opensips-watcherinfo-stack-buffer-overflow)
- [OpenSIPS out-of-bounds read in multipart body parser](https://junglewise.ai/threats/cve-2026-45705-opensips-out-of-bounds-read-in-multipart-body-parser)
- [OpenSIPS construct_uri buffer overflow](https://junglewise.ai/threats/cve-2026-45537-opensips-construct-uri-buffer-overflow)
- [OpenSIPS integer overflow in TCP message framing](https://junglewise.ai/threats/cve-2026-45103-opensips-integer-overflow-in-tcp-message-framing)