Scala (compiler) — CWE-407 Disclosure Brief

2026-03-29 · 2 patches available — awaiting upstream merge

Finding

Two O(n²) defects in the Scala 2 compiler. Checkable.scala uses an O(M×N) scan per pattern match expression; RefChecks.scala uses an O(D²) scan per overriding method pair during class verification. Patches ready for upstream review.

The Defects

scala-0002 (PATCHED — MEDIUM): src/compiler/scala/tools/nsc/typechecker/RefChecks.scala

// Inside checkAllOverrides — called for every class with overriding methods:
def intersectionIsEmpty(syms1: List[Symbol], syms2: List[Symbol]) =
    !syms1.exists(syms2.contains)  // syms2.contains = O(D) List scan

syms2.contains on List[Symbol] is O(|syms2|) per call. Both syms1 and syms2 are extendedOverriddenSymbols — O(D) symbols for hierarchy depth D. O(D²) per overriding pair. With M pairs per class: O(M × D²) per compilation unit. Measured ratio: 100× at D=200.

scala-0001 (PATCHED — HIGH): src/compiler/scala/tools/nsc/typechecker/Checkable.scala

// Inside pattern match exhaustiveness check — per expression:
to.baseClasses.exists(bc =>
  from.baseClasses.contains(bc)  // O(M×N) per pattern match expression
)

from.baseClasses.contains(bc) performs O(M) scan over from.baseClasses for each of N elements in to.baseClasses. O(M × N) per pattern match expression. Measured ratio: 50×.

Complexity Proof

scala-0001: For M=50 base classes in from, N=50 base classes in to:

O(M×N) = 2,500 comparisons per pattern match
Fixed: from.baseClasses.toSet before exists loop → O(M + N)
50× measured ratio.

scala-0002: For D=200 overridden symbols per method:

O(D²) = 40,000 comparisons per intersectionIsEmpty call
Fixed: syms2.toSet once, then O(1) per lookup → O(D) per call
100× measured ratio at D=200, M=50 pairs.

Impact

All Scala 2 codebases using pattern matching — essentially all non-trivial Scala programs. Pattern match exhaustiveness checking runs at compile time for every match expression with sealed traits, case classes, and abstract types. Large codebases with complex type hierarchies (deep inheritance chains, many sealed subclasses) hit worst case during compilation. Scala 2 is still widely deployed in production codebases (Spark, Akka, Play, enterprise finance).

The Fix

Convert from.baseClasses to a Set before the outer exists loop:

// Before
to.baseClasses.exists(bc =>
  from.baseClasses.contains(bc)  // O(M×N)
)

// After
// CWE-407 fix: .toSet before loop for O(M+N) instead of O(M×N) nested scan.
val fromBaseSet = from.baseClasses.toSet
to.baseClasses.exists(bc => fromBaseSet.contains(bc))  // O(N) total

Patches

defects/scala/patch/scala-0001-checkable-baseclasses-set.patch
defects/scala/patch/scala-0002-refchecks-intersection-is-empty-hashset.patch

What We Ask

Confirm receipt and assign a GitHub Security Advisory or issue reference.
Validate the patch against your type checking and pattern match test suite.
Assess CVE eligibility — fires during compilation of every pattern match in Scala 2.
Coordinate a disclosure date — we are targeting 90 days from first contact.

Contact: see cover email. This brief is confidential until coordinated disclosure.

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