The GB18030 4-byte decoder in musl libc's iconv() implementation contains a gap-skipping loop that performs a full linear scan of the gb18030126 lookup table (23,940 entries) on each iteration of an outer loop whose iteration count is input-dependent. For 4-byte sequences whose linear index falls just below the dense CJK Unified Ideographs range, the outer loop executes approximately 20,905 times, resulting in approximately 500 million comparisons per input character. Measured on musl 1.2.6 and 1.2.5: a single 4-byte input character (bytes 0x82 0x35 0x8F 0x33) takes approximately 260ms to decode, compared to approximately 13 microseconds for a benign character — a 19,000x slowdown. A payload of 40kB will take ~43 minutes to decode.
| Vendor | Product | Versions |
|---|---|---|
| musl | libc | 0.8.0 to 1.2.6 |
Updated the patch available to a new version that addresses the issue.
Updated description with detailed technical information and noted that the vulnerability is actively exploited.
Initial creation
