Date: Wed, 24 Sep 2014 15:12:08 -0400 From: Chet Ramey <chet.ramey@...e.edu> To: Michal Zalewski <lcamtuf@...edump.cx>, oss-security@...ts.openwall.com CC: chet.ramey@...e.edu Subject: Re: CVE-2014-6271: remote code execution through bash On 9/24/14, 2:54 PM, Michal Zalewski wrote: >> My main concern with the current patch is that still exposes the bash parser >> and function definition printer to attacks from the network. Bugs in those >> fairly large components could cause another critical issue. > > Yup, that surprised me when testing the patch, too - I can still get a > function called HTTP_COOKIE, for example. I worry about potential side > effects of parsing even in absence of parser bugs. In most > object-oriented languages, such side effects are practically > guaranteed. Bash may be saved by simplicity, but not sure how robust > that assumption is. Lots of code out there uses exported functions. > I've written more code in bash than I should have and never used > function exports, or even realized that they exist. I wonder if they > can be made optional (e.g., gated by a flag on the subprocess) without > breakage. > > Another option may be to export them through specially prefixed > variables, which should be transparent but minimize the risk of > interfering with web servers and such. There are several options for making shell functions inherited via the environment more robust, none of them backwards compatible. I will choose one and implement it for a future bash version. The leading candidates both raise the bar by requiring a potential attacker to be able to create arbitrarily-named environment variables as well as environment variables with specific values. I considered (and implemented) a blacklist approach that would have protected against a set of commonly-named variables (HTTP_*, CGI_*, SSH_*, LC_*, and so on), but the consensus was that that was too easily circumvented. I removed it from the distributed patches. Chet -- ``The lyf so short, the craft so long to lerne.'' - Chaucer ``Ars longa, vita brevis'' - Hippocrates Chet Ramey, ITS, CWRU chet@...e.edu http://cnswww.cns.cwru.edu/~chet/
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