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mksh R50c released, security fix

03.10.2014 by tg@
Tags: android bug debian mksh news pcli release security

The MirBSD Korn Shell has got a new security and maintenance release.

This release fixes one mksh(1)-specific issue when importing values from the environment. The issue has been detected by the main developer during careful code review, looking at whether the shell is affected by the recent “shellshock” bugs in GNU bash, many of which also affect AT&T ksh93. (The answer is: no, none of these bugs affects mksh.) Stephane Chanzelas kindly provided me with an in-depth look at how this can be exploited. The issue has not got a CVE identifier because it was identified as low-risk. The problem here is that the environment import filter mistakenly accepted variables named “FOO+” (for any FOO), which are, by general environ(7) syntax, distinct from “FOO”, and treated them as appending to the value of “FOO”. An attacker who already had access to the environment could so append values to parameters passed through programs (including sudo(8) or setuid) to shell scripts, including indirectly, after those programs intended to sanitise the environment, e.g. invalidating the last $PATH component. It could also be used to circumvent sudo’s environment filter which protected against the vulnerability of an unpatched GNU bash being exploited.

tl;dr: mksh not affected by any shellshock bugs, but we found a bug of our own, with low impact, which does not affect any other shell, during careful code review. Please do update to mksh R50c quickly.

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