commit de76f21e115c451cf0653790fc4b209cd4778a07
Author: Simon Kelley <simon@thekelleys.org.uk>
Date:   Fri Apr 10 22:16:45 2026 +0100

    Fix NSEC bitmap parsing infinite loop. CVE-2026-4890
    
    Report from Royce M <royce@xchglabs.com>.
    
    Location: dnssec.c:1290-1306, dnssec.c:1450-1463
    
    The bitmap window iteration advances by p[1] instead of p[1]+2 (missing the 2-byte window header). With bitmap_length=0, both rdlen and p are
    unchanged, causing an infinite loop and dnsmasq stops responding to all queries.
    
    The same code accesses p[2] after only checking rdlen >= 2 without verifying p[1] >= 1, causing OOB reads at 6 locations.
    
    Both bugs are reachable before RRSIG validation (confirmed by the source comment at line 2125), so no valid DNSSEC signatures are needed.

diff --git a/src/dnssec.c b/src/dnssec.c
index 415c4e5..8672444 100644
--- a/src/dnssec.c
+++ b/src/dnssec.c
@@ -1274,10 +1274,10 @@ static int prove_non_existence_nsec(struct dns_header *header, size_t plen, unsi
 	     packet checked to be as long as rdlen implies in prove_non_existence() */
 	  
 	  /* If we can prove that there's no NS record, return that information. */
-	  if (nons && rdlen >= 2 && p[0] == 0 && (p[2] & (0x80 >> T_NS)) != 0)
+	  if (nons && rdlen >= 2 && p[0] == 0 && p[1] >= 1  && (p[2] & (0x80 >> T_NS)) != 0)
 	    *nons = 0;
 	  
-	  if (rdlen >= 2 && p[0] == 0)
+	  if (rdlen >= 2 && p[0] == 0 && p[1] >= 1)
 	    {
 	      /* A CNAME answer would also be valid, so if there's a CNAME is should 
 		 have been returned. */
@@ -1305,8 +1305,8 @@ static int prove_non_existence_nsec(struct dns_header *header, size_t plen, unsi
 		  break; /* finished checking */
 		}
 	      
-	      rdlen -= p[1];
-	      p +=  p[1];
+	      rdlen -= p[1] + 2;
+	      p +=  p[1] + 2;
 	    }
 	  
 	  return 0;
@@ -1433,7 +1433,7 @@ static int check_nsec3_coverage(struct dns_header *header, size_t plen, int dige
 		p += hash_len; /* skip next-domain hash */
 		rdlen -= p - psave;
 
-		if (rdlen >= 2 && p[0] == 0)
+		if (rdlen >= 2 && p[0] == 0 && p[1] >= 1)
 		  {
 		    /* If we can prove that there's no NS record, return that information. */
 		    if (nons && (p[2] & (0x80 >> T_NS)) != 0)
@@ -1462,8 +1462,8 @@ static int check_nsec3_coverage(struct dns_header *header, size_t plen, int dige
 			break; /* finished checking */
 		      }
 		    
-		    rdlen -= p[1];
-		    p +=  p[1];
+		    rdlen -= p[1] + 2;
+		    p +=  p[1] + 2;
 		  }
 		
 		return 1;