DirectConnection.listen() NPE in SASL.doResponse with sd-bus peer

[Monkopedia]

DirectConnection.listen() crashes with NPE in SASL.doResponse when peer is sd-bus

Summary

dbus-java server-side direct bus listener crashes with:

java.lang.NullPointerException
  at java.text.RuleBasedCollator.compare
  at org.freedesktop.dbus.connections.SASL.doResponse(SASL.java:369)

when the connecting peer is native sd-bus (C/libsystemd).

This looks like a null-handling bug in SASL response processing for AUTH EXTERNAL.

Environment

• dbus-java-core: 5.1.1
• dbus-java-transport-junixsocket: 5.1.1
• Java: 21.0.10
• OS: Linux x86_64 (6.18.7-arch1-1)
• Native peer: libsystemd 259

Minimal Reproducer

I created a standalone repro with:

• Java server: DirectConnectionBuilder.forAddress("unix:path=...,listen=true").build(); connection.listen();
• C client using sd-bus:
  • sd_bus_new
  • sd_bus_set_address("unix:path=...")
  • sd_bus_start

Run:

./repro/dbus-java-sasl-npe/run-repro.sh

Repro files:

• repro/dbus-java-sasl-npe/server/src/main/java/repro/DbusJavaServer.java
• repro/dbus-java-sasl-npe/client/sd_bus_client.c
• repro/dbus-java-sasl-npe/run-repro.sh

Actual Result

Server exits with:

Exception in thread "main" java.lang.NullPointerException
    at java.base/java.text.RuleBasedCollator.compare(RuleBasedCollator.java:359)
    at org.freedesktop.dbus.connections.SASL.doResponse(SASL.java:369)
    at org.freedesktop.dbus.connections.SASL.auth(SASL.java:625)
    at org.freedesktop.dbus.connections.transports.AbstractTransport.authenticate(AbstractTransport.java:254)
    at org.freedesktop.dbus.connections.transports.AbstractTransport.internalConnect(AbstractTransport.java:230)
    at org.freedesktop.dbus.connections.transports.AbstractTransport.listen(AbstractTransport.java:215)
    at org.freedesktop.dbus.connections.impl.DirectConnection.listen(DirectConnection.java:66)
    at repro.DbusJavaServer.main(DbusJavaServer.java:47)

Expected Result

DirectConnection.listen() should not crash; it should either authenticate successfully or reject auth without throwing NPE.

Notes

In the D-Bus auth protocol, AUTH initial response is optional. A missing initial payload should be handled safely:

• https://dbus.freedesktop.org/doc/dbus-specification.html#auth-command-auth

[Monkopedia]

I published the complete standalone reproducer as a public gist:

https://gist.github.com/Monkopedia/dbe1a170944c69c592aa2b75c41a019a

It contains the Java server, C sd-bus client, and run script from the issue description.

[hypfvieh]

Thanks for the report, the provided details and the code to reproduce.

I updated the SASL code in dbus-java 5.x and 6.x to fix the NPE.