CVE-2026-63126

Wire's protobuf decoders did not consistently validate attacker-controlled length-delimited sizes against the current reader bounds before computing cursor, limit, or pointer positions.

In the Kotlin runtime, ProtoAdapter.decode(ByteArray) and ProtoAdapter.decode(ByteString) use the ProtoReader32 fast path implemented by ByteArrayProtoReader32. In ByteArrayProtoReader32.internalNextLengthDelimited(), Wire read an untrusted varint length into an Int and rejected only negative values. A length such as 2147483647 is non-negative, so it passed that check, but pos + length overflowed the signed 32-bit cursor and produced a negative limit. The following if (limit > pushedLimit) guard did not catch this because the overflowed value was negative.

That invalid limit then reached string, bytes, skip, and scalar-reading paths as an invalid byte count or invalid range. Instead of failing as a checked decode error such as IOException, malformed input could throw unchecked runtime exceptions including IllegalArgumentException and ArrayIndexOutOfBoundsException. Applications commonly treat malformed protobuf input as an expected decode failure; unchecked runtime exceptions escaping that boundary can crash request handling or the process.

The original report is a sibling of the negative-length skipped-group bug fixed as CVE-2026-45799. It is not the same bug. The length in this advisory is positive, and the overflow occurs when setting a length-delimited message limit, not only when skipping a group.

While auditing for the same bug class, related boundary flaws were also found and fixed:

• Kotlin ProtoReader now validates logical message limits before varint, fixed32, fixed64, and skip operations. The originally reported byte-array overflow payload did not reproduce as the same signed overflow in ProtoReader, because that reader tracks positions as Long, but the streaming reader still needed consistent current-message-limit enforcement.
• Swift ReadBuffer.readVarint() read pointer.pointee before checking that one byte remained. A tag-only varint field could read past the end of the buffer.
• Swift ReadBuffer.verifyAdditional(count:) formed pointer.advanced(by: count) before proving the requested count fit within the remaining buffer, so pointer arithmetic ran before the bounds were established. (The distinct Swift negative-length skipGroup() crash is tracked separately as GHSA-86wm-r4c5-2rc9 / CVE-2026-61695; this advisory covers the positive/oversized-length boundary failures.)
• Swift nested-message decoding and packed-repeated decoding computed end pointers from untrusted lengths before validating that the bytes were present.
• Swift packed-repeated decoding reserved array capacity from an untrusted length before validating that the length existed in the current buffer.
• Swift size-delimited decoding converted an untrusted UInt64 varint size to Int without exactness or availability checks. On platforms where the value is not representable, this could trap.

The fix enforces a single invariant across the hardened readers: every decoded or skipped byte count must be non-negative and no larger than the remaining bytes in the current logical message limit before any cursor, pointer, limit, allocation, or slice is advanced.

Proof of concept and regression payloads

These payloads are intentionally small and should be treated as malformed protobuf input. After the fix, they must fail with normal decode errors such as IOException, EOFException, or ProtoDecoder.Error.unexpectedEndOfData, not unchecked runtime exceptions, traps, out-of-bounds reads, or large allocations.

Kotlin byte-array known length-delimited field

Hex:

0A FF FF FF FF 07

Meaning:

• 0A: field 1, length-delimited
• FF FF FF FF 07: varint length 2147483647

Pre-fix behavior observed through Person.ADAPTER.decode(byteArray):

java.lang.IllegalArgumentException: startIndex: 6 > endIndex: -2147483643

Expected fixed behavior:

IOException / EOFException

Kotlin byte-array unknown length-delimited field

Hex:

1A FF FF FF FF 07

Meaning:

• 1A: field 3, length-delimited
• FF FF FF FF 07: varint length 2147483647

Pre-fix behavior observed:

ArrayIndexOutOfBoundsException

Expected fixed behavior:

IOException / EOFException

Kotlin byte-array skipped group containing oversized positive length

Hex:

0B 0A FF FF FF FF 07 0C

Meaning:

• 0B: start group, field 1
• 0A: nested field 1, length-delimited
• FF FF FF FF 07: varint length 2147483647
• 0C: end group, field 1

Expected fixed behavior:

IOException / EOFException

Kotlin current-message-limit fixed32 boundary

Hex:

02 0D 05 00 00 00

Meaning:

• 02: outer length-delimited message length is 2 bytes
• 0D: nested field 1, fixed32
• 05 00 00 00: enough bytes remain in the underlying source, but not inside the current logical message limit

Expected fixed behavior:

EOFException

This covers the invariant that scalar reads must not cross the current length-delimited message boundary even when the underlying source has more bytes available.

Swift tag-only varint value

Hex:

08

Meaning:

• 08: field 1, varint
• Missing varint value byte

Pre-fix risk:

• ReadBuffer.readVarint() could dereference pointer.pointee before verifying that a byte remained.

