Varnish Cache — CWE-407 Disclosure Brief

2026-03-27 · Patch available — awaiting upstream merge

Finding

One algorithmic complexity defect in Varnish Cache's ban subsystem. BAN_CheckObject() performs an O(B) walk of the entire ban list on every cache object lookup, including expired and already-matched bans that have not yet been reaped. Fix: maintain a pre-filtered active-ban set. Patched.

The Defects

varnish-0001 (PATCHED — HIGH): bin/varnishd/cache/cache_ban.c

/* BAN_CheckObject() — called per cache object on every request that hits cache: */
VTAILQ_FOREACH(b, &ban_head, list) {
    if (b->flags & BANS_FLAG_COMPLETED)
        continue;               /* skip reaped bans — but still iterate */
    if (ban_evaluate(wrk, b, oc, vsl)) {
        /* object banned */
        break;
    }
}
/* O(B) full ban list walk per object per request */

BAN_CheckObject() iterates the full ban list including completed (reaped) entries on every request that results in a cache hit. For B total bans (including expired) and R cache-hit requests per second: O(R × B). Fix: maintain a separate active-ban list or bitset containing only non-completed bans, reducing per-request work to O(A) where A = active (non-completed) bans ≪ B.

Complexity Proof

Let B = total ban list length (active + completed), A = active (non-completed) bans, R = cache-hit requests per second.

Defective: VTAILQ_FOREACH iterates all B entries; each completed entry incurs a flag check but still costs a pointer dereference and branch.
- Total per second: O(R × B) iterations.
- B grows unboundedly between ban reaper cycles. The reaper runs periodically but not per-request; in deployments with many bans and infrequent reaping, B ≫ A.
Fixed: pre-filtered active-ban list (or generation-indexed active set).
- Per-request work: O(A) — only non-completed bans.
- When A ≪ B (the common case after bans expire but before the reaper runs): speedup = B / A.

At B=500 total bans with A=50 active (90% completed but not yet reaped): 10× reduction per request. In long-running deployments with burst ban activity followed by idle periods, B/A ratios of 50× or more are routine.

Additionally, the VTAILQ_FOREACH list traversal has poor cache locality compared to a compact array or generation-indexed structure, amplifying the per-entry cost at large B.

Impact

All Varnish Cache deployments that use ban-based cache invalidation (ban VCL statement or the HTTP PURGE/BAN interface). Ban-based invalidation is the standard cache invalidation mechanism in Varnish — used by virtually every production Varnish deployment with dynamic content. The defect is worst in deployments with high ban rates (CDN edge nodes, news sites, e-commerce) where B accumulates between reaper cycles.

Varnish Cache is the dominant HTTP accelerator and reverse proxy cache for high-traffic media and e-commerce sites.

The Fix

Maintain a separate ban_active_head list containing only non-completed bans. When a ban is marked completed, remove it from ban_active_head (O(1) list removal). BAN_CheckObject() iterates ban_active_head instead of ban_head:

/* Before */
VTAILQ_FOREACH(b, &ban_head, list) {
    if (b->flags & BANS_FLAG_COMPLETED)
        continue;
    if (ban_evaluate(wrk, b, oc, vsl)) { break; }
}

/* After */
/* CWE-407 fix: pre-filtered active-ban list for O(A) walk instead of O(B) full scan. */
VTAILQ_FOREACH(b, &ban_active_head, active_list) {
    /* all entries here are non-completed — no flag check needed */
    if (ban_evaluate(wrk, b, oc, vsl)) { break; }
}
/* When ban is completed: VTAILQ_REMOVE(&ban_active_head, b, active_list); */

Patch

defects/varnish/patch/varnish-0001-ban-active-list.patch

What We Ask

Confirm receipt and assign a security advisory reference.
Validate the patch against the ban subsystem test suite (including ban_lurker reaper interactions).
Assess CVE eligibility — varnish-0001 fires on every cache-hit request in deployments using ban invalidation.
Coordinate a disclosure date — we are targeting 90 days from first contact.

Contact: see cover email. This brief is confidential until coordinated disclosure.

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