gomarkdown/markdown IsSafeURL panics on short URLs

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The package github.com/gomarkdown/markdown is a Go library for parsing Markdown text and rendering as HTML. Prior to 0.0.0-20260724235538-61af6e1e6971, parser.IsSafeURL in parser/parser.go slices url[:nPath] before the len(url) >= nPath guard. A short destination causes a slice-bounds panic when the loop reaches a relative-path prefix for which len(url) < nPath and cap(url) < nPath. The one-byte destination / is an explicit non-panic counterexample because it matches the first valid path and returns before a longer prefix is sliced. Demonstrated panic cases include empty, ., and .. destinations under those conditions. The panic is reachable when markdown.ToHTML renders untrusted Markdown with html.Safelink enabled, including an empty link destination, and is also reachable by callers that pass a short attacker-controlled URL directly to the exported parser.IsSafeURL function. An uncaught panic can terminate the calling process and cause denial of service. This issue is fixed in 0.0.0-20260724235538-61af6e1e6971.

Source published

16 Oct 2026 15:38:18 UTC

Source modified

26 Oct 2026 17:37:11 UTC

Observed here

26 Oct 2026 18:35:30 UTC

Source terms

CVE Program Terms of Use

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This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: cve-program · CVE Program Terms of Use. Source text is retained; field grouping and change labels are provided by ChangeIntel.

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

The package github.com/gomarkdown/markdown is a Go library for parsing Markdown text and rendering as HTML. Prior to 0.0.0-20260724235538-61af6e1e6971, parser.IsSafeURL in parser/parser.go slices url[:nPath] before the len(url) >= nPath guard. A short destination causes a slice-bounds panic when the loop reaches a relative-path prefix for which len(url) < nPath and cap(url) < nPath. The one-byte destination / is an explicit non-panic counterexample because it matches the first valid path and returns before a longer prefix is sliced. Demonstrated panic cases include empty, ., and .. destinations under those conditions. The panic is reachable when markdown.ToHTML renders untrusted Markdown with html.Safelink enabled, including an empty link destination, and is also reachable by callers that pass a short attacker-controlled URL directly to the exported parser.IsSafeURL function. An uncaught panic can terminate the calling process and cause denial of service. This issue is fixed in 0.0.0-20260724235538-61af6e1e6971.

Source published

Oct 16, 2026, 3:38:18 PM (timezone not stated)

Source modified

Oct 26, 2026, 5:37:11 PM (timezone not stated)

Observed here

7 Nov 2026 04:27:32 UTC

Source terms

ENISA EUVD · source terms and attribution apply

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This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: euvd · ENISA EUVD · source terms and attribution apply. Source text is retained; field grouping and change labels are provided by ChangeIntel.

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

The package github.com/gomarkdown/markdown is a Go library for parsing Markdown text and rendering as HTML. Prior to 0.0.0-20260724235538-61af6e1e6971, parser.IsSafeURL in parser/parser.go slices url[:nPath] before the len(url) >= nPath guard. A short destination causes a slice-bounds panic when the loop reaches a relative-path prefix for which len(url) < nPath and cap(url) < nPath. The one-byte destination / is an explicit non-panic counterexample because it matches the first valid path and returns before a longer prefix is sliced. Demonstrated panic cases include empty, ., and .. destinations under those conditions. The panic is reachable when markdown.ToHTML renders untrusted Markdown with html.Safelink enabled, including an empty link destination, and is also reachable by callers that pass a short attacker-controlled URL directly to the exported parser.IsSafeURL function. An uncaught panic can terminate the calling process and cause denial of service. This issue is fixed in 0.0.0-20260724235538-61af6e1e6971.

Source published

16 Oct 2026 16:16:52 UTC

Source modified

26 Oct 2026 18:17:23 UTC

Observed here

27 Oct 2026 05:23:30 UTC

Source terms

NIST NVD usage terms; underlying source attribution retained

Original source record · Cite this exact observation · Complete original publisher JSON

This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: nvd · NIST NVD usage terms; underlying source attribution retained. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS 3.1 · [email protected] · Secondary · 7.5 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

## 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: ```go func IsSafeURL(url []byte) bool { nLink := len(url) for _, path := range validPaths { // "/", "./", "../" nPath := len(path) linkPrefix := url[:nPath] // line 789: sliced BEFORE the length guard if nLink >= nPath && bytes.Equal(linkPrefix, path) { // line 790: guard is too late ... ``` 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 rendering untrusted Markdown with the html.Safelink flag (the URL-sanitizing hardening flag itself introduces the crash); or (2) any downstream that calls the exported parser.IsSafeURL on a 0/1/2-byte attacker URL, no flag required. No recover() anywhere, so an uncaught crash of the calling goroutine. ## Verification EXECUTED with negative controls. Direct: the empty/1/2-char cases panic with the exact slice-bounds message while "/" returns true. End to end via ToHTML with html.CommonFlags|html.Safelink: `[x]()` panics, while `[x](/)`, `[x](http://example.com)`, plain text, and `[x](javascript:alert(1))` (safely dropped) do not. NOT verified: how many downstream apps enable Safelink or call IsSafeURL directly (the default no-Safelink render path does not reach it). ## Suggested fix Move the slice inside the guard: check `nLink >= nPath` first, then compute url[:nPath] (or use bytes.HasPrefix(url, path), which is length-safe). One-line change, mirroring the already-correct URIs loop. Also checked and sound: CVE-2026-40890 (smartypants smartLeftAngle OOB) is fixed; CVE-2024-44337 (list/paragraph infinite loop) is fixed (deep nesting capped at maxNesting=64); default XSS is by design (Safelink/SkipHTML not in CommonFlags). ## Tooling AI assistance was used while investigating. The panic is executed-verified with negative controls (IsSafeURL on empty/./.. panics with a slice-bounds error while "/" returns true; ToHTML with html.Safelink on `[x]()` panics while valid and javascript: links do not), and I confirmed at HEAD that IsSafeURL computes url[:nPath] before the `nLink >= nPath` guard.

Source published

16 Oct 2026 15:41:39 UTC

Source modified

16 Oct 2026 16:00:45 UTC

Observed here

24 Oct 2026 13:19:41 UTC

Source terms

GitHub Advisory Database · CC-BY-4.0

Original source record · Cite this exact observation · Complete original publisher JSON

This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: osv · GitHub Advisory Database · CC-BY-4.0. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS_V3 · CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

Publisher rating · HIGH