Kpow Custom Serdes and Protobuf v4.31.1

Derek Troy-West

October 12th, 2025

xx min read

Kpow Custom Serdes and Protobuf v4.31.1

Note: The potential compatibility issues described in this post only impacts users who have implemented Custom Serdes that contain generated protobuf classes.

Resolution: If you encounter these compatibility issues, resolve them by re-generating any generated protobuf classes with protoc v31.1.

In the upcoming v94.6 release of Kpow, we're updating all Confluent Serdes dependencies to the latest major version 8.0.1.

In io.confluent/kafka-protobuf-serializer:8.0.1 the protobuf version is advanced from 3.25.5 to 4.31.1, and so the version of protobuf used by Kpow changes.

Confluent protobuf upgrade PR: https://github.com/confluentinc/schema-registry/pull/3569
Related Github issue: https://github.com/confluentinc/schema-registry/issues/3047

This is a major upgrade of the underlying protobuf libraries, and there are some breaking changes related to generated code.

Protobuf 3.26.6 introduces a breaking change that fails at runtime (deliberately) if the makeExtensionsImmutable method is called as part of generated protobuf code.

The decision to break at runtime was taken because earlier versions of protobuf were found to be vulnerable to the footmitten CVE.

Protobuf footmitten CVE and breaking change announcement: https://protobuf.dev/news/2025-01-23/
Apache protobuf discussion thread: https://lists.apache.org/thread/87osjw051xnx5l5v50dt3t81yfjxygwr
Comment on a Schema Registry ticket: https://github.com/confluentinc/schema-registry/issues/3360

We found that when we advanced to the 8.0.1 version of the libraries; we encountered issues with some test classes generated by 3.x protobuf libraries.

Compilation issues:

Compiling 14 source files to /home/runner/work/core/core/target/kpow-enterprise/classes
/home/runner/work/core/core/modules/kpow/src-java-dev/factorhouse/serdes/MyRecordOuterClass.java:129: error: cannot find symbol
        makeExtensionsImmutable();
        ^
  symbol:   method makeExtensionsImmutable()
  location: class MyRecord

Runtime issues:

Bad type on operand stack
Exception Details:
Location:
io/confluent/kafka/schemaregistry/protobuf/ProtobufSchema.toMessage(Lcom/google/protobuf/DescriptorProtos$FileDescriptorProto;Lcom/google/protobuf/DescriptorProtos$DescriptorProto;)Lcom/squareup/wire/schema/internal/parser/MessageElement; : invokestatic
Reason:
Type 'com/google/protobuf/DescriptorProtos$MessageOptions' (current frame, stack[1]) is not assignable to 'com/google/protobuf/GeneratedMessage$ExtendableMessage'
Current Frame:
bci:
flags: { }
locals: { 'com/google/protobuf/DescriptorProtos$FileDescriptorProto', 'com/google/protobuf/DescriptorProtos$DescriptorProto', 'java/lang/String', 'com/google/common/collect/ImmutableList$Builder', 'com/google/common/collect/ImmutableList$Builder', 'com/google/common/collect/ImmutableList$Builder', 'com/google/common/collect/ImmutableList$Builder', 'java/util/LinkedHashMap', 'java/util/LinkedHashMap', 'java/util/List', 'com/google/common/collect/ImmutableList$Builder' }
stack: { 'com/google/common/collect/ImmutableList$Builder', 'com/google/protobuf/DescriptorProtos$MessageOptions' }
Bytecode:
0000000: 2bb6 0334 4db2 0072 1303 352c b903 3703
0000010: 00b8 0159 4eb8 0159 3a04 b801 593a 05b8
0000020: 0159 3a06 bb02 8959 b702 8b3a 07bb 0289