What causes "random string length must be between 0 and %d" in shadow1ng/fscan?

Use a length between 0 and maxRandomStringLength (see the %d in the message).

random string length must be between 0 and %d

Error message

random string length must be between 0 and %d

What it means

validateRandomStringLength bounds the requested length for the randomStr/random string expression function to [0, maxRandomStringLength]. Passing a negative or excessively large length returns this error naming the allowed maximum.

Source

Thrown at webscan/lib/eval_random.go:119

			},
		},
	}
}

func randomIntSpan(min, max int64) (int64, error) {
	if max <= min {
		return 0, fmt.Errorf("randomInt: max(%d) must be greater than min(%d)", max, min)
	}
	const maxInt64 = int64(^uint64(0) >> 1)
	if min < 0 && max > maxInt64+min {
		return 0, fmt.Errorf("randomInt: range too large")
	}
	return max - min, nil
}

func validateRandomStringLength(n types.Int) (int, error) {
	if n < 0 || n > maxRandomStringLength {
		return 0, fmt.Errorf("random string length must be between 0 and %d", maxRandomStringLength)
	}
	return int(n), nil
}

View on GitHub (pinned to 95cc12e753)

Solutions

Use a length between 0 and maxRandomStringLength (see the %d in the message).
For larger payloads, build them by concatenating multiple randomStr calls.
Clamp the computed length with a min/max helper before calling the function.

Example fix

// before
s, err := randomStr(1 << 30) // too large

// after
n := 1 << 30
if n > maxRandomStringLength { n = maxRandomStringLength }
s, err := randomStr(n)

Defensive patterns

Strategy: validation

Validate before calling

func saneLen(n int) (int, error) {
    if n < 0 || n > maxRandomStringLength {
        return 0, fmt.Errorf("length %d out of [0,%d]", n, maxRandomStringLength)
    }
    return n, nil
}

Try / catch

n, err := validateRandomStringLength(want)
if err != nil {
    n = maxRandomStringLength // clamp instead of failing
}

Prevention

Clamp computed lengths before calling random string functions.
Build large payloads by concatenating multiple bounded random strings.
Keep POC template lengths within documented limits.

When it happens

Trigger: Calling the random-string function with n < 0 or n > maxRandomStringLength, typically via a POC expression like randomStr(999999999).

Common situations: POC templates with oversized random payloads for buffer-overflow style tests; negative lengths from computed expressions; copy-pasted templates written for libraries with larger limits.

Understand the failure class

Background: "value must be between 0 and 1" / "out of range" / "must not be negative" errors: fixing range-validation failures across open-source libraries — this error's family across 42 libraries.

Related errors

AI-assisted analysis of shadow1ng/fscan@95cc12e753 (2026-09-06). Data as JSON: /api/errors/d9e0a7c55e605bd1. Report an issue: GitHub.