Out-of-bounds read in portableproplib on a truncated XML plist preamble

The bug is in lib/portableproplib/prop_object.c at line 722, where a memcmp call reads out-of-bounds. Start by examining the MATCH macro and the surrounding parser logic in prop_object.c. Reproduce the issue with the minimal input '\x3c' and an ASan build. Check input length validation before fixed-length comparisons. The fix must be applied to the portableproplib code used by XBPS, and tests should be run to ensure repository index parsing (lib/repo.c) and archive handling (lib/archive.c) remain safe.

I found a reproducible out-of-bounds read in XBPS's bundled portableproplib XML plist parser. In lib/portableproplib/prop_object.c, MATCH(str) calls memcmp(&xml[1], str, sizeof(str) - 1) before confirming that the input contains that many bytes. A one-byte input consisting only of < reaches this comparison.

Affected code tested: XBPS 0.60.7 and a frozen master revision at c5f41d5. With AddressSanitizer-enabled dictionary internalization builds, the one-byte input reports a five-byte heap-buffer-overflow read in _prop_object_internalize_context_alloc() at prop_object.c:722. I reproduced it again on both revisions on 23 September 2026; earlier testing also reached the same parser defect through the array internalization API. The test adapter allocates input_size + 1 bytes, copies the input, and appends a NUL terminator before invoking the upstream API. The invalid read therefore occurs despite the terminator.

Minimal input (hex): 3c (ASCII <). The relevant sanitizer trace begins:

ERROR: AddressSanitizer: heap-buffer-overflow
READ of size 5
  #2 _prop_object_internalize_context_alloc .../lib/portableproplib/prop_object.c:722
  #3 _prop_generic_internalize .../lib/portableproplib/prop_object.c:639

XBPS also passes repository index and metadata plist buffers to xbps_dictionary_internalize() in lib/repo.c, and archive dictionaries through lib/archive.c. This establishes product paths to the parser. I have not established a specific remote attacker model, native process termination, or information disclosure; the observed failure is the sanitizer-detected read while rejecting malformed input.

Please check the remaining input length before each fixed-length comparison and assess the affected release range. I can provide the full sanitizer log and adapter source if helpful.

I submitted a CVE ID request to MITRE CNA-LR under tracking number CAN-2026-2052927. This is a request tracking number, not an assigned CVE ID. Please coordinate on this existing request rather than filing a duplicate for the same root cause. For public credit, please identify He Huang as the finder and NexusSan as the discovery tool.