ProtobufParser.readStringKey() reads the length prefix of a protobuf map entry's string key into an attacker-controlled int and then checks (inputPtr + len) <= inputEnd using signed arithmetic. For values near Integer.MAXVALUE the addition overflows to a negative value and the check passes even when the input contains no key data. The parser then calls finishShortText(len), which expands the text buffer to the declared length via TextBuffer.expandCurrentSegment and Arrays.copyOf, throwing OutOfMemoryError: Requested array size exceeds VM limit.

An 8-byte malformed message containing a map entry with a declared key length of 0x7FFFFFFF is sufficient. Configured StreamReadConstraints, including maxStringLength, maxNameLength, maxDocumentLength and maxNestingDepth, do not prevent the failure. Applications running with -XX:+ExitOnOutOfMemoryError terminate the JVM.

This is the same integer-overflow pattern addressed for other call sites in the remediation associated with CVE-2026-104016 and CVE-2026-104017; the map-key call site was corrected on the 3.x branches only, leaving the corresponding 2.x branches unguarded. Release lines 2.18, 2.19 and 2.20 do not contain readStringKey() and are not affected by this vector. No privileges or user interaction beyond the ability to submit data for parsing are required. The 3.0 release line and 3.1.0-3.1.5 / 3.2.0-3.2.1 do not contain readStringKey() and are therefore not affected; the method first appears in 3.1.6 and 3.2.2, mirroring its introduction at 2.21.6 on the 2.x side.