BSD compress implemented an LZW compressor and decompressor. This decompressor implementation did not correctly handle compressed streams that contain code words that were not yet added to the decompression table. LZW decompression has a special case (a KwKwK string) when code word may match the first free entry in the decompression table. The implementation used in BSD compress allow code words not only matching, but also exceeding the first free entry.

It seems this compress implementation first appeared in BSD around 1985, and was later used in various other code base, such as ncompress and gzip. Other components that contain affected code will be listed below. Following page list the version of the code as was used in 4.3BSD:

http://minnie.tuhs.org/cgi-bin/utree.pl?file=4.3BSD-Reno/src/usr.bin/compress/compress.c

Relevant code appears in the decompress() routine:

/ Special case for KwKwK string. / if ( code >= freeent ) { stackp++ = finchar; code = oldcode; }

This allows creating a loop in the decompression table, which leads to an "infinite" loop:

/ Generate output characters in reverse order / #ifdef SIGNEDCOMPARESLOW while ( ((unsigned long)code) >= ((unsigned long)256) ) { #else while ( code >= 256 ) { #endif stackp++ = tabsuffixof(code); code = tabprefixof(code); }

where tabprefixof is:

unsigned short codetab [HSIZE]; #define codetabof(i) codetab[i] #define tabprefixof(i) codetabof(i)

This overflows destack "buffer" (part of the htab[]):

countint htab [HSIZE]; # define tabsuffixof(i) ((chartype )(htab))[i] # define destack ((chartype )&tabsuffixof(1<<BITS))

Depending on the relative htab[] and codetab[] positions, destack overflow may overwrite codetab[] entries, which may break infinite loop and let program continue its execution with possibly corrupted memory.

Array index error in the maketable function in unlzh.c in the LZH decompression component in gzip 1.3.5, when running on certain platforms, allows context-dependent attackers to cause a denial of service (crash) and possibly execute arbitrary code via a crafted GZIP archive that triggers an out-of-bounds write, aka a "stack modification vulnerability."

unlzh.c in the LHZ component in gzip 1.3.5 allows context-dependent attackers to cause a denial of service (infinite loop) via a crafted GZIP archive.

Buffer overflow in the maketable function in the LHZ component in gzip 1.3.5 allows context-dependent attackers to execute arbitrary code via a crafted decoding table in a GZIP archive.

Buffer underflow in the buildtree function in unpack.c in gzip 1.3.5 allows context-dependent attackers to execute arbitrary code via a crafted leaf count table that causes a write to a negative index.

Flaw bug created to hold information about an old flaw we knew something about. For more details see the MITRE CVE description.