CVE-2026-85769 in Red Hat
Summary
by MITRE • 09/04/2026
A flaw was found in libtpms, a library that provides software TPM 2.0 emulation. When restoring TPM 2.0 state (for example during a virtual machine's power-on or state/migration restore), a malformed state blob can supply an oversized skip-block length that is not validated against the remaining size of the input buffer. This can drive an internal size counter negative, which bypasses a subsequent bounds check due to an unsafe signed-to-unsigned conversion, causing the parser to read memory outside the bounds of the heap buffer holding the state data. Successful exploitation can crash the process hosting libtpms (such as swtpm), resulting in a denial of service of the emulated TPM device and the virtual machine that depends on it. No data corruption or information disclosure was confirmed.
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Analysis
by VulDB Data Team • 09/04/2026
The vulnerability identified within libtpms, a widely utilized library for software-based Trusted Platform Module 2.0 emulation, represents a critical memory safety flaw rooted in improper input validation during state restoration processes. This specific defect manifests when the system attempts to restore TPM 2.0 state data, such as during virtual machine power-on events or migration restores involving saved states. The core technical issue lies in the parsing logic for skip-block lengths within the serialized state blob. When a malformed state file is processed, it can contain an oversized value designated as the length of a block to be skipped. Crucially, this input value is not adequately validated against the remaining size of the input buffer before being used in arithmetic operations that drive internal counters negative.
The exploitation mechanism relies on a classic integer overflow scenario exacerbated by unsafe type conversion. The library fails to check if the skip-block length exceeds available data, allowing an attacker-controlled large unsigned value to be subtracted from or added to signed integers representing buffer offsets or remaining sizes. This operation results in a negative internal size counter. Subsequently, this negative signed value undergoes an implicit cast to an unsigned integer type for bounds checking purposes. Due to the nature of two's complement representation and unsigned arithmetic, the large positive equivalent of the negative number bypasses standard upper-bound checks designed to prevent out-of-bounds access. Consequently, the parser proceeds to read memory locations outside the allocated heap buffer that holds the TPM state data.
From a classification perspective, this vulnerability aligns with CWE-190 Integer Overflow or Wraparound and CWE-787 Out-of-Bounds Read. The lack of proper validation against input boundaries constitutes CWE-20 Improper Input Validation. In terms of offensive security frameworks, this flaw facilitates an initial access vector that leads to a denial of service, mapping closely to the ATT&CK technique T1499 Endpoint Denial of Service via resource exhaustion or crash-induced instability. While the immediate impact is limited to process termination rather than arbitrary code execution or data exfiltration, the reliability of the emulated TPM device and the associated virtual machine are severely compromised.
The operational impact of this vulnerability is primarily a denial of service condition for both the host process running libtpms, such as swtpm, and any guest operating systems relying on that specific TPM instance for cryptographic operations or secure boot validation. An attacker with the ability to supply malicious state blobs during migration or restoration phases can trigger these crashes repeatedly, effectively disrupting availability. Although no confirmed data corruption or information disclosure has been reported in this specific context, the potential for memory safety violations often serves as a precursor to more severe exploits if similar patterns exist elsewhere in the codebase. Mitigation strategies should focus on applying vendor-provided patches that enforce strict bounds checking before arithmetic operations involving buffer sizes and implementing rigorous input validation routines that verify all length fields against actual available data limits prior to processing.