Elo Ruby backend has Remote Code Execution via unescaped `#{...}` interpolation in emitted string literals and subtype-constraint labels

Summary

Elo's Ruby backend emits attacker-controlled text into Ruby double-quoted strings without escaping the # character. Because # begins Ruby's interpolation sequences — #{expr}, #@ivar, #@@cvar, #$gvar — and any expression wrapped in #{...} may use backticks for shell execution, an Elo program that contains a crafted string literal or a labelled subtype constraint compiles to Ruby that evaluates attacker-chosen Ruby, and through backticks runs attacker-chosen shell commands, at runtime.

The same Elo source compiled to JavaScript, Python, or SQL is benign, so the bug also breaks the cross-target portability guarantee the rest of the compiler honours.

Vulnerable sinks

Two sinks exist in src/compilers/ruby.ts at commit 749af7b:

1. String-literal emitter — lines 180–184. The emitter escapes backslash, double quote, and control characters, but not #. Any #{...} sequence in an Elo string literal flows straight into the emitted Ruby source.

2. Labelled subtype-constraint branch — lines 405–412. The error message passed to p_fail(p, "...") is wrapped in a Ruby double-quoted string with no escape pass at all. Because the parser at src/parser.ts:1339–1348 accepts single-quoted Elo strings as constraint labels, a label such as '#{ `id` }' flows unchanged into the emitted Ruby.

Attack surfaces

All three surfaces share the same root cause (the missing # escape in the Ruby double-quoted-string emission path) and are addressed by the same patch:

Proof of Concept

String-literal sink:

# poc.elo
'RES=#{`id 2>&1`}'

$ eloc -t ruby -x poc.elo
# emitted Ruby contains:  "RES=#{`id 2>&1`}"
# running it returns the output of id(1).

Replacing `id 2>&1` with `touch /tmp/pwned` produces a trivially observable filesystem side effect.

Subtype-constraint label sink:

# poc-subtype.elo
let X = Int(i | '#{`touch /tmp/pwned2`}': i > 0) in X(-1)

The predicate fails for -1, causing p_fail(p, "#{...}") to interpolate and execute the backtick command at the Ruby layer.

Both flows are automated in the reproduction script submitted with the original report and were verified against Ruby 3.x.

Root cause

The Ruby-emitter's double-quoted-string escape routine omitted # from its character set, and one emission path (the subtype-constraint label branch) performed no escape pass at all.