IO-475: Fix UNC path prefix validation in FilenameUtils #825

MiriamCohenDev · 2026-01-26T11:56:34Z

Fixes a bug in FilenameUtils#normalizeNoEndSeparator when handling UNC paths that do not end with a slash.

What:

The function normalizeNoEndSeparator incorrectly returned null for valid UNC paths like //server because it treated them as invalid.
This was caused by FilenameUtils#getPrefixLength returning -1 for UNC paths without a trailing slash.

How:

Updated getPrefixLength to correctly handle UNC paths without a trailing slash, returning the proper prefix length instead of -1.
Updated and added unit tests to reflect the correct behavior, since many existing tests assumed null would be returned for these paths.

Why:

Previous behavior caused valid UNC paths to be treated as invalid, which could break downstream logic that relies on normalizeNoEndSeparator.
This change ensures proper handling of UNC paths and maintains consistency across FilenameUtils methods.

Testing:

Updated tests in FilenameUtilsTest to cover UNC paths without trailing slashes.
Verified that all tests pass with the new logic.

Files changed:

src/main/java/org/apache/commons/io/FilenameUtils.java
src/test/java/org/apache/commons/io/FilenameUtilsTest.java

…ling slash

…e correct logic

garydgregory

Hello @MiriamCohenDev

Thank you for your PR. Please see my comments. Run mvn by itself before you push and fix any issues that come up.

garydgregory · 2026-01-26T12:37:57Z

src/main/java/org/apache/commons/io/FilenameUtils.java

 */
 package org.apache.commons.io;

+import java.io.Console;


Remove this import.

garydgregory · 2026-01-26T12:38:14Z

src/main/java/org/apache/commons/io/FilenameUtils.java

        if (!isSeparator(ch0) || !isSeparator(ch1)) {
            return isSeparator(ch0) ? 1 : 0;
        }
+


Remove extra blank line.

garydgregory · 2026-01-26T12:38:43Z

src/test/java/org/apache/commons/io/FilenameUtilsTest.java

        }
        try (BufferedOutputStream output3 =
-                new BufferedOutputStream(Files.newOutputStream(testFile1))) {
+                     new BufferedOutputStream(Files.newOutputStream(testFile1))) {


Don't make spurious edits, it makes the PR larger and takes longer to review.

src/test/java/org/apache/commons/io/FilenameUtilsTest.java

garydgregory · 2026-01-26T12:40:09Z

src/test/java/org/apache/commons/io/FilenameUtilsTest.java

        assertEquals("D:\\a\\b\\c", FilenameUtils.separatorsToWindows("D:/a/b/c"));
    }
-}
+}


Files end in an empty line.

garydgregory · 2026-01-26T12:40:24Z

src/test/java/org/apache/commons/io/FilenameUtilsTest.java

        assertEquals(SEP + SEP + "server.example.org" + SEP + "a" + SEP + "b" + SEP + "c.txt", FilenameUtils.normalize("\\\\server.example.org\\a\\b\\c.txt"));
        assertEquals(SEP + SEP + "server.sub.example.org" + SEP + "a" + SEP + "b" + SEP + "c.txt",
-            FilenameUtils.normalize("\\\\server.sub.example.org\\a\\b\\c.txt"));
+                FilenameUtils.normalize("\\\\server.sub.example.org\\a\\b\\c.txt"));


Don't make spurious edits, it makes the PR larger and takes longer to review.

src/test/java/org/apache/commons/io/FilenameUtilsTest.java

garydgregory · 2026-01-26T13:00:17Z

Jira ticket is https://issues.apache.org/jira/browse/IO-475

garydgregory

Hello @MiriamCohenDev

Thank you for your updates. You missed a few spots. It's simpler to review a PR if you don't delete assertions. Instead, change them to reflect the new behavior.

