gomarkdown/markdown IsSafeURL panics on short URLs

Why it is Yellow

• CVSS 7.5 high
• Remotely exploitable without logging in
• On your watchlist: Go

Affected packages

References

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

Original 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.