gomarkdown/markdown IsSafeURL panics on short URLs

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-2026-102941: 0.28% estimated exploitation activity in the next 30 days; percentile 20.41 (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 IsSafeURL slices url[:nPath] before checking that len(url) = nPath, so a URL shorter than a path prefix (0, 1 or 2 bytes) triggers a slice-bounds panic, and this is reached end to end when the html.Safelink hardening flag is enabled and the input contains an empty or very short link destination, giving an unrecoverable crash DoS. Confirmed at HEAD (e08cff8; present since 2022). The defect parser/parser.go, IsSafeURL: url[:nPath] is evaluated before the nLink = nPath check. When len(url) < nPath and cap(url) < nPath, the slice expression panics. Only the relative-path loop is affected; the adjacent URIs loop is already correctly guarded (the signal that this one deviates, the same signal as the anchor). Input: IsSafeURL("") panics ([:1] cap 0), IsSafeURL(".") panics ([:2] cap 1), IsSafeURL("..") panics ([:3] cap 2), while IsSafeURL("/") returns true. End to end: markdown.ToHTML(input, html.CommonFlags|html.Safelink) with input x panics. Attacker model: (1) any service…

Published by the source

16 Oct 2026, 15:41 UTC

Last modified by the source

16 Oct 2026, 16:00 UTC

Source last checked

26 Oct 2026, 15:32 UTC

1. Revision 2 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-cv23-7vc5-jfh7

2. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-cv23-7vc5-jfh7

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# Patch Next advisory brief: CVE-2026-102941

Advisory: gomarkdown/markdown IsSafeURL panics on short URLs
IDs: CVE-2026-102941, GHSA-cv23-7vc5-jfh7
Record: https://patchnext.com/advisories/cve-2026-102941 (revision 2, source checked 26 Oct 2026, 15:32 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 0.0.0-20260724235538-61af6e1e6971. A vendor fix does not mean a deployment is fixed.

## Affected components
- Go github.com/gomarkdown/markdown: affected < 0.0.0-20260724235538-61af6e1e6971; fixed in 0.0.0-20260724235538-61af6e1e6971

## 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://osv.dev/vulnerability/GHSA-cv23-7vc5-jfh7 (OSV record GHSA-cv23-7vc5-jfh7 lists affected ranges: Go github.com/gomarkdown/markdown >=0 <0.0.0-20260724235538-61af6e1e6971 (fixed))
- Verified: https://github.com/advisories/GHSA-cv23-7vc5-jfh7 (GitHub-reviewed advisory GHSA-cv23-7vc5-jfh7 lists affected ranges: Go github.com/gomarkdown/markdown >=0 <0.0.0-20260724235538-61af6e1e6971)

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