CPU Exhaustion in Avro Decoder via Unbounded Block-Count Iteration

Summary

The Avro array and map decoders looped over an attacker-controlled block-count value without checking the underlying reader's error state inside the loop body. Reader.ReadBlockHeader returns the count as a Go int, which is 64-bit on amd64 / arm64 targets — so a producer can declare a block of up to math.MaxInt64 (~9.2 × 10¹⁸) elements followed by EOF (or any truncated payload), and the decoder will attempt that many no-op iterations before propagating the error. The realistic ceiling is "indefinite until the worker is killed externally" — a single hostile payload pins a CPU core until the process is OOM-killed, deadline-cancelled, or terminated. Remote, unauthenticated denial-of-service.

The fix exits the loop on the first inner-decode error. It does not bound the loop length itself; for full coverage on untrusted inputs, also configure Config.MaxSliceAllocSize and Config.MaxMapAllocSize (the latter introduced in v2.33.0).

Description

Avro arrays and maps are encoded as one or more blocks; each block declares an element count followed by that many encoded elements. The decoder reads the block count as a zigzag-encoded long, then iterates that many times calling an inner decoder.

Three iteration sites trusted the block count without checking the reader's accumulated error state between iterations:

• codec_skip.go sliceSkipDecoder.Decode — skip helper for arrays.
• codec_skip.go mapSkipDecoder.Decode — skip helper for maps.
• reader_generic.go Reader.ReadArrayCB and Reader.ReadMapCB — callback-based decoders used by generic and unmarshaling code paths.

Because the inner Decode(nil, r) call is a no-op when r has already errored (it returns immediately without consuming bytes), the loop would run to completion even after the first iteration's EOF. On amd64 / arm64, Reader.ReadBlockHeader returns the count as int (= int64), so the loop bound is whatever the wire payload specified, up to math.MaxInt64. A modest 200-million-count payload (well under 2³¹) already burns several seconds; a math.MaxInt − 2 payload (the value used in the regression test TestDecoder_ArrayMultiBlockExceedsMaxInt from PR #9) effectively pins the goroutine until external kill.

This overlaps with GHSA-mc57-h6j3-3hmv: the same large-block-count payload that drives the unbounded loop here also drives the cumulative-arithmetic overflow there (cross-platform), and on a 32-bit target additionally triggers the union-index / byte-slice narrowing.

Affected components

These are the audited and patched sites. Any other code path that iterates over an attacker-controlled count while calling a Reader-style decoder is structurally susceptible to the same pattern; reviewers of consumer code should grep for for range l / for i := 0; i < int(l); i++ near Reader method calls and confirm an in-loop error check.

Technical details

Vulnerable pattern:

for range l {
    d.decoder.Decode(nil, r)
    // r.Error may have been set by Decode; loop continues regardless.
}

After r.Error != nil, subsequent Decode calls short-circuit and return without consuming bytes or doing useful work, but the loop control variable still runs to l. With l = math.MaxInt64, the loop body executes ~9.2 × 10¹⁸ times — effectively infinite for any realistic timeout.

Fixed pattern (b124caa, 2ce4242):

for range l {
    d.decoder.Decode(nil, r)
    if r.Error != nil {
        break
    }
}

The fix terminates the loop on the first inner error. It does not bound l itself — a well-formed payload that actually contains N encoded null elements still iterates N times. The MaxSliceAllocSize / MaxMapAllocSize caps are the policy-level bound on that case (see Mitigation).

Fixed behavior

The reader's accumulated error is checked after every inner Decode in the four affected loops. Decoder errors now surface in O(1) iterations instead of O(blockCount) when the underlying read fails mid-stream.

Affected versions

• github.com/hamba/avro/v2 — all versions up to and including v2.31.0 (repository is read-only upstream).
• github.com/iskorotkov/avro/v2 — all versions prior to v2.33.0.

Fixed versions

github.com/iskorotkov/avro/v2 v2.33.0 and later. There is no upstream fix for github.com/hamba/avro/v2 — module path is archived. Migrate to the fork as described under Mitigation.

Mitigation

Migrate from github.com/hamba/avro/v2 to github.com/iskorotkov/avro/v2 >= v2.33.0. Replace the import path and run go mod tidy:

go get github.com/iskorotkov/avro/v2@latest

Or, for consumers that prefer the original import path, a replace directive in go.mod:

replace github.com/hamba/avro/v2 => github.com/iskorotkov/avro/v2 v2.33.0

replace is honoured only for the main module of a build — transitive consumers must add their own replace, or migrate the import path directly.

The error-propagation fix runs on the existing decode path and requires no configuration.

For defense-in-depth against well-formed but oversized payloads (where the fix above does not help, because no error fires), set explicit allocation caps:

cfg := avro.Config{
    MaxByteSliceSize:  102_400,
    MaxSliceAllocSize: 10_000,
    MaxMapAllocSize:   10_000,
}.Freeze()

decoder := cfg.NewDecoder(schema, reader)

MaxMapAllocSize is new in v2.33.0 and opt-in (default zero, which leaves the previous unbounded behavior). Without setting it, a producer that ships a math.MaxInt64-count block still consumes the corresponding memory and CPU; see GHSA-mx64-mj3q-7prj for the cumulative-allocation enforcement details.

If you cannot upgrade immediately, the structural workarounds are application-level: per-request decode timeouts, isolated decoder workers under CPU quotas, and rejection of payloads whose advertised block count exceeds a known sane bound for your schema.

Proof-of-concept input

A minimal payload that triggers the bug for an array of int:

zigzag-encoded long: math.MaxInt64   (block element count)
EOF                                  (no further bytes)

The decoder reads the block-count header, enters the loop, fails to read the first element (EOF), records the error, and then iterates math.MaxInt64 − 1 further times calling the inner decoder as a no-op. Wall-clock cost on commodity hardware: indefinite — the goroutine pins one CPU core until the process is OOM-killed, deadline-cancelled, or terminated externally. The classic "a few seconds per request" characterisation applies only to small-but-still-pathological block counts in the 10⁸–10⁹ range (e.g. 200_999_000 in TestDecoder_SkipArrayEOF); the architectural ceiling is math.MaxInt64.

A negative block count (-N) is also legal in Avro (signals an N-element block with an explicit byte length); the same iteration pattern applies once the count is negated.

References

• Fix PR: iskorotkov/avro#4 (callback path)
• Fix commits: b124caa (skip helpers), 2ce4242 (callback path)
• Release: v2.33.0
• Security policy: SECURITY.md
• Related advisories on this fork: GHSA-mc57-h6j3-3hmv (integer overflow — same large-block-count payload also triggers cumulative-arithmetic overflow there), GHSA-mx64-mj3q-7prj (unbounded map allocation — the policy-level bound on well-formed huge inputs)
• Cross-module precedent on hamba/avro: GO-2023-1930 / CVE-2023-37475 / GHSA-9x44-9pgq-cf45
• Upstream (read-only): hamba/avro

Credits

• Discovery and fixes (commits b124caa skip helpers and 2ce4242 callback path, PR #4): Daniel Błażewicz (@klajok)
• Release authorship: Ivan Korotkov (@iskorotkov)

Timeline

• 2026-04-28 — Skip-decoder fix (b124caa) merged.
• 2026-04-30 — Callback-decoder fix (PR #4, 2ce4242) merged.
• 2026-05-06 — v2.33.0 tagged and released.
• 2026-05-11 — Advisory published.
• 2026-05-15 — Advisory revised.

References

• GHSA-w8j3-pq8g-8m7w
• https://nvd.nist.gov/vuln/detail/CVE-2026-46385