garydgregory · 2026-01-26T14:46:22Z

src/test/java/org/apache/commons/io/FilenameUtilsTest.java

    void testEqualsNormalizedError_IO_128() {
        assertFalse(FilenameUtils.equalsNormalizedOnSystem("//file.txt", "file.txt"));
        assertFalse(FilenameUtils.equalsNormalizedOnSystem("file.txt", "//file.txt"));
-        assertFalse(FilenameUtils.equalsNormalizedOnSystem("//file.txt", "//file.txt"));


Don't delete assertions, instead change it to show the new behavior here.

garydgregory · 2026-01-26T14:46:28Z

src/test/java/org/apache/commons/io/FilenameUtilsTest.java

        assertNull(FilenameUtils.getFullPathNoEndSeparator("1:"));
        assertNull(FilenameUtils.getFullPathNoEndSeparator("1:a"));
        assertNull(FilenameUtils.getFullPathNoEndSeparator("///a/b/c.txt"));
-        assertNull(FilenameUtils.getFullPathNoEndSeparator("//a"));


Don't delete assertions, instead change it to show the new behavior here.

garydgregory · 2026-01-26T14:46:34Z

src/test/java/org/apache/commons/io/FilenameUtilsTest.java

        assertNull(FilenameUtils.getPathNoEndSeparator("1:"));
        assertNull(FilenameUtils.getPathNoEndSeparator("1:a"));
        assertNull(FilenameUtils.getPathNoEndSeparator("///a/b/c.txt"));
-        assertNull(FilenameUtils.getPathNoEndSeparator("//a"));


Don't delete assertions, instead change it to show the new behavior here.

garydgregory · 2026-01-26T14:46:39Z

src/test/java/org/apache/commons/io/FilenameUtilsTest.java

        assertNull(FilenameUtils.getPrefix("1:"));
        assertNull(FilenameUtils.getPrefix("1:a"));
        assertNull(FilenameUtils.getPrefix("\\\\\\a\\b\\c.txt"));
-        assertNull(FilenameUtils.getPrefix("\\\\a"));


Don't delete assertions, instead change it to show the new behavior here.

garydgregory · 2026-01-26T14:46:43Z

src/test/java/org/apache/commons/io/FilenameUtilsTest.java

        assertEquals(-1, FilenameUtils.getPrefixLength("1:"));
        assertEquals(-1, FilenameUtils.getPrefixLength("1:a"));
        assertEquals(-1, FilenameUtils.getPrefixLength("\\\\\\a\\b\\c.txt"));
-        assertEquals(-1, FilenameUtils.getPrefixLength("\\\\a"));


Don't delete assertions, instead change it to show the new behavior here.

garydgregory · 2026-01-26T14:46:47Z

src/test/java/org/apache/commons/io/FilenameUtilsTest.java

        assertNull(FilenameUtils.normalize("1:"));
        assertNull(FilenameUtils.normalize("1:a"));
        assertNull(FilenameUtils.normalize("\\\\\\a\\b\\c.txt"));
-        assertNull(FilenameUtils.normalize("\\\\a"));


Don't delete assertions, instead change it to show the new behavior here.

garydgregory · 2026-01-26T14:46:54Z

src/test/java/org/apache/commons/io/FilenameUtilsTest.java

        assertNull(FilenameUtils.normalizeNoEndSeparator("1:"));
        assertNull(FilenameUtils.normalizeNoEndSeparator("1:a"));
        assertNull(FilenameUtils.normalizeNoEndSeparator("\\\\\\a\\b\\c.txt"));
-        assertNull(FilenameUtils.normalizeNoEndSeparator("\\\\a"));


Don't delete assertions, instead change it to show the new behavior here.

garydgregory

See my previous comments, and run mvn without anything else and fix:

