# GHSA-mx64-mj3q-7prj: iskorotkov/avro: Denial-of-Service Vulnerability in Decoder

> Attackers can cause denial of service via memory exhaustion in the Avro decoder unless developers explicitly configure the new map allocation limit.

- **Severity:** Not scored
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 18 May 2026
- **Also known as:** GO-2026-5048

## What is GHSA-mx64-mj3q-7prj?

GHSA-mx64-mj3q-7prj is a denial of service vulnerability in github.com/hamba/avro/v2, published on 18 May 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

# Memory Exhaustion via Unbounded Map Allocations in Avro Decoder

## Summary

The Avro map decoder accepted attacker-controlled block-element counts from the wire format and grew the destination map without enforcing an upper bound. The slice decoder already had `Config.MaxSliceAllocSize` for the equivalent attack against arrays; the map decoder had no analogous limit, so a producer could declare an arbitrarily large map (in one block, or chunked across many sub-limit blocks) and exhaust process memory until the OOM killer fired.

The fix introduces `Config.MaxMapAllocSize` with cumulative enforcement across block boundaries. **The new limit is opt-in**: the field defaults to zero, which preserves the previous unbounded behavior for backward compatibility. **Upgrading to `v2.33.0` alone does not mitigate the issue** — consumers of untrusted Avro data must explicitly set `MaxMapAllocSize` on their `avro.Config`.

## Description

Avro maps are encoded as a sequence of blocks; each block declares a `long` element count followed by that many key/value pairs. The decoder uses these counts both to size the destination map and as the loop bound for reading entries.

Pre-fix, the map decoder enforced no upper limit at any layer:

- No per-block element-count check.
- No cumulative across-block element-count check.
- No memory-budget check before `make(map[...]..., n)` or before growing the map.

The slice decoder had been hardened via `Config.MaxSliceAllocSize` and tracked cumulatively across blocks; the map decoder was a missing-by-symmetry gap. Even a partial per-block bound on maps would have been insufficient on its own — Avro permits encoding a logical map as many small blocks, so a producer could split a 10 GB map into 10,000 sub-MaxMapAllocSize blocks and still drive total allocation past any single-block threshold. The fix tracks cumulative entry count at block-header boundaries — *before* the block's entries are decoded into the map — and errors out before allocation when the running total would exceed the configured cap.

Two decoder variants were affected, both in `codec_map.go`:

- `mapDecoder.Decode` — string-keyed maps.
- `mapDecoderUnmarshaler.Decode` — `encoding.TextUnmarshaler`-keyed maps (e.g. `map[CustomKey]V` where `*CustomKey` implements `UnmarshalText`).

## Affected components

| File | Symbol | Pre-fix behavior | Post-fix behavior |
|------|--------|------------------|-------------------|
| `config.go` | `Config.MaxMapAllocSize` | Field did not exist | New `int` field; default zero means unlimited (back-compat) |
| `codec_map.go` | `mapDecoder.Decode` | Read block count, grew map unbounded | Validates cumulative count against `MaxMapAllocSize` at each block header |
| `codec_map.go` | `mapDecoderUnmarshaler.Decode` | Same | Same |

PR [#5](https://github.com/iskorotkov/avro/pull/5) (`fix/map-alloc-chunking-bypass`) covers both decoders and adds chunking-attack tests for both. The same PR also adds the previously-missing chunking-attack test coverage for the slice path in `534c7518` — the slice *logic* was already correct, only its test coverage was incomplete.

## Technical details

The fix mirrors the slice decoder's pattern:

1. At each block header, read the element count as `int64`.
2. Add it to a running total maintained across the block loop.
3. If the running total exceeds `Config.MaxMapAllocSize` (when nonzero), return an error before allocating any of that block's entries.
4. Otherwise, decode the block's entries into the map.

Per-block enforcement alone would be bypassable by chunking; cumulative tracking closes that. The check sits at the block-header read, *before* per-entry allocation, so a single oversized block also cannot allocate first and then fail post-hoc.

`Config.MaxMapAllocSize` semantics match `Config.MaxSliceAllocSize`: zero means unlimited, any positive value is the cumulative cap on element count (not byte size).

## Fixed behavior

`v2.33.0` adds the `MaxMapAllocSize` configuration field and the cumulative-enforcement logic in both map decoders. Both decoders return a descriptive error when the cumulative entry count would exceed the configured cap; no entries are allocated past the limit.

Tests added in PR #5 cover, for both `mapDecoder` and `mapDecoderUnmarshaler`:

- Single-block allocation exceeding the limit (rejected before allocation).
- Chunking attack: multiple sub-limit blocks whose cumulative count exceeds the limit (rejected at the block-header that crosses the threshold).
- Multi-block under the limit (decoded normally).

