OpenSIPS prior 3.6.6/4.0.0-rc1 Base64 Encoding s.b64encode buffer overflow

Summaryinfo

A vulnerability identified as very critical has been detected in OpenSIPS. Affected is the function s.b64encode of the component Base64 Encoding. Performing a manipulation results in buffer overflow. This vulnerability is cataloged as CVE-2026-45100. It is possible to initiate the attack remotely. There is no exploit available.

Detailsinfo

A vulnerability was found in OpenSIPS. It has been classified as very critical. This affects the function s.b64encode of the component Base64 Encoding. The manipulation with an unknown input leads to a buffer overflow vulnerability. CWE is classifying the issue as CWE-120. The product copies an input buffer to an output buffer without verifying that the size of the input buffer is less than the size of the output buffer, leading to a buffer overflow. This is going to have an impact on confidentiality, integrity, and availability. The summary by CVE is:

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.

The advisory is shared at github.com. This vulnerability is uniquely identified as CVE-2026-45100 since 05/08/2026. The exploitability is told to be easy. It is possible to initiate the attack remotely. Technical details are known, but no exploit is available. The price for an exploit might be around USD $0-$5k at the moment (estimation calculated on 08/05/2026).

Upgrading to version 3.6.6 or 4.0.0-rc1 eliminates this vulnerability.

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Productinfo

Name

• OpenSIPS

Website

• Product: https://github.com/OpenSIPS/opensips/

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