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Response snapshot Oct 27, 2026, 16:01 UTC

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VulnerabilityCVE-2026-102554

Google Guava: CompactHashMap/CompactHashSet/MapMakerInternalMap deserialization triggers uncontrolled memory allocation

`CompactHashMap`, `CompactHashSet`, and `MapMakerInternalMap` implement `readObject(ObjectInputStream)` by reading an attacker-controlled `elementCount` or `initialCapacity` from the stream.

Published Updated

At a glance

Exploited
Not known
Severity
8.2 High (CVSS 4.0)
Chance of exploitation
Not estimated
Sources
1 source

What to do

GitHub lists first patched versions: com.google.guava:guava: 33.7.2. Confirm the affected range and upgrade guidance in the exact advisory.

Read GitHub-reviewed advisory GHSA-xxph-c9ww-hj94 (opens a new tab)

What happened

# CompactHashMap/CompactHashSet/MapMakerInternalMap native deserialization triggers uncontrolled memory allocation

## Summary

`CompactHashMap`, `CompactHashSet`, and `MapMakerInternalMap` implement `readObject(ObjectInputStream)` by reading an attacker-controlled `elementCount` or `initialCapacity` from the stream.

On the very first insertion during or after deserialization (for the `CompactHash*` collections, inside `allocArrays()`) or immediately during deserialization (for `MapMakerInternalMap`), the implementation eagerly allocates arrays sized to that full, unvalidated value (up to `CompactHashing.MAX_SIZE` = 2^30-1, ~1.07 billion, for example) before the rest of the stream's actual content has been consumed.

Continue reading

A 72-byte malicious serialized stream containing a single tiny element can force a multi-gigabyte allocation and crash a JVM with `OutOfMemoryError`, dynamically confirmed against the current release.

## Details

`CompactHashMap.readObject()` (`guava/src/com/google/common/collect/CompactHashMap.java`):

```java private void readObject(ObjectInputStream stream) throws IOException, ClassNotFoundException { stream.defaultReadObject(); int elementCount = stream.readInt(); if (elementCount = 0, "Expected size must be >= 0"); this.metadata = Ints.constrainToRange(expectedSize, 1, CompactHashing.MAX_SIZE); } ```

The real allocation is deferred to `allocArrays()`, which fires lazily on the *first* `put()` call — i.e. after only one key/value pair has actually been read off the wire:

```java int allocArrays() { ... int expectedSize = metadata; int buckets = CompactHashing.tableSize(expectedSize); this.table = CompactHashing.createTable(buckets); setHashTableMask(buckets - 1); this.entries = new int[expectedSize]; this.keys = new Object[expectedSize]; this.values = new Object[expectedSize]; return expectedSize; } ```

With `expectedSize` near `CompactHashing.MAX_SIZE` (2^30-1), this single call allocates a hash `table` plus three `expectedSize`-length arrays — multiple gigabytes — triggered by a stream that has, at that point, only supplied one small real element. `CompactHashSet.readObject()`/`allocArrays()` have the identical shape. `CompactLinkedHashMap`/`CompactLinkedHashSet` extend these classes and inherit the same unguarded path.

The problem with `MapMakerInternalMap` is similar, with [a size read from the serialized data and stored in a `MapMaker` instance](https://github.com/google/guava/blob/1278ad9c2257f48dc82fb7b7c625ab563605aa24/guava/src/com/google/common/collect/MapMakerInternalMap.java#L2848-L2851) for [later user during deserialization](https://github.

com/google/guava/blob/1278ad9c2257f48dc82fb7b7c625ab563605aa24/guava/src/com/google/common/collect/MapMakerInternalMap.java#L2897), when [the `MapMaker` is passed back to `MapMakerInternalMap`](https://github.com/google/guava/blob/1278ad9c2257f48dc82fb7b7c625ab563605aa24/guava/src/com/google/common/collect/MapMaker.java#L287) and [segments are allocated](https://github.

com/google/guava/blob/1278ad9c2257f48dc82fb7b7c625ab563605aa24/guava/src/com/google/common/collect/MapMakerInternalMap.java#L173-L200) by [calling `newSegment`](https://github.com/google/guava/blob/1278ad9c2257f48dc82fb7b7c625ab563605aa24/guava/src/com/google/common/collect/MapMakerInternalMap.java#L1143), whose implementations ([example](https://github.

com/google/guava/blob/1278ad9c2257f48dc82fb7b7c625ab563605aa24/guava/src/com/google/common/collect/MapMakerInternalMap.java#L443)) create `Segment` instances ([example](https://github.com/google/guava/blob/1278ad9c2257f48dc82fb7b7c625ab563605aa24/guava/src/com/google/common/collect/MapMakerInternalMap.java#L2019)) that [allocate arrays of the requested capacity](https://github.

com/google/guava/blob/1278ad9c2257f48dc82fb7b7c625ab563605aa24/guava/src/com/google/common/collect/MapMakerInternalMap.java#L1234) through [the `newTable`](https://github.com/google/guava/blob/1278ad9c2257f48dc82fb7b7c625ab563605aa24/guava/src/com/google/common/collect/MapMakerInternalMap.java#L1262) helper.

