[OTR-users] Security Advisory: upgrade to libotr 4.1.1

[Ian Goldberg]

Off-the-Record Messaging (OTR) Security Advisory 2016-01

Integer overflow vulnerability in libotr

Versions 4.1.0 and earlier of libotr in 64-bit builds contain an integer
overflow security flaw.  This flaw could potentially be exploited by a
remote attacker to cause a heap buffer overflow and subsequently for
arbitrary code to be executed on the user's machine.

In several places in proto.c, the sizes of portions of incoming messages
were stored in variables of type int or unsigned int instead of size_t.
If a message arrives with very large sizes (for example unsigned int
datalen = UINT_MAX), then constructions like malloc(datalen+1) will turn
into malloc(0), which on some architectures returns a non-NULL pointer,
but UINT_MAX bytes will get written to that pointer.

CVE-2016-2851 has been assigned to this issue.

The recommended course of action is to upgrade libotr to version
4.1.1 immediately.  The new version can be obtained here:

Source code:
    https://otr.cypherpunks.ca/libotr-4.1.1.tar.gz
gpg signature:
    https://otr.cypherpunks.ca/libotr-4.1.1.tar.gz.asc

git repository:
    git://git.otr.im/libotr.git (tag 4.1.1)

A new Windows binary of pidgin-otr (4.0.2) linked to libotr 4.1.1 is
available; however, as the Windows version is a 32-bit build, it is not
susceptible to the above flaw.  Windows users may wish to upgrade in any
event in order to take advantages of other fixes in libotr 4.1.1 and
pidgin-otr 4.0.2, however (see the respective NEWS files).

pidgin-otr 4.0.2 Source code:
    https://otr.cypherpunks.ca/pidgin-otr-4.0.2.tar.gz
gpg signature:
    https://otr.cypherpunks.ca/pidgin-otr-4.0.2.tar.gz.asc

Windows installer:
    https://otr.cypherpunks.ca/binaries/windows/pidgin-otr-4.0.2.exe
gpg signature:
    https://otr.cypherpunks.ca/binaries/windows/pidgin-otr-4.0.2.exe.asc

Windows zip file:
    https://otr.cypherpunks.ca/binaries/windows/pidgin-otr-4.0.2.zip
gpg signature:
    https://otr.cypherpunks.ca/binaries/windows/pidgin-otr-4.0.2.zip.asc


Linux and *BSD vendors and package maintainers have been notified, and
updated packages should be available from them.

If upgrading to version 4.1.1 is not possible, please apply the
following patch to 4.1.0:

--- a/src/proto.c
+++ b/src/proto.c
@@ -715,7 +715,7 @@ gcry_error_t otrl_proto_accept_data(char **plaintextp, OtrlTLV **tlvsp,
     unsigned int sender_keyid, recipient_keyid;
     gcry_mpi_t sender_next_y = NULL;
     unsigned char ctr[8];
-    unsigned int datalen, reveallen;
+    size_t datalen, reveallen;
     unsigned char *data = NULL;
     unsigned char *nul = NULL;
     unsigned char givenmac[20];
@@ -916,7 +916,7 @@ OtrlFragmentResult otrl_proto_fragment_accumulate(char **unfragmessagep,
 
     if (k > 0 && n > 0 && k <= n && start > 0 && end > 0 && start < end) {
 	if (k == 1) {
-	    int fraglen = end - start - 1;
+	    size_t fraglen = end - start - 1;
 	    size_t newsize = fraglen + 1;
 	    free(context->context_priv->fragment);
 	    context->context_priv->fragment = NULL;
@@ -937,7 +937,7 @@ OtrlFragmentResult otrl_proto_fragment_accumulate(char **unfragmessagep,
 	    }
 	} else if (n == context->context_priv->fragment_n &&
 		k == context->context_priv->fragment_k + 1) {
-	    int fraglen = end - start - 1;
+	    size_t fraglen = end - start - 1;
 	    char *newfrag = NULL;
 	    size_t newsize = context->context_priv->fragment_len + fraglen + 1;
 	    /* Check for overflow */
@@ -989,10 +989,10 @@ gcry_error_t otrl_proto_fragment_create(int mms, int fragment_count,
 	char ***fragments, ConnContext *context, const char *message)
 {
     char *fragdata;
-    int fragdatalen = 0;
+    size_t fragdatalen = 0;
     int curfrag = 0;
-    int index = 0;
-    int msglen = strlen(message);
+    size_t index = 0;
+    size_t msglen = strlen(message);
     /* Should vary by number of msgs */
     int headerlen = context->protocol_version == 3 ? 37 : 19;
 
@@ -1012,7 +1012,7 @@ gcry_error_t otrl_proto_fragment_create(int mms, int fragment_count,
 	int i;
 	char *fragmentmsg;
 
-	if (msglen - index < mms - headerlen) {
+	if (msglen - index < (size_t)(mms - headerlen)) {
 	    fragdatalen = msglen - index;
 	} else {
 	    fragdatalen = mms - headerlen;
 

Our heartfelt thanks to Markus Vervier of X41 D-Sec GmbH
<markus.vervier at x41-dsec.de> for finding and alerting us to this flaw.

Followups to the otr-users mailing list <otr-users at lists.cypherpunks.ca>,
please.

Thanks,
    Ian Goldberg <iang at cs.uwaterloo.ca>
    for the OTR development team <otr at cypherpunks.ca>