PYSEC-2026-3045

Impact

A maliciously crafted QPY file containing a malformed symengine serialization stream as part of the larger QPY serialization of a ParameterExpression object can cause a segfault within the symengine library, allowing an attacker to terminate the hosting process deserializing the QPY payload.

Patches

This issue is addressed in 1.3.0 when using QPY format version 13. QPY format versions 10, 11, and 12 are all still inherently vulnerable if they are using symengine symbolic encoding and symengine <= 0.13.0 is installed in the deserializing environment (as of publishing there is no newer compatible release of symengine available). Using QPY 13 is strongly recommended for this reason.

The symengine 0.14.0 release has addressed the segfault issue, but it is backward incompatible and will not work with any Qiskit release; it also prevents loading a payload generated with any other version of symengine. Using QPY 13 is strongly recommended for this reason.

It is also strongly suggested to patch the locally installed version of symengine in the deserializing environment to prevent the specific segfault. The commit [1] can be applied on top of symengine 0.13.0 and used to build a patched python library that will not segfault in the presence of a malformed payload and instead raise a RuntimeError which will address the vulnerability.

Workarounds

As QPY is backwards compatible qiskit.qpy.load() function will always attempt to deserialize the symengine-serialized payloads in QPY format versions 10, 11, and 12. These are any payloads generated with the use_symengine argument on qiskit.qpy.dump() set to True (which is the default value starting in Qiskit 1.0.0. The only option is to disallow parsing if those QPY formats are being read and the use_symengine flag was set in the file's header. You can detect whether a payload is potentially vulnerable by using the following function built using the Python standard library: