vulnerability / NVI-00012110 / Revision 2
gomarkdown/markdown IsSafeURL panics on short URLs
GitHub lists this reviewed advisory for github.com/gomarkdown/markdown. Published version ranges and source prerequisites are shown below; this is not an inspection of a visitor’s application.
This report has not inspected your inventory, configuration, deployed build, reachability or evidence. A catalog listing and a severity score do not establish exposure or compromise.
01 / Meaning
Why it matters
Published affected context helps an owner decide what evidence to gather. This report has not examined your application.
7.5 high
GitHub source severity; exploitability and local consequence require separate evidence.
As of · GitHub Advisory Database
0.279%
FIRST EPSS model v2026.06.15; exploitation probability over 30 days, percentile 0.20376. Missing scores are unknown.
As of · FIRST EPSS
02 / Applicability
The affected context
Affected versions, configuration prerequisites and known exclusions are public. Confirm each one against the deployed service.
github.com/gomarkdown/markdown
source listed< 0.0.0-20260724235538-61af6e1e6971
## 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 ([
03 / Response
What to do next
- investigate
Identify the exact component and deployed version, examine the linked publisher’s conditions, and retain evidence of any mitigation.
Confirm current vendor support, compatibility and deployment evidence. This report has not tested a fix in your system. FIRST EPSS
A fixed release is not deployment evidence.
Check current vendor support and compatibility. After making a change, verify the deployed build and relevant runtime conditions. Keep the evidence, owner and time. An updated lockfile alone does not prove what is running.
04 / Limits
What this report cannot establish
- Customer inventory, deployment, reachability and exploitation have not been inspected.
05 / Provenance
Sources you can inspect
Each assertion retains its source. Mirrored records may repeat one underlying publisher; additional links are not independent corroboration.
- GHSA-cv23-7vc5-jfh7 ↗
GitHub Advisory Database · vendor advisory
Observed . GitHub Advisory Database; CC-BY-4.0. Each advisory links its original attribution. Null normalizes fields.
- CVE-2026-102941 ↗
FIRST EPSS · official catalog
Observed . FIRST EPSS; probability of exploitation in the next 30 days, not local compromise. Dataset date/model retained; FIRST services terms apply.
Public revision history
Only published revisions appear here. Routine fetches do not create a material-change event.
- Revision 2 ·
Dated EPSS estimate updated; this is not a new disclosure or proof of exploitation.
- Revision 1 ·
First indexed by Null; source disclosure date is shown separately.