CVE-2026-5089: Buffer Underwrite

YAML::Syck versions before 1.38 for Perl has an out-of-bounds read.

The base60 (sexagesimal) parsing code in perl_syck.h has a buffer underflow bug in both int#base60 and float#base60 handlers. When processing the leftmost segment of a colon-separated value (e.g., the 1 in 1:30:45), the inner while loop can decrement a pointer past the start of the string buffer:

while ( colon >= ptr && *colon != ':' ) { colon--; } if ( *colon == ':' ) *colon = '\0'; // colon may be ptr-1 here

When no colon is found (final/leftmost segment), colon becomes ptr-1, and the subsequent *colon dereference reads one byte before the allocated buffer.