CVE-2016-9840 (published 2017-05-23T04:29Z)

inftrees.c in zlib 1.2.8 might allow context-dependent attackers to have unspecified impact by leveraging improper pointer arithmetic.

Severity: MEDIUM
CVSS v2 base score: 6.8 (AV:N/AC:M/Au:N/C:P/I:P/A:P)
CVSS v3 base score: 8.8, HIGH (CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H)

Weakness information:
cwe_id: NVD-CWE-noinfo
cwe_name: Insufficient Information
description: There is insufficient information about the issue to classify it; details are unkown or unspecified.
extended_description: Insufficient Information

Fix information:
repo_url: https://github.com/madler/zlib
commit_details: repo_url: https://github.com/madler/zlib
author: Mark Adler
author_date: 2016-09-21 23:35:50-07:00
msg: Remove offset pointer optimization in inftrees.c.

inftrees.c was subtracting an offset from a pointer to an array,
in order to provide a pointer that allowed indexing starting at
the offset. This is not compliant with the C standard, for which
the behavior of a pointer decremented before its allocated memory
is undefined. Per the recommendation of a security audit of the
zlib code by Trail of Bits and TrustInSoft, in support of the
Mozilla Foundation, this tiny optimization was removed, in order
to avoid the possibility of undefined behavior.
repository: repo_url: https://github.com/madler/zlib
repo_name: madler/zlib
description: A massively spiffy yet delicately unobtrusive compression library.
repo_language: C
owner: madler
file_changes: filename: inftrees.c
change_type: ModificationType.MODIFY
diff: @@ -54,7 +54,7 @@ unsigned short FAR *work;
     code FAR *next;             /* next available space in table */
     const unsigned short FAR *base;     /* base value table to use */
     const unsigned short FAR *extra;    /* extra bits table to use */
-    int end;                    /* use base and extra for symbol > end */
+    unsigned match;             /* use base and extra for symbol >= match */
     unsigned short count[MAXBITS+1];    /* number of codes of each length */
     unsigned short offs[MAXBITS+1];     /* offsets in table for each length */
     static const unsigned short lbase[31] = { /* Length codes 257..285 base */
@@ -181,19 +181,17 @@ unsigned short FAR *work;
     switch (type) {
     case CODES:
         base = extra = work;    /* dummy value--not used */
-        end = 19;
+        match = 20;
         break;
     case LENS:
         base = lbase;
-        base -= 257;
         extra = lext;
-        extra -= 257;
-        end = 256;
+        match = 257;
         break;
     default:            /* DISTS */
         base = dbase;
         extra = dext;
-        end = -1;
+        match = 0;
     }
 
     /* initialize state for loop */
@@ -216,13 +214,13 @@ unsigned short FAR *work;
     for (;;) {
         /* create table entry */
         here.bits = (unsigned char)(len - drop);
-        if ((int)(work[sym]) < end) {
+        if (work[sym] + 1 < match) {
             here.op = (unsigned char)0;
             here.val = work[sym];
         }
-        else if ((int)(work[sym]) > end) {
-            here.op = (unsigned char)(extra[work[sym]]);
-            here.val = base[work[sym]];
+        else if (work[sym] >= match) {
+            here.op = (unsigned char)(extra[work[sym] - match]);
+            here.val = base[work[sym] - match];
         }
         else {
             here.op = (unsigned char)(32 + 64);         /* end of block */

programming_language: C