Decimal: Unbounded exponent in `Decimal.new` enables unauthenticated DoS

Summary decimal doesn't bound the exponent on parsed input, so something like "1e10000000" is parsed fine but then explodes the memory to more than 7GB if you run e.g. Decimal.add(Decimal.parse("1e10000000"), 1) because for positive exp, the function tail-recurses with coef * 10 and exp - 1 per iteration, growing the bignum coefficient by one digit each step. In the worst case, one request is enough to OOM the BEAM.

Details

Decimal.new/parse/cast happily store huge exponents. After that, a bunch of core paths allocate proportional to exp:

• add/sub/div go through add_align, which calls pow10(exp1 - exp2) and builds a giant bignum (lib/decimal.ex:1734-1738, 1827).
• to_string/2 with :normal (also :xsd and the String.Chars impl) does :lists.duplicate(exp, ?0) (lib/decimal.ex:1506, 1513).
• to_integer/1 recurses coef * 10, exp - 1 once per unit of exp (lib/decimal.ex:1603-1605).
• round/3 does the same :lists.duplicate trick on the exp difference (lib/decimal.ex:1850, 1874).
• compare/3 with a threshold argument loops back into add/sub, so it's vulnerable too (lib/decimal.ex:331-332).

PoC

Any of these will hang or OOM the BEAM:

Decimal.add(Decimal.new("1"), Decimal.new("1e1000000000"))
Decimal.to_string(Decimal.new("1e1000000000"), :normal)
Decimal.to_integer(Decimal.new("1e1000000000"))
Decimal.round(Decimal.new("1e1000000000"))

Impact

Unauthenticated remote DoS. Anything that takes a user-supplied decimal (JSON, form field, Ecto :decimal field — basically everywhere) and then does arithmetic, rounding, to_integer, or to_string on it is exposed. One request can kill the node with a Out-of-Memory exception.

Note on the security fixes done in version 2.4.0

While 2.4.0 has the changes to mitigate this issue it's not considered as a patched version because it doesn't have them enabled by default.