[INFO] --- checkstyle:3.6.0:check (default-cli) @ commons-io ---
[INFO] There are 16 errors reported by Checkstyle 13.0.0 with /Users/garygregory/git/commons/commons-io/src/conf/checkstyle.xml ruleset.
[ERROR] src/main/java/org/apache/commons/io/FilenameUtils.java:[940,13] (coding) FinalLocalVariable: Variable 'pos' should be declared final.
[ERROR] src/main/java/org/apache/commons/io/FilenameUtils.java:[941,16] (coding) FinalLocalVariable: Variable 'hostnamePart' should be declared final.
[ERROR] src/main/java/org/apache/commons/io/FilenameUtils.java:[942,9] (whitespace) WhitespaceAfter: 'if' is not followed by whitespace.
[ERROR] src/main/java/org/apache/commons/io/FilenameUtils.java:[942,9] (whitespace) WhitespaceAround: 'if' is not followed by whitespace.
[ERROR] src/main/java/org/apache/commons/io/FilenameUtils.java:[942,56] (whitespace) WhitespaceAround: '{' is not preceded with whitespace.
[ERROR] src/main/java/org/apache/commons/io/FilenameUtils.java:[945,9] (blocks) RightCurly: '}' at column 9 should be on the same line as the next part of a multi-block statement (one that directly contains multiple blocks: if/else-if/else, do/while or try/catch/finally).
[ERROR] src/main/java/org/apache/commons/io/FilenameUtils.java:[946,9] (whitespace) WhitespaceAfter: 'else' is not followed by whitespace.
[ERROR] src/main/java/org/apache/commons/io/FilenameUtils.java:[946,9] (whitespace) WhitespaceAround: 'else' is not followed by whitespace.
[ERROR] src/main/java/org/apache/commons/io/FilenameUtils.java:[946,13] (whitespace) WhitespaceAround: '{' is not preceded with whitespace.
[ERROR] src/test/java/org/apache/commons/io/FilenameUtilsTest.java:[410,24] (whitespace) WhitespaceAfter: ',' is not followed by whitespace.
[ERROR] src/test/java/org/apache/commons/io/FilenameUtilsTest.java:[457,24] (whitespace) WhitespaceAfter: ',' is not followed by whitespace.
[ERROR] src/test/java/org/apache/commons/io/FilenameUtilsTest.java:[561,54] (whitespace) ParenPad: '(' is followed by whitespace.
[ERROR] src/test/java/org/apache/commons/io/FilenameUtilsTest.java:[780,21] (whitespace) ParenPad: '(' is followed by whitespace.
[ERROR] src/test/java/org/apache/commons/io/FilenameUtilsTest.java:[919,21] (whitespace) ParenPad: '(' is followed by whitespace.
[ERROR] src/test/java/org/apache/commons/io/FilenameUtilsTest.java:[992,21] (whitespace) ParenPad: '(' is followed by whitespace.
[ERROR] src/test/java/org/apache/commons/io/FilenameUtilsTest.java:[1132,21] (whitespace) ParenPad: '(' is followed by whitespace.

MiriamCohenDev added 2 commits January 26, 2026 13:17

[IO-745] Fix FilenameUtils#getPrefixLength for UNC paths without trai…

ba525f1

…ling slash

[IO-745] Fix FilenameUtilsTest Adding and modifying tests to match th…

318c919

…e correct logic

garydgregory requested changes Jan 26, 2026

View reviewed changes

MiriamCohenDev force-pushed the IO-475-fix-unc-prefix-check branch from 51f04e9 to 090b9f9 Compare January 26, 2026 14:20

IO-475: Address all review comments from Grigory

64038e1

MiriamCohenDev force-pushed the IO-475-fix-unc-prefix-check branch from 090b9f9 to 64038e1 Compare January 26, 2026 14:32

garydgregory reviewed Jan 26, 2026

View reviewed changes

garydgregory requested changes Jan 26, 2026

View reviewed changes

IO-475: Fix UNC path prefix validation in FilenameUtils #825

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IO-475: Fix UNC path prefix validation in FilenameUtils #825

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MiriamCohenDev commented Jan 26, 2026

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garydgregory commented Jan 26, 2026

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