From f438804c181e8652df4f3428f90011909d3d51e0 Mon Sep 17 00:00:00 2001
From: Christian Grothoff <christian@grothoff.org>
Date: Tue, 28 Jul 2026 10:55:44 +0200
Subject: [PATCH] do not abort on nonce slot collisions with different
 algorithms

---
 src/microhttpd/digestauth.c | 29 +++++++++++++++++++++--------
 1 file changed, 21 insertions(+), 8 deletions(-)

diff --git a/src/microhttpd/digestauth.c b/src/microhttpd/digestauth.c
index 09eb49c6..38f51fc4 100644
--- a/src/microhttpd/digestauth.c
+++ b/src/microhttpd/digestauth.c
@@ -857,19 +857,32 @@ check_nonce_nc (struct MHD_Connection *connection,
 
   MHD_mutex_lock_chk_ (&daemon->nnc_lock);
 
-  mhd_assert (0 == nn->nonce[noncelen]); /* The old value must be valid */
-
-  if ( (0 != memcmp (nn->nonce, nonce, noncelen)) ||
-       (0 != nn->nonce[noncelen]) )
-  { /* The nonce in the slot does not match nonce from the client */
+  /* The nonces recorded in the array do not all have the same length: the
+   * daemon may serve several digest algorithms at the same time and the
+   * length of the nonce depends on the size of the digest (44 characters
+   * for MD5, 76 for SHA-256 and SHA-512/256).  The slot selected by the
+   * hash of the client's nonce may therefore hold a nonce of a completely
+   * different length, which must not be inspected with the client's
+   * length. */
+  if (strlen (nn->nonce) != noncelen)
+  {
     if (0 == nn->nonce[0])
     { /* The slot was never used, while the client's nonce value should be
        * recorded when it was generated by MHD */
       ret = MHD_CHECK_NONCENC_WRONG;
     }
-    else if (0 != nn->nonce[noncelen])
-    { /* The value is the slot is wrong */
-      ret =  MHD_CHECK_NONCENC_STALE;
+    else
+    { /* The slot holds a nonce generated for another digest algorithm.
+       * The client's nonce is not (or no longer) recorded. */
+      ret = MHD_CHECK_NONCENC_STALE;
+    }
+  }
+  else if (0 != memcmp (nn->nonce, nonce, noncelen))
+  { /* The nonce in the slot does not match nonce from the client */
+    if (0 == nn->nonce[0])
+    { /* The slot was never used, while the client's nonce value should be
+       * recorded when it was generated by MHD */
+      ret = MHD_CHECK_NONCENC_WRONG;
     }
     else
     {