CVE-2026-102941gomarkdown/markdown IsSafeURL panics on short URLs

Official description

The package github.com/gomarkdown/markdown is a Go library for parsing Markdown text and rendering as HTML. Prior to 0.0.0-20260724235538-61af6e1e6971, parser.IsSafeURL in parser/parser.go slices url[:nPath] before the len(url) >= nPath guard. A short destination causes a slice-bounds panic when the loop reaches a relative-path prefix for which len(url) < nPath and cap(url) < nPath. The one-byte destination / is an explicit non-panic counterexample because it matches the first valid path and returns before a longer prefix is sliced. Demonstrated panic cases include empty, ., and .. destinations under those conditions. The panic is reachable when markdown.ToHTML renders untrusted Markdown with html.Safelink enabled, including an empty link destination, and is also reachable by callers that pass a short attacker-controlled URL directly to the exported parser.IsSafeURL function. An uncaught panic can terminate the calling process and cause denial of service. This issue is fixed in 0.0.0-20260724235538-61af6e1e6971.

Affected packages & fixed versions

Affected products (as reported)

References

• https://github.com/gomarkdown/markdown/security/advisories/GHSA-cv23-7vc5-jfh7 x_refsource_CONFIRM
• https://github.com/gomarkdown/markdown/commit/61af6e1e69710c5ca13cfbaaaa1bc11bc763a8a9 x_refsource_MISC