diff --git a/ChangeLog b/ChangeLog
index 7bb1727..7845080 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -18,6 +18,8 @@
 Bugfix
    * Fix potential undefined behaviour in Camellia.
    * Fix memory leaks in PKCS#5 and PKCS#12.
+   * Stack buffer overflow if ctr_drbg_update() is called with too large
+     add_len (found by Jean-Philippe Aumasson) (not triggerable remotely).
 
 Changes
    * Blind RSA private operations even when POLARSSL_RSA_NO_CRT is defined.
diff --git a/include/polarssl/ctr_drbg.h b/include/polarssl/ctr_drbg.h
index d5459d9..a2f1a61 100644
--- a/include/polarssl/ctr_drbg.h
+++ b/include/polarssl/ctr_drbg.h
@@ -154,6 +154,10 @@
  * \param ctx           CTR_DRBG context
  * \param additional    Additional data to update state with
  * \param add_len       Length of additional data
+ *
+ * \note                If add_len is greater than CTR_DRBG_MAX_SEED_INPUT,
+ *                      only the first CTR_DRBG_MAX_SEED_INPUT bytes are used,
+ *                      the remaining ones are silently discarded.
  */
 void ctr_drbg_update( ctr_drbg_context *ctx,
                       const unsigned char *additional, size_t add_len );
diff --git a/library/ctr_drbg.c b/library/ctr_drbg.c
index 1b46f45..980d954 100644
--- a/library/ctr_drbg.c
+++ b/library/ctr_drbg.c
@@ -118,6 +118,9 @@
 
     int i, j, buf_len, use_len;
 
+    if( data_len > CTR_DRBG_MAX_SEED_INPUT )
+        return( POLARSSL_ERR_CTR_DRBG_INPUT_TOO_BIG );
+
     memset( buf, 0, CTR_DRBG_MAX_SEED_INPUT + CTR_DRBG_BLOCKSIZE + 16 );
 
     /*
@@ -233,6 +236,11 @@
 
     if( add_len > 0 )
     {
+        /* MAX_INPUT would be more logical here, but we have to match
+         * block_cipher_df()'s limits since we can't propagate errors */
+        if( add_len > CTR_DRBG_MAX_SEED_INPUT )
+            add_len = CTR_DRBG_MAX_SEED_INPUT;
+
         block_cipher_df( add_input, additional, add_len );
         ctr_drbg_update_internal( ctx, add_input );
     }