Expected fixed behavior:

ProtoDecoder.Error.unexpectedEndOfData

Swift nested message with oversized positive length

Hex:

12 FF FF FF FF 07

Meaning:

• 12: field 2, length-delimited
• FF FF FF FF 07: varint length 2147483647

Pre-fix risk:

• Nested message decoding computed an end pointer from an untrusted length before proving that the buffer contained that many bytes.

Expected fixed behavior:

ProtoDecoder.Error.unexpectedEndOfData

Swift packed repeated field with oversized positive length

Hex:

0A FF FF FF FF 07

Meaning:

• 0A: field 1, length-delimited packed repeated field
• FF FF FF FF 07: varint length 2147483647

Pre-fix risk:

• Packed repeated decoding could reserve capacity based on an untrusted length before proving the bytes were present.

Expected fixed behavior:

ProtoDecoder.Error.unexpectedEndOfData

Swift size-delimited stream with unrepresentable size

Hex:

FF FF FF FF FF FF FF FF FF 01

Meaning:

• Size-delimited message length varint UInt64.max

Pre-fix risk:

• ProtoDecoder.decodeSizeDelimited(_:from:) converted the untrusted UInt64 to Int without exactness checking.

Expected fixed behavior:

ProtoDecoder.Error.unexpectedEndOfData

Root cause

The vulnerable code mixed three operations that must remain separate:

1. Decode an untrusted protobuf length.
2. Validate that the length is non-negative and fits within the current logical message boundary.
3. Advance the cursor, pointer, limit, slice, or allocation based on that length.

In the vulnerable paths, step 3 happened before step 2 was complete. For Kotlin ByteArrayProtoReader32, this caused signed integer wraparound in pos + length. For Swift, related pointer and allocation operations could be performed before proving the requested bytes existed.

Workarounds

The recommended remediation is to upgrade to a patched release.

Partial mitigations if an immediate upgrade is not possible:

• Reject or cap untrusted protobuf message sizes before passing bytes to Wire.
• Prefer decoding from a bounded source where possible rather than decoding unbounded attacker-controlled byte arrays.
• Treat unchecked runtime exceptions from protobuf decode as malformed-input failures at service trust boundaries so they cannot crash the process.
• For Swift, do not pass untrusted size-delimited streams or Data directly to affected decoders without an outer size cap and exception/error boundary.

These mitigations reduce exposure but do not fully fix the parser bugs.

Detection

A crash or error may contain one of the following symptoms when processing malformed protobuf bytes:

IllegalArgumentException: startIndex: 6 > endIndex: -2147483643
ArrayIndexOutOfBoundsException
IndexOutOfBoundsException
unexpected unchecked RuntimeException during ProtoAdapter.decode(ByteArray)
Swift trap during Int conversion from an untrusted protobuf size
Swift unexpected pointer/buffer failure while reading malformed varints or length-delimited values

The absence of these exact messages does not prove safety. Any unchecked exception, trap, or process crash while decoding malformed length-delimited protobuf input should be investigated.

Verification

Regression tests added:

• ProtoReader32Test.lengthDelimitedRejectsPositiveLengthOverflow
• ProtoReader32Test.fixed32CannotReadPastLengthDelimitedLimit
• ProtoReaderTest.fixed32CannotReadPastLengthDelimitedLimit
• ProtoReaderTests.testReadVarintRejectsMissingValue
• ProtoReaderTests.testNestedMessageRejectsOversizedLength
• ProtoReaderTests.testPackedRepeatedRejectsOversizedLengthBeforePreallocation
• ProtoDecoderTests.testDecodeSizeDelimitedRejectsUnrepresentableSize

Focused verification command:

./gradlew :wire-runtime:jvmTest :wire-runtime-swift:test

Expected result:

BUILD SUCCESSFUL

Relationship to related advisories

This is a distinct vulnerability from the negative-length issues. It is a
different bug class, a positive, non-negative length (for example
2147483647) that passes the existing length < 0 check but still overflows
the signed 32-bit cursor or crosses the current message boundary, and it has
a separate fix (PR #3635, not the negative-length PRs).

• CVE-2026-45799 / GHSA-7xpr-hc2w-34m9 fixed the original Kotlin/JVM
  negative-length skipped-group crash (Wire 6.3.0). The non-negative
  overflow described here was not covered by that check and remained
  exploitable through 6.4.4.
• GHSA-86wm-r4c5-2rc9 / CVE-2026-61695 covers the Swift negative-length
  skipGroup() crash (PR #3616). The Swift hardening in this advisory
  (PR #3635) instead addresses positive/oversized-length overflow, buffer
  over-read, and unrepresentable-size conversions in the Swift readers.