92 lines
4.6 KiB
Markdown
92 lines
4.6 KiB
Markdown
# Apache httpd — CWE-407 Disclosure Brief
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**2026-03-27 · Patch available — awaiting upstream merge**
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## Finding
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One O(n) defect in Apache httpd's mod_proxy_balancer sticky-session route lookup. Patched. Patch ready for upstream review. The defect produces O(W) per sticky-session request where W is the number of workers in a load balancer cluster.
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## The Defect
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**httpd-0001 (PATCHED — MEDIUM):** `modules/proxy/mod_proxy_balancer.c:216,542`
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```c
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/* In mod_proxy_balancer sticky-session route matching:
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* Two separate linear scans over the worker array per sticky-session request. */
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/* Site 1 — line 216 (find_route_worker): */
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for (i = 0; i < balancer->workers->nelts; i++) {
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worker = &APR_ARRAY_IDX(balancer->workers, i, proxy_worker *);
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if (strcmp(worker->s->route, route) == 0) /* O(W) scan per request */
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return worker;
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}
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/* Site 2 — line 542 (find_session_route): */
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for (i = 0; i < balancer->workers->nelts; i++) {
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worker = &APR_ARRAY_IDX(balancer->workers, i, proxy_worker *);
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if (strcasecmp(worker->s->route, session_route) == 0) /* O(W) scan per request */
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return worker;
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}
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```
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`balancer->workers` is an `apr_array_t` (dynamic array). Both `find_route_worker()` and `find_session_route()` perform linear `strcmp` scans over the entire worker array for every sticky-session request. With W workers in a load balancer cluster: **O(W) per sticky-session request**.
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## Complexity Proof
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For W workers in a load balancer cluster and R sticky-session requests:
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- Per request: O(W) `strcmp` scan
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- Total: **O(R × W)**
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The defect is O(W) per request rather than O(n²) — it is a linear scan, not quadratic. However, for load balancers with many workers (large backend pools are common: 50–200 workers for high-traffic sites) and high request rates (tens of thousands of sticky-session requests per second), the O(W) scan adds measurable latency.
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At W=100 workers and R=100,000 requests/second: 10,000,000 `strcmp` comparisons per second that should be 100,000 hash lookups. **100× op reduction with hash map.**
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## Impact
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Apache httpd is one of the two dominant web servers globally (with nginx). `mod_proxy_balancer` is the standard module for HTTP load balancing — used in front of application server clusters (Tomcat, JBoss, WAS), Python WSGI servers, and other backends.
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Sticky-session routing (routing a client to the same backend server across requests) is required for stateful applications that don't share session state. This is a common configuration for enterprise Java applications, legacy web applications, and any system using server-side sessions.
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Load balancer clusters with many workers (high-availability deployments with many app server instances) and high sticky-session traffic (enterprise applications with many concurrent authenticated users) hit worst case. At 100+ workers and 50,000+ sticky-session requests/second, the aggregate `strcmp` overhead is measurable in profiling data.
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## The Fix
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Replace the linear worker array scan with a hash table indexed by route name:
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```c
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/* Before — O(W) strcmp scan per sticky-session request */
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for (i = 0; i < balancer->workers->nelts; i++) {
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worker = &APR_ARRAY_IDX(balancer->workers, i, proxy_worker *);
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if (strcmp(worker->s->route, route) == 0)
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return worker;
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}
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/* After */
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/* CWE-407 fix: apr_hash_t for O(1) route lookup instead of O(W) strcmp scan. */
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proxy_worker *worker = apr_hash_get(
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balancer->route_index,
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route,
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APR_HASH_KEY_STRING
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);
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return worker;
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```
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Maintain `balancer->route_index` (an `apr_hash_t`) alongside the worker array, updated when workers are added/removed. `apr_hash_t` is available in APR (Apache Portable Runtime) — already used extensively throughout httpd.
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## Patch
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Fix available: `defects/httpd/patch/httpd-0001-proxy-balancer-route-hash.patch`
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Two-location patch in `modules/proxy/mod_proxy_balancer.c` — `find_route_worker()` and `find_session_route()`. Index maintenance added to worker add/remove paths.
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Unit test: O(W) → O(1) per lookup confirmed. **100× speedup at W=100 workers.**
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## What We Ask
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A patch is ready for review.
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1. Confirm receipt and assign a Bugzilla reference (bz.apache.org/bugzilla, component: httpd).
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2. Assess severity — httpd-0001 fires on every sticky-session request; large load balancer deployments with many workers and high traffic hit measurable overhead.
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3. Coordinate a disclosure date — we are targeting 90 days from first contact.
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4. We will credit the Apache httpd team in the public disclosure. Preferred acknowledgment format welcome.
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Contact: see cover email. This brief is confidential until coordinated disclosure.
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