[BUG] Snapshot rename replacement unbounded length rename

[meet-v25]

Describe the bug

OpenSearch's snapshot restore functionality is not checking the length of the rename index in the renameIndex() method. It can lead to processing of a huge string of index.

Location: server/src/main/java/org/opensearch/snapshots/RestoreService.java:1301
renamedIndex = index.replaceAll(request.renamePattern(), request.renameReplacement());

The String.replaceAll() method is called without validating:

1. The length of the replacement string
2. The resulting index name length after replacement

Related component

Storage:Snapshots

To Reproduce

1. Create an index with a name containing many repeated characters (e.g., "a" * 255)
2. Let it / create a snapshot of the index
3. Restore the snapshot with malicious rename parameters:
   i. rename_pattern: "a" (matches each character)
   ii. rename_replacement: "b" * 1,000,000 (1 million characters per match)
4. Result: 255 matches * 1M characters = 255 million character string allocation
5. JVM runs out of memory and crashes

opensearch  | java.lang.OutOfMemoryError: Java heap space
opensearch  | Dumping heap to data/java_pid1.hprof ...
opensearch  | Unable to create data/java_pid1.hprof: File exists
opensearch  | [2026-01-05T10:00:57,140][ERROR][o.o.b.OpenSearchUncaughtExceptionHandler] [e9baa2091a80] fatal error in thread [opensearch[e9baa2091a80][generic][T#3]], exiting
opensearch  | java.lang.OutOfMemoryError: Java heap space

Expected behavior

There should not be any JVM crashes on any nodes due to an index renaming during index restore from snapshot.

Solution: Add validation for the rename_replacement field and cap the field size to 255 bytes. This size limit is the same limit of index names as in OpenSearch.

Additional Details

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• OS: [e.g. iOS]
• Version [e.g. 22]

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