$SAFE escaping vulnerability about Exception#to_s / NameError#to_s (CVE-2012-4464, CVE-2012-4466)

Vulnerabilities found for Exception#to_s, NameError#to_s, and name_err_mesg_to_s() which is Ruby interpreter-internal API. A malicious user code can bypass $SAFE check by utilizing one of those security holes.


Ruby's $SAFE mechanism enables untrusted user codes to run in $SAFE >= 4 mode. This is a kind of sandboxing so some operations are restricted in that mode to protect other data outside the sandbox.

The problem found was around this mechanism. Exception#to_s, NameError#to_s, and name_err_mesg_to_s() interpreter-internal API was not correctly handling the $SAFE bits so a String object which is not tainted can destructively be marked as tainted using them. By using this an untrusted code in a sandbox can modify a formerly-untainted string destructively.

Ruby 1.8 once had a similar security issue. It fixed Exception#to_s and NameError#to_s, but name_err_mesg_to_str() issue survived previous security fix.

Affected versions

  • all Ruby 1.8.7 prior to patchlevel 371
  • all Ruby 1.9.3 prior to patchlevel 286
  • all development branches of Ruby 2.0 prior to revision r37068


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This vulnerability was found by Tyler Hickes.


  • Originally published at 2012-10-12 19:17:50 JST.