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
### References
- https://github.com/thephpleague/flysystem/security/advisories/GHSA-cxf4-7mrp-vvpr
- https://github.com/thephpleague/flysystem/commit/ef4a9a557d769b5d472c403125716706a0d9cc77
- https://github.com/thephpleague/flysystem/releases/tag/3.35.3
- https://nvd.nist.gov/vuln/detail/CVE-2026-102601