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From 4fe6744a220eddd3f1749b40cac3dfc510787de6 Mon Sep 17 00:00:00 2001
From: Simon Kelley <simon@thekelleys.org.uk>
Date: Fri, 19 Jan 2018 12:26:08 +0000
Subject: [PATCH] DNSSEC fix for wildcard NSEC records. CVE-2017-15107
applies.
It's OK for NSEC records to be expanded from wildcards,
but in that case, the proof of non-existence is only valid
starting at the wildcard name, *.<domain> NOT the name expanded
from the wildcard. Without this check it's possible for an
attacker to craft an NSEC which wrongly proves non-existence
in a domain which includes a wildcard for NSEC.
---
src/dnssec.c | 117 +++++++++++++++++++++++++++++++++++++++++++++++++++-------
2 files changed, 114 insertions(+), 15 deletions(-)
--- a/src/dnssec.c
+++ b/src/dnssec.c
@@ -424,15 +424,17 @@ static void from_wire(char *name)
static int count_labels(char *name)
{
int i;
-
+ char *p;
+
if (*name == 0)
return 0;
- for (i = 0; *name; name++)
- if (*name == '.')
+ for (p = name, i = 0; *p; p++)
+ if (*p == '.')
i++;
- return i+1;
+ /* Don't count empty first label. */
+ return *name == '.' ? i : i+1;
}
/* Implement RFC1982 wrapped compare for 32-bit numbers */
@@ -1412,8 +1414,8 @@ static int hostname_cmp(const char *a, c
}
}
-static int prove_non_existence_nsec(struct dns_header *header, size_t plen, unsigned char **nsecs, int nsec_count,
- char *workspace1, char *workspace2, char *name, int type, int *nons)
+static int prove_non_existence_nsec(struct dns_header *header, size_t plen, unsigned char **nsecs, unsigned char **labels, int nsec_count,
+ char *workspace1_in, char *workspace2, char *name, int type, int *nons)
{
int i, rc, rdlen;
unsigned char *p, *psave;
@@ -1426,6 +1428,9 @@ static int prove_non_existence_nsec(stru
/* Find NSEC record that proves name doesn't exist */
for (i = 0; i < nsec_count; i++)
{
+ char *workspace1 = workspace1_in;
+ int sig_labels, name_labels;
+
p = nsecs[i];
if (!extract_name(header, plen, &p, workspace1, 1, 10))
return 0;
@@ -1434,7 +1439,27 @@ static int prove_non_existence_nsec(stru
psave = p;
if (!extract_name(header, plen, &p, workspace2, 1, 10))
return 0;
-
+
+ /* If NSEC comes from wildcard expansion, use original wildcard
+ as name for computation. */
+ sig_labels = *labels[i];
+ name_labels = count_labels(workspace1);
+
+ if (sig_labels < name_labels)
+ {
+ int k;
+ for (k = name_labels - sig_labels; k != 0; k--)
+ {
+ while (*workspace1 != '.' && *workspace1 != 0)
+ workspace1++;
+ if (k != 1 && *workspace1 == '.')
+ workspace1++;
+ }
+
+ workspace1--;
+ *workspace1 = '*';
+ }
+
rc = hostname_cmp(workspace1, name);
if (rc == 0)
@@ -1832,24 +1857,26 @@ static int prove_non_existence_nsec3(str
static int prove_non_existence(struct dns_header *header, size_t plen, char *keyname, char *name, int qtype, int qclass, char *wildname, int *nons)
{
- static unsigned char **nsecset = NULL;
- static int nsecset_sz = 0;
+ static unsigned char **nsecset = NULL, **rrsig_labels = NULL;
+ static int nsecset_sz = 0, rrsig_labels_sz = 0;
int type_found = 0;
- unsigned char *p = skip_questions(header, plen);
+ unsigned char *auth_start, *p = skip_questions(header, plen);
int type, class, rdlen, i, nsecs_found;
/* Move to NS section */
if (!p || !(p = skip_section(p, ntohs(header->ancount), header, plen)))
return 0;
+
+ auth_start = p;
for (nsecs_found = 0, i = ntohs(header->nscount); i != 0; i--)
{
unsigned char *pstart = p;
- if (!(p = skip_name(p, header, plen, 10)))
+ if (!extract_name(header, plen, &p, daemon->workspacename, 1, 10))
return 0;
-
+
GETSHORT(type, p);
GETSHORT(class, p);
p += 4; /* TTL */
@@ -1866,7 +1893,69 @@ static int prove_non_existence(struct dn
if (!expand_workspace(&nsecset, &nsecset_sz, nsecs_found))
return 0;
- nsecset[nsecs_found++] = pstart;
+ if (type == T_NSEC)
+ {
+ /* If we're looking for NSECs, find the corresponding SIGs, to
+ extract the labels value, which we need in case the NSECs
+ are the result of wildcard expansion.
+ Note that the NSEC may not have been validated yet
+ so if there are multiple SIGs, make sure the label value
+ is the same in all, to avoid be duped by a rogue one.
+ If there are no SIGs, that's an error */
+ unsigned char *p1 = auth_start;
+ int res, j, rdlen1, type1, class1;
+
+ if (!expand_workspace(&rrsig_labels, &rrsig_labels_sz, nsecs_found))
+ return 0;
+
+ rrsig_labels[nsecs_found] = NULL;
+
+ for (j = ntohs(header->nscount); j != 0; j--)
+ {
+ if (!(res = extract_name(header, plen, &p1, daemon->workspacename, 0, 10)))
+ return 0;
+
+ GETSHORT(type1, p1);
+ GETSHORT(class1, p1);
+ p1 += 4; /* TTL */
+ GETSHORT(rdlen1, p1);
+
+ if (!CHECK_LEN(header, p1, plen, rdlen1))
+ return 0;
+
+ if (res == 1 && class1 == qclass && type1 == T_RRSIG)
+ {
+ int type_covered;
+ unsigned char *psav = p1;
+
+ if (rdlen1 < 18)
+ return 0; /* bad packet */
+
+ GETSHORT(type_covered, p1);
+
+ if (type_covered == T_NSEC)
+ {
+ p1++; /* algo */
+
+ /* labels field must be the same in every SIG we find. */
+ if (!rrsig_labels[nsecs_found])
+ rrsig_labels[nsecs_found] = p1;
+ else if (*rrsig_labels[nsecs_found] != *p1) /* algo */
+ return 0;
+ }
+ p1 = psav;
+ }
+
+ if (!ADD_RDLEN(header, p1, plen, rdlen1))
+ return 0;
+ }
+
+ /* Must have found at least one sig. */
+ if (!rrsig_labels[nsecs_found])
+ return 0;
+ }
+
+ nsecset[nsecs_found++] = pstart;
}
if (!ADD_RDLEN(header, p, plen, rdlen))
@@ -1874,7 +1963,7 @@ static int prove_non_existence(struct dn
}
if (type_found == T_NSEC)
- return prove_non_existence_nsec(header, plen, nsecset, nsecs_found, daemon->workspacename, keyname, name, qtype, nons);
+ return prove_non_existence_nsec(header, plen, nsecset, rrsig_labels, nsecs_found, daemon->workspacename, keyname, name, qtype, nons);
else if (type_found == T_NSEC3)
return prove_non_existence_nsec3(header, plen, nsecset, nsecs_found, daemon->workspacename, keyname, name, qtype, wildname, nons);
else
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