CVE-2026-71225 Overview
A flaw in libkcapi weakens confidentiality guarantees for symmetric cipher operations on large inputs. When applications invoke one-shot symmetric encryption in stateful modes such as Counter (CTR) or Cipher Block Chaining (CBC), the library improperly reuses the Initialization Vector (IV) across internal data chunks when the input exceeds 64 KiB. The defect maps to [CWE-330] (Use of Insufficiently Random Values) and impacts any workload that routes bulk cryptographic operations through libkcapi's userspace API to the Linux kernel crypto interface.
Critical Impact
IV reuse across chunks in CTR and CBC modes exposes plaintext relationships and can corrupt cryptographic processing, undermining data confidentiality and integrity for inputs over 64 KiB.
Affected Products
- libkcapi (Linux Kernel Crypto API userspace library)
- Applications performing one-shot symmetric cipher operations via libkcapi
- Red Hat Enterprise Linux distributions shipping affected libkcapi builds
Discovery Timeline
- 2026-08-05 - CVE CVE-2026-71225 published to NVD
- 2026-08-05 - Last updated in NVD database
Technical Details for CVE-2026-71225
Vulnerability Analysis
libkcapi provides a userspace wrapper over the Linux kernel Crypto API through the AF_ALG socket interface. For one-shot symmetric operations, the library internally partitions large buffers into chunks before submitting them to the kernel. The defect occurs because each chunk is dispatched with the original caller-supplied IV rather than the ciphertext block or counter state produced by the preceding chunk.
In CTR mode, this restarts the counter for every chunk, producing identical keystreams across chunk boundaries. Attackers observing two ciphertext chunks encrypted under the same key and starting counter can XOR them to recover plaintext relationships. In CBC mode, resetting the IV breaks the chaining property between chunks, weakening semantic security and producing incorrect plaintext on decryption.
Root Cause
The root cause is improper state propagation in libkcapi's chunked processing path for one-shot cipher calls. The library treats each chunk as an independent operation with the initial IV rather than carrying forward the updated cryptographic state. The 64 KiB threshold reflects the internal buffering boundary that triggers chunked submission to the kernel.
Attack Vector
A remote attacker who can influence an application to process specially crafted large inputs through libkcapi can force the vulnerable chunked path. Exploitation does not require authentication or user interaction, but attack complexity is high because the adversary must control input sizing and observe or influence resulting ciphertext. Successful exploitation degrades confidentiality of encrypted data and may cause integrity failures on decryption.
No public proof-of-concept code is available. See the Red Hat CVE-2026-71225 Advisory and Red Hat Bug Report #2462011 for technical details.
Detection Methods for CVE-2026-71225
Indicators of Compromise
- Repeated identical ciphertext blocks across chunk boundaries in data encrypted with libkcapi CTR mode
- Decryption failures or plaintext corruption at 64 KiB boundaries in CBC-encrypted artifacts
- Applications linking against vulnerable libkcapi.so versions and processing inputs larger than 64 KiB
Detection Strategies
- Inventory installed libkcapi package versions across Linux hosts and compare against fixed releases published by distribution vendors.
- Audit application code paths that invoke kcapi_cipher_encrypt or kcapi_cipher_decrypt with input sizes exceeding 64 KiB.
- Perform cryptographic validation testing on encrypted datasets produced after libkcapi upgrades to detect anomalous ciphertext patterns.
Monitoring Recommendations
- Track package management events for libkcapi installations and updates via distribution package logs.
- Monitor AF_ALG socket usage on production systems to identify workloads that route bulk encryption through the kernel crypto API.
- Alert on integrity check failures in downstream systems consuming data encrypted by libkcapi-dependent services.
How to Mitigate CVE-2026-71225
Immediate Actions Required
- Identify all systems and applications that link against libkcapi and process cryptographic inputs larger than 64 KiB.
- Apply vendor-supplied libkcapi updates as soon as they are available from your Linux distribution.
- Re-encrypt any stored data produced with vulnerable libkcapi builds using a patched implementation and fresh IVs.
Patch Information
Refer to the Red Hat CVE-2026-71225 Advisory for authoritative patch status and affected package versions. Downstream distribution advisories will list the fixed libkcapi package release for each supported Red Hat Enterprise Linux stream. Track the Red Hat Bug Report #2462011 for upstream fix commits.
Workarounds
- Constrain application input sizes to below 64 KiB per cipher operation until patched libkcapi is deployed.
- Route bulk symmetric encryption through alternative userspace libraries such as OpenSSL or libgcrypt that do not exhibit this chunking defect.
- Explicitly manage IV progression in application code by invoking libkcapi's streaming API with caller-controlled state instead of the one-shot interface.
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
