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Treat backslash as a path separator in XModem filename validation #11085

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Merged
caveman99 merged 2 commits into from
Jul 20, 2026
Merged

Treat backslash as a path separator in XModem filename validation #11085

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142780c
Treat backslash as a path separator in XModem filename validation
caveman99 Jul 20, 2026
9f1bbd6
Restrict drive-qualifier rejection to ASCII drive letters
caveman99 Jul 20, 2026
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5 changes: 4 additions & 1 deletion src/xmodem.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -62,10 +62,13 @@ bool XModemAdapter::isValidFilename(const char *name)
{
if (!name || name[0] == '\0')
return false;
const bool driveLetter = (name[0] >= 'A' && name[0] <= 'Z') || (name[0] >= 'a' && name[0] <= 'z');
if (driveLetter && name[1] == ':')
return false;
// Reject any ".." path component. Absolute paths and subdirectories are fine; they stay within
// the filesystem root, so only traversal out of it needs blocking.
for (const char *seg = name; *seg;) {
const char *slash = strchr(seg, '/');
const char *slash = strpbrk(seg, "/\\");
const size_t len = slash ? (size_t)(slash - seg) : strlen(seg);
if (len == 2 && seg[0] == '.' && seg[1] == '.')
return false;
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21 changes: 21 additions & 0 deletions test/test_xmodem/test_main.cpp
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Expand Up @@ -21,6 +21,22 @@ void test_xmodem_rejects_dotdot_traversal(void)
TEST_ASSERT_FALSE(XModemAdapter::isValidFilename("/.."));
}

void test_xmodem_rejects_backslash_traversal(void)
{
TEST_ASSERT_FALSE(XModemAdapter::isValidFilename("..\\secret"));
TEST_ASSERT_FALSE(XModemAdapter::isValidFilename("..\\..\\Windows\\System32\\drivers\\etc\\hosts"));
TEST_ASSERT_FALSE(XModemAdapter::isValidFilename("dir\\..\\..\\x"));
TEST_ASSERT_FALSE(XModemAdapter::isValidFilename("dir/..\\x"));
TEST_ASSERT_FALSE(XModemAdapter::isValidFilename("dir\\.."));
}

void test_xmodem_rejects_drive_qualified(void)
{
TEST_ASSERT_FALSE(XModemAdapter::isValidFilename("C:\\Windows\\System32\\x"));
TEST_ASSERT_FALSE(XModemAdapter::isValidFilename("C:/Windows/System32/x"));
TEST_ASSERT_FALSE(XModemAdapter::isValidFilename("c:relative.txt"));
}

void test_xmodem_rejects_empty(void)
{
TEST_ASSERT_FALSE(XModemAdapter::isValidFilename(""));
Expand All @@ -36,6 +52,9 @@ void test_xmodem_allows_legit_paths(void)
// ".." only inside a name (not a whole component) is a valid filename, not traversal.
TEST_ASSERT_TRUE(XModemAdapter::isValidFilename("my..file"));
TEST_ASSERT_TRUE(XModemAdapter::isValidFilename("..."));
// A colon that cannot form a drive qualifier is a legal filename on the POSIX daemon.
TEST_ASSERT_TRUE(XModemAdapter::isValidFilename("1:30pm.txt"));
TEST_ASSERT_TRUE(XModemAdapter::isValidFilename("dir/1:30pm.txt"));
}

#endif // FSCom
Expand All @@ -46,6 +65,8 @@ void setup()
UNITY_BEGIN();
#ifdef FSCom
RUN_TEST(test_xmodem_rejects_dotdot_traversal);
RUN_TEST(test_xmodem_rejects_backslash_traversal);
RUN_TEST(test_xmodem_rejects_drive_qualified);
RUN_TEST(test_xmodem_rejects_empty);
RUN_TEST(test_xmodem_allows_legit_paths);
#endif
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