[exim-cvs] SPF: harden against crafted DNS responses

Gitweb: https://git.exim.org/exim.git/commitdiff/4f07f38374f8662c318699fb30432273ffcfe0d3

Commit:     4f07f38374f8662c318699fb30432273ffcfe0d3
Parent:     35916b2f03491ce505f69b15096bfd8d00e08286
Author:     Jeremy Harris <jgh146exb@???>
AuthorDate: Tue Oct 10 12:45:27 2023 +0100
Committer:  Jeremy Harris <jgh146exb@???>
CommitDate: Tue Oct 10 12:45:27 2023 +0100

    SPF: harden against crafted DNS responses
---
 src/src/spf.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/src/src/spf.c b/src/src/spf.c
index ef55520b1..16c840d71 100644
--- a/src/src/spf.c
+++ b/src/src/spf.c
@@ -122,6 +122,7 @@ for (dns_record * rr = dns_next_rr(dnsa, &dnss, RESET_ANSWERS); rr;
     switch(rr_type)
       {
       case T_MX:
+    if (rr->size < 2) continue;
     s += 2;    /* skip the MX precedence field */
       case T_PTR:
     {
@@ -137,6 +138,7 @@ for (dns_record * rr = dns_next_rr(dnsa, &dnss, RESET_ANSWERS); rr;
     gstring * g = NULL;
     uschar chunk_len;

+    if (rr->size < 1+6) continue;        /* min for version str */
     if (strncmpic(rr->data+1, US SPF_VER_STR, 6) != 0)
       {
       HDEBUG(D_host_lookup) debug_printf("not an spf record: %.*s\n",
@@ -144,9 +146,12 @@ for (dns_record * rr = dns_next_rr(dnsa, &dnss, RESET_ANSWERS); rr;
       continue;
       }

-    for (int off = 0; off < rr->size; off += chunk_len)
+    /* require 1 byte for the chunk_len */
+    for (int off = 0; off < rr->size - 1; off += chunk_len)
       {
-      if (!(chunk_len = s[off++])) break;
+      if (  !(chunk_len = s[off++])
+         || rr->size < off + chunk_len    /* ignore bogus size chunks */
+         ) break;
       g = string_catn(g, s+off, chunk_len);
       }
     if (!g)

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