diff --git a/CHANGES b/CHANGES
index 8ae835854..e662c60de 100644
--- a/CHANGES
+++ b/CHANGES
@@ -14,6 +14,12 @@ Changes with FreeUnit 1.35.4                                     xx xxx 2026
     *) Bugfix: replace removed cgi module with email.parser in Python upload
        test fixture; fixes test suite compatibility with Python 3.13.
 
+    *) Bugfix: harden HTTP/URI parser and string helper bounds — proxy
+       response Content-Length underflow guarded in nxt_h1proto.c, oversized
+       upstream Content-Length values are now logged and flagged inconsistent
+       in nxt_http_proxy.c, nxt_is_complex_uri_encoded() off-by-one fixed for
+       trailing "%" sequences, and nxt_rmemstrn() rejects length > buffer.
+
 Changes with FreeUnit 1.35.3                                     07 Apr 2026
 
     *) Feature: migrate contrib package mirror from packages.nginx.org to
diff --git a/src/nxt_h1proto.c b/src/nxt_h1proto.c
index dda3900e3..331368faf 100644
--- a/src/nxt_h1proto.c
+++ b/src/nxt_h1proto.c
@@ -2880,11 +2880,60 @@ nxt_h1p_peer_body_process(nxt_task_t *task, nxt_http_peer_t *peer,
 
     } else if (h1p->remainder > 0) {
         length = nxt_buf_chain_length(out);
-        h1p->remainder -= length;
 
-        if (h1p->remainder == 0) {
+        /*
+         * Cast to nxt_off_t can wrap to negative on 64-bit if length
+         * exceeds NXT_OFF_T_MAX; check that explicitly as well as the
+         * overrun condition.
+         */
+        if (nxt_slow_path((nxt_off_t) length < 0
+                          || (nxt_off_t) length > h1p->remainder))
+        {
+            nxt_buf_t  *b;
+            size_t     trimmed;
+
+            /*
+             * Upstream sent more body bytes than its Content-Length
+             * declared.  Truncate the buf chain to remainder bytes so
+             * we never forward the excess past the Content-Length we
+             * already advertised downstream, then flag inconsistent
+             * and close.
+             */
+            nxt_log(task, NXT_LOG_WARN,
+                    "upstream sent %uz body bytes past Content-Length "
+                    "(remainder %O)", length, h1p->remainder);
+
+            trimmed = 0;
+            for (b = out; b != NULL; b = b->next) {
+                size_t  bsz;
+
+                if (nxt_buf_is_sync(b)) {
+                    continue;
+                }
+                bsz = b->mem.free - b->mem.pos;
+                if (trimmed + bsz <= (size_t) h1p->remainder) {
+                    trimmed += bsz;
+                    continue;
+                }
+                /* Trim this buf to fit, drop everything after it. */
+                b->mem.free = b->mem.pos
+                              + (size_t) h1p->remainder - trimmed;
+                b->next = NULL;
+                break;
+            }
+
+            peer->request->inconsistent = 1;
+            h1p->remainder = 0;
             nxt_buf_chain_add(&out, nxt_http_buf_last(peer->request));
             peer->closed = 1;
+
+        } else {
+            h1p->remainder -= length;
+
+            if (h1p->remainder == 0) {
+                nxt_buf_chain_add(&out, nxt_http_buf_last(peer->request));
+                peer->closed = 1;
+            }
         }
     }
 
diff --git a/src/nxt_http_proxy.c b/src/nxt_http_proxy.c
index 7f6ad6866..6ea87dcc1 100644
--- a/src/nxt_http_proxy.c
+++ b/src/nxt_http_proxy.c
@@ -418,6 +418,20 @@ nxt_http_proxy_content_length(void *ctx, nxt_http_field_t *field,
 
     if (nxt_fast_path(n >= 0)) {
         r->resp.content_length_n = n;
+
+    } else {
+        /*
+         * n == -2 means the upstream Content-Length value overflows
+         * nxt_off_t; n == -1 is a generic parse error.  Both are
+         * inconsistent with a usable response body length, so log and
+         * mark the response inconsistent rather than silently leaving
+         * content_length_n at -1.
+         */
+        nxt_log(&r->task, NXT_LOG_WARN,
+                "upstream Content-Length \"%*s\" is invalid",
+                (size_t) field->value_length, field->value);
+
+        r->inconsistent = 1;
     }
 
     return NXT_OK;
diff --git a/src/nxt_string.c b/src/nxt_string.c
index a23ee058f..551ba72b9 100644
--- a/src/nxt_string.c
+++ b/src/nxt_string.c
@@ -316,6 +316,10 @@ nxt_rmemstrn(const u_char *s, const u_char *end, const char *ss, size_t length)
     u_char        c1, c2;
     const u_char  *s1, *s2;
 
+    if (nxt_slow_path(length == 0 || length > (size_t) (end - s))) {
+        return NULL;
+    }
+
     s1 = end - length;
     s2 = (u_char *) ss;
     c2 = *s2++;
@@ -715,7 +719,12 @@ nxt_is_complex_uri_encoded(u_char *src, size_t length)
 
         if (nxt_uri_escape[ch >> 5] & (1U << (ch & 0x1f))) {
             if (ch == '%') {
-                if (end - src < 2) {
+                /*
+                 * The two pre-increments below read src+1 and src+2,
+                 * so at least three bytes ('%' plus two hex digits)
+                 * must remain in the buffer.
+                 */
+                if (end - src < 3) {
                     return 0;
                 }