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I was trying to implement the experiment at http://www.cypherspace.org/adam/rsa/link.html, however I am experiencing this bug (it is trivial to replicate):

$ cd ~/public_html
$ mkdir -vp 'print pack"C*",split/\D+/,`echo "16iII*o\U@{$//=$z;[(pop,pop,unpack"H*",<>)]}\EsMsKsN0[lN*1lK[d2%Sa2/d0<X+d*lMLa^*lN%0]dsXx++lMlN/dsM0<J]dsJxp"|dc`;$'
$ curl -sLv curl -sLv 'http://www.ishygddt.xyz/~james/print%20pack%22C*%22,split/%5CD%2b/,%60echo%20%2216iII*o%5CU@%7B$/=%24z;%5B%28pop,pop,unpack%22H*%22,%3C%3E%29%5D%7D%5CEsMsKsN0%5BlN*1lK%5Bd2%25Sa2/' 
*   Trying 2002:4475:be1e:0:ff6e:1661:85f1:eece...
* TCP_NODELAY set
* Connected to www.ishygddt.xyz (2002:4475:be1e:0:ff6e:1661:85f1:eece) port 80 (#0)
> GET /~james/print%20pack%22C*%22,split/%5CD%2b/,%60echo%20%2216iII*o%5CU@%7B$/=%24z;%5B%28pop,pop,unpack%22H*%22,%3C%3E%29%5D%7D%5CEsMsKsN0%5BlN*1lK%5Bd2%25Sa2/ HTTP/1.1
> Host: www.ishygddt.xyz
> User-Agent: curl/7.53.0
> Accept: */*
> 
< HTTP/1.1 200 OK
< Date: Fri, 12 May 2017 22:54:55 GMT
< Server: Hiawatha v10.6
< Connection: keep-alive
< Content-Type: text/html
* Connection #0 to host www.ishygddt.xyz left intact
$ sudo tail -n 1 /var/log/hiawatha/error.log
2002:4475:be1e:0:ff6e:1661:85f1:eece|Fri 12 May 2017 17:54:55 -0500|/home/james/public_html/print pack"C*",split/\D+/,`echo "16iII*o\U@{$/=$z;[(pop,pop,unpack"H*",<>)]}\EsMsKsN0[lN*1lK[d2%Sa2/|data is invalid XML

Solution: don't do this. URLs are not meant to be this way.

It seems that this bug is generally triggered by any directory name containing the less-than character (%3C).

…I thought directory listing pages are supposed to escape special characters, rather than choking on them?