CVE-2026-34155: RAUC Integer Overflow Vulnerability

CVE-2026-34155 Overview

CVE-2026-34155 is an integer overflow vulnerability affecting RAUC, an open-source update framework that controls the update process on embedded Linux systems. Prior to version 1.15.2, RAUC bundles using the 'plain' format that exceed a payload size of 2 GiB cause an integer overflow which results in a signature that covers only the first few bytes of the payload. Given such a bundle with a legitimate signature, an attacker can modify the portion of the payload that is not covered by the signature, potentially allowing for malicious firmware or software updates to be installed on embedded devices.

Critical Impact

Attackers can bypass signature verification on embedded Linux systems by exploiting the integer overflow, enabling modification of update bundles while maintaining valid signatures. This could lead to complete compromise of embedded devices through malicious firmware updates.

Affected Products

• RAUC versions prior to 1.15.2
• Embedded Linux systems using RAUC 'plain' format bundles exceeding 2 GiB

Discovery Timeline

• March 27, 2026 - RAUC releases security patch in version 1.15.2
• March 31, 2026 - CVE-2026-34155 published to NVD
• April 1, 2026 - Last updated in NVD database

Technical Details for CVE-2026-34155

Vulnerability Analysis

This vulnerability is classified under CWE-196 (Unsigned to Signed Conversion Error), which relates to integer handling issues. The flaw exists in how RAUC handles payload size calculations for bundles using the 'plain' format. When a bundle's payload exceeds 2 GiB, an integer overflow occurs during signature calculation, causing the cryptographic signature to only cover the initial bytes of the payload rather than the entire content.

The attack can be performed over the network and requires an attacker to have access to a legitimately signed bundle. Once obtained, the attacker can modify any portion of the payload beyond the first few bytes without invalidating the signature. This fundamentally breaks the integrity guarantee that signature verification is supposed to provide for update bundles.

Root Cause

The root cause is an integer overflow vulnerability in RAUC's handling of payload sizes for 'plain' format bundles. When the payload size exceeds 2 GiB (2,147,483,648 bytes), the size value overflows during signature calculation, resulting in only a small portion of the payload being included in the signature computation. This is a classic unsigned to signed integer conversion error where the size calculation wraps around to a much smaller value.

Attack Vector

The attack vector involves the following scenario:

1. An attacker obtains a legitimate RAUC bundle in 'plain' format with a valid signature
2. The bundle's payload must exceed 2 GiB to trigger the integer overflow condition
3. Due to the overflow, only the first few bytes of the payload are covered by the signature
4. The attacker modifies the remainder of the payload (malicious firmware, backdoors, etc.)
5. The modified bundle still passes signature verification on the target embedded system
6. The compromised update is installed, giving the attacker control over the embedded device

The security patch addresses this by rejecting bundles using the 'plain' format which exceed a payload size of 2 GiB during both signing and verification operations:

+Release 1.15.2 (released Mar 27, 2026)
+--------------------------------------
+
+.. rubric:: Security
+
+* Reject bundles using the 'plain' format which exceed a payload size of 2 GiB
+  during signing and verification.
+  This fixes an integer overflow which results in a signature which covers only
+  the first few bytes of the payload (CVE-2026-34155).
+  See the advisory for more details:
+  https://github.com/rauc/rauc/security/advisories/GHSA-6hj7-q844-m2hx
+
+Contributions from: Jan Lübbe (@jluebbe)
+
 Release 1.15.1 (released Jan 5, 2026)
 -------------------------------------

Source: GitHub Commit Details

Detection Methods for CVE-2026-34155

Indicators of Compromise

• Presence of RAUC bundles in 'plain' format exceeding 2 GiB in size
• Unusual update bundle files on embedded systems with mismatched hash values for payload content
• Evidence of modified firmware or software components following an update operation
• Log entries showing successful signature verification for abnormally large 'plain' format bundles

Detection Strategies

• Monitor RAUC update operations for bundles using the 'plain' format with payloads exceeding 2 GiB
• Implement file integrity monitoring on embedded systems to detect unauthorized changes post-update
• Review RAUC version across all embedded Linux deployments and flag systems running versions prior to 1.15.2
• Deploy network monitoring to detect transfer of unusually large update bundles to embedded devices

Monitoring Recommendations

• Establish baseline monitoring for RAUC update operations and bundle sizes in your environment
• Configure alerting for update bundle processing events on critical embedded systems
• Implement centralized logging for all embedded device update activities
• Monitor for changes to critical system files on embedded devices after update operations

How to Mitigate CVE-2026-34155

Immediate Actions Required

• Upgrade RAUC to version 1.15.2 or later on all affected embedded Linux systems immediately
• Audit existing RAUC bundles in 'plain' format for sizes exceeding 2 GiB and verify their integrity
• Consider migrating to alternative bundle formats (such as 'verity' or 'crypt') that are not affected by this vulnerability
• Review update logs on embedded systems for any suspicious update operations prior to patching

Patch Information

The vulnerability has been addressed in RAUC version 1.15.2, released on March 27, 2026. The fix rejects bundles using the 'plain' format that exceed a payload size of 2 GiB during both signing and verification operations. Organizations should update to this version or later to remediate the vulnerability.

For detailed patch information, refer to the GitHub Security Advisory GHSA-6hj7-q844-m2hx and the GitHub Release v1.15.2.

Workarounds

• Avoid using the 'plain' bundle format for payloads exceeding 2 GiB until systems can be updated
• Switch to alternative RAUC bundle formats such as 'verity' or 'crypt' which provide additional integrity protections
• Implement additional integrity verification mechanisms outside of RAUC signature checking for large bundles
• Restrict network access to embedded systems to prevent unauthorized bundle delivery

# Check RAUC version on embedded systems
rauc --version

# Verify bundle format and size before deployment
rauc info bundle.raucb

# Update RAUC to patched version (example using package manager)
apt-get update && apt-get install rauc=1.15.2