This is the same amplification shape as the already-fixed **CVE-2018-10237** (`AtomicDoubleArray`/`CompoundOrdering`: attacker declares a large size, library eagerly allocates before validating against actual stream content). That fix rewrote `AtomicDoubleArray.readObject()` to grow an `ImmutableLongArray.

Builder` incrementally per element actually read, rather than pre-allocating from the declared size — Guava's own `HashBiMap.readObject()` documents the same defensive principle explicitly in a comment: `init(16); // resist hostile attempts to allocate gratuitous heap`. `CompactHashMap`/`CompactHashSet` do not follow either of these two patterns already present elsewhere in the same codebase.

**Reachability**: All classes are top-level, `implements ... Serializable`, and require no special classpath setup beyond Guava being present (near-ubiquitous on JVM backends). A native Java deserialization attacker names the class directly in a hand-crafted stream — no particular Guava factory method needs to have been used by the victim application, exactly as with the precedent CVE.

## PoC

Full source and run instructions in the attached `poc-guava-compacthash-dos.zip` (Maven project, `guava:33.7.1-jre` — the current latest release — as the only dependency). The PoC:

1. Uses reflection only to legitimately obtain a `CompactHashSet` instance (it is package-private; a real attacker does not need this step, since they craft the serialized bytes directly rather than calling Guava's own serialization code). 2. Serializes one set with 1 element and one with 2 elements, diffs the two byte streams to locate the exact offset of the `elementCount` field (byte-precise, not guessed). 3.

Patches that one 4-byte field in the 1-element stream from `1` to `0x3FFFFFFF` (`CompactHashing.MAX_SIZE`), leaving the rest of the 72-byte stream (one real `"A"` element) untouched. 4. Feeds the resulting 72-byte malicious stream into a plain `ne

Affected package ranges reported by GitHub: com.google.guava:guava: >= 4.0, <= 33.7.1

Scores and how they were measured

CVSS base severity

8.2

Version 4.0As of

CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N

Measurement source (opens a new tab)

How bad it is if exploited. Not a prediction of exploitation or proof you are exposed.

EPSS probability

Not available

A model's estimate of exploitation in the next 30 days, not certainty or your own risk.

Related technology

Sources

  1. Primary source

    GitHub-reviewed advisory GHSA-xxph-c9ww-hj94 (opens a new tab)

    The public advisory supplies the description, affected package/version ranges, and any listed CVSS metric. GitHub review is not local review.

    Published Checked

  2. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-xxph-c9ww-hj94, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

Cite this record

For factual claims, cite the primary sources above; cite KEVWire for its compilation, dates, and classifications.

APA

KEVWire. (2026, October 27). Google Guava: CompactHashMap/CompactHashSet/MapMakerInternalMap deserialization triggers uncontrolled memory allocation (CVE-2026-102554). https://kevwire.com/record/CVE-2026-102554

MLA

"Google Guava: CompactHashMap/CompactHashSet/MapMakerInternalMap deserialization triggers uncontrolled memory allocation (CVE-2026-102554)." KEVWire, 27 Oct. 2026, kevwire.com/record/CVE-2026-102554.

Chicago

KEVWire. "Google Guava: CompactHashMap/CompactHashSet/MapMakerInternalMap deserialization triggers uncontrolled memory allocation (CVE-2026-102554)." Last modified October 27, 2026. https://kevwire.com/record/CVE-2026-102554.

Evidence summary (plain text)

Google Guava: CompactHashMap/CompactHashSet/MapMakerInternalMap deserialization triggers uncontrolled memory allocation (Oct 27, 2026).
`CompactHashMap`, `CompactHashSet`, and `MapMakerInternalMap` implement `readObject(ObjectInputStream)` by reading an attacker-controlled `elementCount` or `initialCapacity` from the stream. On the very first insertion during or after deserialization (for the `CompactHash*` collections, inside `allocArrays()`) or immediately during deserialization (for `MapMakerInternalMap`), the implementation eagerly allocates arrays sized to that full, unvalidated value (up to `CompactHashing.MAX_SIZE` = 2^30-1, ~1.07 billion, for example) before the rest of the stream's actual content has been consumed. A 72-byte malicious serialized stream containing a single tiny element can force a multi-gigabyte allocation and crash a JVM with `OutOfMemoryError`, dynamically confirmed against the current release.

`CompactHashMap.readObject()` (`guava/src/com/google/common/collect/CompactHashMap.java`):
CVSS 8.2 (version 4.0; evidence as of Oct 27, 2026; https://github.com/advisories/GHSA-xxph-c9ww-hj94).
Sources:
GitHub-reviewed advisory GHSA-xxph-c9ww-hj94: https://github.com/advisories/GHSA-xxph-c9ww-hj94
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc
Record ID: CVE-2026-102554.
Record origin: Ingested record.
AI evidence: Not assessed.
Known exploitation: Not assessed.
Zero-day status: Not assessed. KEV membership alone does not establish zero-day status.
About this data

This record was ingested from a data source. Retrieved: Oct 27, 2026, 16:00 UTC. Ingestion is not an independent editorial investigation.

A CISA KEV listing documents known exploitation, not necessarily a zero-day; absence from the catalog does not prove a vulnerability has never been exploited.

Technology tags indicate possible relevance, not verified exposure. Confirm the affected product, version, and configuration against your own inventory; KEVWire does not scan your systems.

A personal reading checklist on this device, not verified remediation.

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