Go Markdown has an Out-of-bounds Read in SmartypantsRenderer

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

7.5 · CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

EPSS

• CVE-2026-40890: 0.35% estimated exploitation activity in the next 30 days; percentile 28.18 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

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 Impact This vulnerability will lead to a Denial of Service / panic on the processing service. -- The Datadog Security Team

Published by the source

14 Apr 2026, 22:38 UTC

Last modified by the source

27 Apr 2026, 16:17 UTC

Source last checked

26 Oct 2026, 15:19 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-77fj-vx54-gvh7

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# Patch Next advisory brief: CVE-2026-40890

Advisory: Go Markdown has an Out-of-bounds Read in SmartypantsRenderer
IDs: CVE-2026-40890, GHSA-77fj-vx54-gvh7
Record: https://patchnext.com/advisories/cve-2026-40890 (revision 1, source checked 26 Oct 2026, 15:19 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: High (CVSS 7.5)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: github.com/gomarkdown/markdown 0.0.0-20260411013819-759bbc3e3207. A vendor fix does not mean a deployment is fixed.

## Affected components
- Go github.com/gomarkdown/markdown: affected < 0.0.0-20260411013819-759bbc3e3207; fixed in 0.0.0-20260411013819-759bbc3e3207

## Suggested inspection (read-only)
- Go github.com/gomarkdown/markdown: check the installed version with your package manager.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on github.com/gomarkdown/markdown.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-77fj-vx54-gvh7 (GitHub-reviewed advisory GHSA-77fj-vx54-gvh7 lists affected ranges: Go github.com/gomarkdown/markdown >=0 <0.0.0-20260411013819-759bbc3e3207)

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