CVE-2026-58262: Klever-Go Signature Verification Vulnerability

CVE-2026-58262 Overview

CVE-2026-58262 is a consensus safety vulnerability in Klever-Go, the Go implementation of the Klever blockchain protocol. Versions prior to 1.7.20 miscount padding bits in the PubKeysBitmap when validating header signatures. The verification routine treats these unused bits as valid signer positions during the two-thirds validator quorum check. A malicious or compromised block producer can set the padding bits to inflate the apparent signer count while gathering fewer real BLS signatures than the protocol requires. Nodes then accept the header as correctly signed, weakening consensus safety and undermining finality. The issue is fixed in version 1.7.20 and is tracked under [CWE-345] Insufficient Verification of Data Authenticity.

Critical Impact
Attackers with block producer access can forge quorum on Klever-Go headers, breaking finality guarantees without gathering the required two-thirds validator signatures.

Affected Products

Klever-Go blockchain node implementation
All versions prior to 1.7.20
Networks running Klever consensus with affected nodes

Discovery Timeline

2026-08-07 - CVE-2026-58262 published to NVD
2026-08-12 - Last updated in NVD database

Technical Details for CVE-2026-58262

Vulnerability Analysis

The Klever-Go header signature verification routine validates aggregate BLS signatures against a PubKeysBitmap. Each bit position in the bitmap maps to a validator in the consensus group. Because bitmaps are byte-aligned, the final byte contains padding bits when the consensus group size is not a multiple of eight. The vulnerable code counts every set bit in the bitmap toward the two-thirds quorum threshold without masking off the padding positions.

The BLS aggregate-signature check itself ignores these padding bits since they correspond to no validator public key. This mismatch between the quorum count and the cryptographic check is the root of the flaw. A block producer can flip padding bits to inflate numOfOnesInBitmap and pass the quorum test with fewer genuine signatures than the protocol demands. Downstream nodes accept the header as finalized.

Root Cause

The root cause is missing validation of reserved bitmap positions in core/process/headerCheck/headerSignatureVerify.go. The routine performs a size check on the bitmap but does not enforce that bits beyond consensusSize remain zero. Because BLS aggregation only consumes bits that reference real keys, the two independent checks disagree on what constitutes a valid signer set.

Attack Vector

Exploitation requires the low-privilege ability to produce block headers, meaning a compromised or malicious block producer. The attacker crafts a header whose PubKeysBitmap sets non-zero padding bits in the final byte. The aggregate signature is computed over a smaller-than-quorum set of real validators, but the bitmap count reaches the two-thirds threshold. Network-reachable nodes that import or intercept the header accept it as correctly signed.

The patch introduces ErrBitmapWithPaddingNotZero and rejects any bitmap whose padding bits are set before performing the quorum count:

// Reject bitmaps whose padding bits (positions >= consensusSize in the final byte) are set.
// Those positions map to no real validator, but the quorum count below would otherwise count
// them as signatures, letting a malicious leader inflate the apparent signer set (KLR-04).
if remainder := consensusSize % 8; remainder != 0 {
    allowedLastByteMask := byte((1 << uint(remainder)) - 1)
    if bitmap[len(bitmap)-1]&^allowedLastByteMask != 0 {
        log.Debug("bitmap has non-zero padding bits beyond consensus size",
            "consensus size", consensusSize)
        return ErrBitmapWithPaddingNotZero
    }
}

numOfOnesInBitmap := 0
for index := range bitmap {
    numOfOnesInBitmap += bits.OnesCount8(bitmap[index])

Source: Klever-Go security patch commit

Detection Methods for CVE-2026-58262

Indicators of Compromise

Block headers whose PubKeysBitmap final byte contains bits set beyond the consensus group size
Header acceptance events where the count of set bitmap bits exceeds the number of distinct BLS public keys aggregated in the signature
Discrepancies between local quorum accounting and peer node quorum accounting for the same block height

Detection Strategies

Inspect ingested headers and compute bitmap[len-1] &^ allowedLastByteMask to flag any non-zero result on nodes running versions prior to 1.7.20
Cross-check the number of ones in PubKeysBitmap against the number of validator signatures actually aggregated for each finalized block
Compare finality decisions across independent nodes to identify headers accepted by unpatched nodes but rejected by patched peers

Monitoring Recommendations

Enable debug logging on header verification paths to capture the new bitmap has non-zero padding bits beyond consensus size message after upgrading
Alert on validator set changes and block producer rotations, correlating with any bitmap anomalies observed on the network
Monitor consensus group size transitions since padding bits only appear when consensusSize % 8 != 0

How to Mitigate CVE-2026-58262

Immediate Actions Required

Upgrade all Klever-Go nodes and validators to version 1.7.20 or later without delay
Audit historical headers for evidence of non-zero padding bits in PubKeysBitmap on affected chains
Rotate or restrict access for any block producer credentials suspected of misuse before the upgrade

Patch Information

The fix is delivered in Klever-Go 1.7.20. The relevant changes are in core/process/headerCheck/errors.go and core/process/headerCheck/headerSignatureVerify.go, adding the ErrBitmapWithPaddingNotZero sentinel and a mask check that rejects bitmaps with non-zero padding bits before performing the quorum count. Reference the GitHub Security Advisory GHSA-f9h7-4mmq-vgcq and the upstream commit for full patch details.

Workarounds

No configuration-based workaround exists; the flaw is in signature verification logic and requires the code patch
Operators unable to upgrade immediately should reduce exposure by limiting peer connections to trusted, patched nodes
Coordinate with the validator set to reject headers from any producer emitting bitmaps with non-zero padding bits

bash

# Upgrade Klever-Go to the patched release
git clone https://github.com/klever-io/klever-go.git
cd klever-go
git checkout v1.7.20
make build
# Restart the node service after replacing the binary
systemctl restart klever-node