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Fix GH-23106: mb_strpos() reads past the end of a haystack ending in a truncated UTF-8 sequence - #25123
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Fix GH-23106: mb_strpos() reads past the end of a haystack ending in a truncated UTF-8 sequence#25123VishnuSrivatsava wants to merge 1 commit into
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…in a truncated UTF-8 sequence Backport of master commit cc22056 (PR php#23107), requested in phpGH-25025. On this branch the offset walk lives in mbfl_find_offset_utf8() in libmbfl. The forward walk can land past the end of the haystack on a truncated multi-byte sequence, and the returned pointer is then used by zend_memnstr() despite violating its end >= pointer invariant. Clamp the pointer to the end of the haystack.
VishnuSrivatsava
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November 1, 2026 11:09
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Fixes the PHP-8.2 part of #25025 and GH-23106.
Backport of master commit cc22056 (PR #23107, credited to Lazizbek Ergashev). As analyzed in #25025, on this branch the offset walk lives in
mbfl_find_offset_utf8()in libmbfl, so the master patch had to be rewritten for the different function andconst unsigned char *pointers rather than applied directly.The forward walk can land past the end of the haystack on a truncated multi-byte sequence, and the returned pointer is then used by
zend_memnstr()despite violating itsend >= pinvariant. The clamp restores the invariant for the forward path and for the needle-offset walk feedingzend_memnrstr()on the reverse path.Changes:
mbfl_find_offset_utf8()to the end of the haystack after the forward offset walk.ext/mbstring/tests/gh23106.phpt.Verification on this branch (macOS arm64,
--enable-debug+ AddressSanitizer):mb_strpos("AA\xf0\x90", "xyz", 3)aborts withAssertion failed: (end >= p), function zend_memnstr, file zend_operators.h, line 179.ext/mbstringsuite passes (327 passed, 0 failed).Not touching NEWS, following the branch convention where release managers add entries at packaging time. 8.2 is in security-only support until 2026-12-31; since this is an out-of-bounds read, it should qualify for a security release, but that call belongs to release managers. The PHP-8.3 backport is in #25121.