FSTCompiler silently writes a corrupt FST when a fixed-length-arc node would expand to 2^31 bytes or more

Finding LFV-007Area: util / FSTPriority: lowImpact: data corruptionNovelty: new

Finding page for LFV-007

AI-assisted finding, confirmed by automated checks, not yet triaged by a human Lucene committer. This site files nothing upstream.

What it shows

You build an FST in which one node has many arcs and at least one large output, so that its fixed-length arc area numArcs*maxBytesPerArc reaches 2^31 bytes. You expect a correct FST or an error. Instead, with assertions off, compile() returns an FST whose node header was written over the unexpanded arc list: lookups through that node throw, and enumeration can return a key that was never added.

The test shows that a default BYTE2 FSTCompiler with 2048 sparse root arcs, one of them 2^20 bytes (a 1,048,570-byte output), so the fixed-length root node would take 2048 x 2^20 = 2^31 bytes and the int size wraps.

Expected vs actual

Expected: Util.get returns key 0's output (or compile() fails). Actual: with -da, compile() returns a corrupt FST and the lookup throws IndexOutOfBoundsException: Index -32704 out of bounds for length 33; with -ea, Lucene's assert destPos >= srcPos (FSTCompiler.java:666) fires inside compile().

Files

• src/org/apache/lucene/util/fst/TestFSTNodeExpandedSizeOverflow.java (JUnit test)

Run it

• Tested on Lucene main e1faeba08bde (2026-11-02): still reproduces.
• Tested on Lucene main 645d4fe14a20 (2026-10-28): reproduced.
• Recorded outcome of run 1 on Lucene main 645d4fe14a20: 1 test(s) run, 1 failed.
• How to read the outcome: a failing test is the finding reproducing (the failure message states expected against actual).
• Seeds: a rerun with the same seed replays the same random choices. Assertions off (-da -dsa -Dtests.asserts=false) is the production-like setting.

# Lucene checkout at the tested commit, built with: ./gradlew --no-daemon assemble   (JDK 25)
LUCENE=/path/to/lucene; V=12.0.0-SNAPSHOT
CP=$LUCENE/lucene/core/build/libs/lucene-core-$V.jar
CP=$CP:$LUCENE/lucene/codecs/build/libs/lucene-codecs-$V.jar
CP=$CP:$LUCENE/lucene/test-framework/build/libs/lucene-test-framework-$V.jar
CP=$CP:randomizedtesting-runner-2.9.2.jar:junit-4.13.2.jar:hamcrest-3.0.jar   # test-framework dependencies (Maven Central)
CP=$CP:junit-platform-console-standalone-6.1.3.jar   # JUnit Platform console launcher (Maven Central)

javac -d classes -cp "$CP" $(find src -name '*.java')

# run 1: assertions on, seed DEADBEEF
java -ea -esa --add-modules jdk.incubator.vector --enable-native-access=ALL-UNNAMED -Dtests.seed=DEADBEEF \
     -cp "classes:$CP" org.junit.platform.console.ConsoleLauncher \
     execute --select-class org.apache.lucene.util.fst.TestFSTNodeExpandedSizeOverflow

Gradle alternative: copy the test into the matching module's src/test/ tree of the checkout and run ./gradlew -p lucene/<module> test --tests <class> -Ptests.seed=<seed> (add -Ptests.asserts=false for a run without assertions).

Formal models and scripts this finding's part of the hunt used: study-fst-verification

Feedback

Open an issue in lucene-ai-bug-hunt. Please do not open issues or pull requests in apache/lucene about this finding unless a Lucene committer agrees it should be filed.

Snapshot of 2026-11-04. AI-assisted; not yet triaged by a human Lucene committer. Feedback: open an issue in lucene-ai-bug-hunt.