Flysystem: WhitespacePathNormalizer's control-character (CorruptedPathDetected) check is bypassed by malformed UTF-8 in the path, affecting every adapter

An attacker who can supply file paths can bypass Flysystem's control-character rejection when a path contains malformed UTF-8. This affects every adapter using the default WhitespacePathNormalizer, allowing paths containing control characters to pass that check. Path-traversal protection is unaffected.

Check composer.lock for league/flysystem at version <= 3.35.2; 3.35.3 is fixed. Check whether untrusted paths reach Filesystem using the default WhitespacePathNormalizer. If your application supplies a custom PathNormalizer, inspect its validation rather than assuming this default-normaliser flaw applies.

The description recommends changing the control-character check in src/WhitespacePathNormalizer.php to reject any preg_match() result other than 0, using $matched !== 0. This treats malformed UTF-8 errors as corrupted paths too.

Related public issue (context, not a duplicate)

Closed issue #1429 ("Handle non-UTF-8 paths", 2024-03-24) raised exactly this general concern and even suggested detection via preg_match('//u', $path) !== 1 -- note the reporter's suggested check explicitly compares !== 1, which would correctly treat PCRE's false return as "reject." The maintainer's reply pointed to the PathNormalizer interface as the place to implement this. The control-character check that ended up shipping in WhitespacePathNormalizer (if (preg_match('#\p{C}+#u', $unixPath))) addresses the general concern but does not use the !== 1-style comparison the original issue suggested -- it uses a bare truthy check, which is exactly the gap this report demonstrates. So this is not a duplicate of #1429; it's a concrete bypass surviving in the fix that issue's concern led to.

Vulnerability Details

File: src/WhitespacePathNormalizer.php, lines 22-28 (normalizePath()) -- the default PathNormalizer used by Filesystem for every adapter (Local, FTP, SFTP, S3, AsyncAwsS3, Azure, GCS, ZipArchive, GridFS, InMemory) unless the application supplies a custom one.

Root Cause

public function normalizePath(string $path): string
{
    $unixPath = str_replace('\\', '/', $path);

    if (preg_match('#\p{C}+#u', $unixPath)) {
        throw CorruptedPathDetected::forPath($path);
    }
    ...

preg_match() returns false (a PHP engine error) rather than 0 when the subject string is not valid UTF-8 and the pattern uses the /u modifier -- PCRE can't even attempt the match. false and 0 are both falsy in PHP, and if (preg_match(...)) does not distinguish them. So a path containing any single invalid UTF-8 byte anywhere in the string makes preg_match() fail with a "Malformed UTF-8 characters" engine error, the if evaluates false, and CorruptedPathDetected is silently not thrown -- even when the same string also contains literal control characters this exact check exists to catch.

the identical payload IS correctly rejected once it's valid UTF-8:

$n->normalizePath("foo\x1bbar");         // valid UTF-8, contains ESC -> throws CorruptedPathDetected (correct)
$n->normalizePath("foo\x80\x1bbar");    // 0x80 = invalid lone UTF-8 continuation byte -> NOT thrown (bypass)

The path-traversal protection is unaffected -- it's exact byte-string comparison on /-delimited segments, independent of UTF-8 validity:

$n->normalizePath("\x80/../../etc/passwd");   // still throws PathTraversalDetected

Recommended Fix

public function normalizePath(string $path): string
{
    $unixPath = str_replace('\\', '/', $path);
    $matched = preg_match('#\p{C}+#u', $unixPath);

    if ($matched !== 0) {
        // $matched === false means malformed UTF-8 -- must also be treated as corrupted,
        // not silently allowed through.
        throw CorruptedPathDetected::forPath($path);
    }
    ...

Verification

Dynamically confirmed on league/flysystem HEAD 6837e1d / tag 3.35.2, PHP 8.4.22 CLI, end-to-end through LocalFilesystemAdapter:

[1] write() succeeded -- normalizer did NOT reject the path.
[2] Actual bytes on disk: ...801b5b386d6e6f726d616c2d6c6f6f6b696e672d66696c652e7478741b5b306d...
[3] Filesystem::listContents() path contains raw ESC (0x1b): YES
[4] Raw terminal output (via `cat -v`): M-^@^[[8mnormal-looking-file.txt^[[0m^[[2K^[[1Aurgent-invoice.pdf