- Fix CVE-2026-42923, Degradation of service with unbounded NSEC3

hash calculations. Thanks to Qifan Zhang, Palo Alto Networks, for
  the report.

doc/Changelog

@@ -16,6 +16,9 @@
 	- Fix CVE-2026-42534, Jostle logic bypass degrades resolution
 	  performance. Thanks to Qifan Zhang, Palo Alto Networks, for the
 	  report.
+	- Fix CVE-2026-42923, Degradation of service with unbounded NSEC3
+	  hash calculations. Thanks to Qifan Zhang, Palo Alto Networks, for
+	  the report.
 
 23 April 2026: Wouter
 	- Merge #1441: Fix buffer overrun in

validator/val_neg.c

@@ -62,6 +62,13 @@
 #include "sldns/rrdef.h"
 #include "sldns/sbuffer.h"
 
+/**
+ * The maximum salt length that the negative cache is willing to use.
+ * Larger salt increases the computation time, while recommendations are
+ * for zero salt length for zones.
+ */
+#define MAX_SALT_LENGTH 64
+
 int val_neg_data_compare(const void* a, const void* b)
 {
 	struct val_neg_data* x = (struct val_neg_data*)a;
@@ -826,7 +833,11 @@ void neg_insert_data(struct val_neg_cache* neg,
 			(slen != 0 && zone->nsec3_salt && s
 			  && memcmp(zone->nsec3_salt, s, slen) != 0))) {
 
-			if(slen > 0) {
+			if(slen > MAX_SALT_LENGTH) {
+				/* RFC 9276 s3.1: operators SHOULD NOT use a salt; large
+				 * salts inflate per-hash block count. Decline to cache. */
+				return;
+			} else if(slen > 0) {
 				uint8_t* sa = memdup(s, slen);
 				if(sa) {
 					free(zone->nsec3_salt);
@@ -1165,6 +1176,15 @@ neg_find_nsec3_ce(struct val_neg_zone* zone, uint8_t* qname, size_t qname_len,
 	uint8_t hashce[NSEC3_SHA_LEN];
 	uint8_t b32[257];
 	size_t celen, b32len;
+	int hashmax = MAX_NSEC3_CALCULATIONS;
+	if(qlabs > hashmax) {
+		/* strip leading labels so the walk costs at most
+		 * MAX_NSEC3_CALCULATIONS hashes, mirroring val_nsec3.c */
+		while(qlabs > hashmax) {
+			dname_remove_label(&qname, &qname_len);
+			qlabs--;
+		}
+	}
 
 	*nclen = 0;
 	while(qlabs > 0) {
@@ -1265,6 +1285,12 @@ neg_nsec3_proof_ds(struct val_neg_zone* zone, uint8_t* qname, size_t qname_len,
 	if(!zone->nsec3_hash) 
 		return NULL; /* not nsec3 zone */
 
+	if(!topname && qlabs > zone->labs + 1)
+		return NULL; /* iterator caller; opt-out proof would be discarded
+			     * at the !topname check below anyway.
+			     * The qlabs check allows the exact-match for
+			     * the one-label-below-zone case. */
+
 	if(!(data=neg_find_nsec3_ce(zone, qname, qname_len, qlabs, buf,
 		hashnc, &nclen))) {
 		return NULL;

validator/val_nsec3.c

@@ -59,11 +59,6 @@
 #include "sldns/sbuffer.h"
 #include "util/config_file.h"
 
-/**
- * Max number of NSEC3 calculations at once, suspend query for later.
- * 8 is low enough and allows for cases where multiple proofs are needed.
- */
-#define MAX_NSEC3_CALCULATIONS 8
 /**
  * When all allowed NSEC3 calculations at once resulted in error treat as
  * bogus. NSEC3 hash errors are not cached and this helps breaks loops with

validator/val_nsec3.h

@@ -98,6 +98,12 @@ struct sldns_buffer;
 /** The SHA1 hash algorithm for NSEC3 */
 #define NSEC3_HASH_SHA1	0x01
 
+/**
+ * Max number of NSEC3 calculations at once, suspend query for later.
+ * 8 is low enough and allows for cases where multiple proofs are needed.
+ */
+#define MAX_NSEC3_CALCULATIONS 8
+
 /**
 * Cache table for NSEC3 hashes.
 * It keeps a *pointer* to the region its items are allocated.