[UNDERTOW-2655] Fix text corruption in FileUtils.readFile when reading multi-byte characters #1834
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fl4via
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Oct 29, 2025
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Hi @finalchild ! thanks for your PR, can you please create a test for the fix?
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…g multi-byte characters The readFile method was reading the InputStream into a fixed-size byte buffer and decoding each chunk independently. This caused multi-byte UTF-8 character sequences to be split across buffer boundaries, resulting in text corruption with replacement characters. Replaced BufferedInputStream with InputStreamReader to handle buffering and character decoding together in a streaming fashion, ensuring multi-byte character sequences are never split. This issue became more significant after UNDERTOW-2337, as large form-data field values are now processed by this function. Originally reported in Spring Framework issue #35292. 🤖 Generated with [Claude Code](https://claude.com/claude-code) Co-Authored-By: Claude <[email protected]>
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Summary
Fixes text corruption in
FileUtils.readFilewhen reading multi-byte UTF-8 characters.Problem: The original implementation read the
InputStreaminto a fixed-size byte buffer (1024 bytes) and decoded each chunk independently. When a multi-byte character sequence was split across a buffer boundary, the decoder received incomplete character data, resulting in replacement characters (�) in the final string.Solution: Replaced
BufferedInputStreamwithInputStreamReaderto handle buffering and character decoding together in a streaming fashion, ensuring multi-byte character sequences are never split.Note: The implementation is copied from Java 25's
InputStreamReader#readAllAsString.This issue became more significant after fixing UNDERTOW-2337, as large form-data field values are now processed by this vulnerable function. Originally reported in Spring Framework issue #35292.
Issue: UNDERTOW-2655