## 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`. Note: `v2.33.0` and later are vulnerable *by default* and only protected when `MaxMapAllocSize` is explicitly configured — see Mitigation.

## Fixed versions

`github.com/iskorotkov/avro/v2` `v2.33.0` and later, **with `Config.MaxMapAllocSize` explicitly set to a non-zero value**.

A bare upgrade to `v2.33.0` without setting `MaxMapAllocSize` leaves the decoder in the same unbounded state as `v2.32.0`. This is a backward-compatibility choice; a future major version may flip the default. Until then, treat this advisory as requiring both an upgrade *and* a configuration change.

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` **and** configure an allocation cap appropriate for your schema. The recommended approach for processes that decode untrusted input is a dedicated frozen config, used at every relevant call site, rather than mutating `avro.DefaultConfig`:

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

decoder := cfg.NewDecoder(schema, reader)
```

Choose the values based on the largest legitimate map your schema produces; a value 2–10× that ceiling provides headroom for benign variance while still bounding worst-case memory.

For consumers that prefer the original import path, a `replace` directive in `go.mod` is supported:

```
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.

If you cannot upgrade immediately, the only structural workarounds are out-of-band: run decoders in memory-constrained child processes or cgroups so an OOM is contained, reject inputs from sources without resource controls, and apply per-request decode deadlines so a runaway decode at least times out before the OOM killer fires.

## Proof-of-concept input

Two attack shapes, both targeting `map[string]int`:

**Single-block, oversize block count.** Emit one block header declaring `n = 2³¹ − 1` (or any value whose `n × averageEntrySize` exceeds available memory) followed by truncated entries. Pre-fix, the decoder pre-allocates `make(map[string]int, n)`, which fails or stalls long before EOF is reached.

**Chunking bypass.** Emit `k` blocks each declaring `n / k` elements, with `n / k` below any plausible per-block threshold but `n` itself well into the GB range. Pre-fix, the decoder happily grows the map block-by-block until the OS kills the process. Post-fix with `MaxMapAllocSize = 10_000`, the decoder rejects whichever block-header read pushes cumulative count past 10,000.

Either shape can be produced by hand-crafting the wire bytes; no `iskorotkov/avro` writer is needed to generate them.

## References

- Fix PR: [iskorotkov/avro#5](https://github.com/iskorotkov/avro/pull/5)
- Fix commit: [`5192df9`](https://github.com/iskorotkov/avro/commit/5192df96a158999344ac96ebcb1f7461d626f6d7) (`codec_map.go`, `config.go`, tests)
- Slice-path chunking-attack t

## Which versions are affected by GHSA-mx64-mj3q-7prj?

4 open source packages ship an affected version, including github.com/hamba/avro/v2 and github.com/iskorotkov/avro/v2.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `github.com/hamba/avro/v2` | Go | All versions | None |
| `github.com/iskorotkov/avro/v2` | Go | All versions | 2.33.0 |
| `github.com/iskorotkov/avro/v2` | Go | All versions | 2.33.0 |
| `github.com/iskorotkov/avro/v2` | Go | All versions | 2.33.0 |

## How do I fix GHSA-mx64-mj3q-7prj?

Change a configuration setting.

## Is GHSA-mx64-mj3q-7prj being exploited?

There is no known exploitation of GHSA-mx64-mj3q-7prj in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for GHSA-mx64-mj3q-7prj and fix it if it is affected.

What it is: GHSA-mx64-mj3q-7prj is a denial of service vulnerability in github.com/hamba/avro/v2, published on 18 May 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of GHSA-mx64-mj3q-7prj in the wild and no public exploit code.

Affected packages and fixed versions:
- github.com/hamba/avro/v2 (Go): affected all versions; no fix listed
- github.com/iskorotkov/avro/v2 (Go): affected all versions; fixed in 2.33.0
- github.com/iskorotkov/avro/v2 (Go): affected all versions; fixed in 2.33.0
- github.com/iskorotkov/avro/v2 (Go): affected all versions; fixed in 2.33.0

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/GHSA-mx64-mj3q-7prj/ (Markdown: https://insights.sbomify.com/GHSA-mx64-mj3q-7prj/index.md)
```

## Sources

- OSV: https://osv.dev/vulnerability/GHSA-mx64-mj3q-7prj

---
From sbomify Insights: https://insights.sbomify.com/GHSA-mx64-mj3q-7prj/
Summaries are generated and may contain mistakes; the official description above is the record.