CVE-2025-12829: Amazon Ion-C Information Disclosure Flaw

CVE-2025-12829 Overview

CVE-2025-12829 is an uninitialized stack read vulnerability in Amazon Ion-C versions prior to v1.1.4. The flaw resides in the Ion text serialization path, where a threat actor can craft input data that, when serialized, exposes uninitialized stack memory through UTF-8 escape sequences in the output. This can leak sensitive data residing in process memory. The issue is tracked under CWE-125 (Out-of-bounds Read) and was disclosed in AWS Security Bulletin AWS-2025-027. Amazon addressed the issue in Ion-C v1.1.4.

Critical Impact
Attackers with local access who can submit crafted data to an Ion-C serializer can extract sensitive information from process stack memory via UTF-8 escape sequences in the generated Ion text output.

Affected Products

Amazon Ion-C versions prior to v1.1.4
Applications and services linking vulnerable Ion-C builds for Ion text serialization
Downstream consumers of Amazon Ion data interchange libraries built on Ion-C

Discovery Timeline

2025-11-07 - CVE-2025-12829 published to the National Vulnerability Database
2026-06-17 - Last updated in NVD database

Technical Details for CVE-2025-12829

Vulnerability Analysis

Amazon Ion is a richly typed, self-describing data serialization format with interchangeable binary and text representations. Ion-C is the C implementation of the Ion reader and writer. The vulnerability affects the Ion text writer path. When specific crafted inputs are serialized to Ion text, the writer emits UTF-8 escape sequences using stack buffers that were not fully initialized before being read.

As a result, the generated Ion text output embeds bytes from uninitialized stack memory. An attacker who controls input to the serializer and can observe the serialized output can harvest residual stack contents. This may include fragments of pointers, cryptographic material, authentication tokens, or other sensitive runtime state present on the stack of the calling thread.

The weakness is classified as CWE-125, an out-of-bounds read, because the writer consumes bytes beyond the valid initialized region of the backing buffer when assembling the escape sequence.

Root Cause

The root cause is incomplete initialization of a stack-allocated buffer used when encoding characters as UTF-8 escape sequences during Ion text serialization. The affected code path reads from buffer positions that were never written, causing stale stack bytes to be incorporated into the serialized escape output.

Attack Vector

Exploitation requires local access and the ability to supply crafted data to an application that calls Ion-C to serialize values to Ion text. No privileges or user interaction are required. The attacker observes the resulting Ion text output and parses the UTF-8 escape sequences to reconstruct leaked stack bytes.

The vulnerability does not have a public proof-of-concept. See the
[GitHub Security Advisory GHSA-7mgf-6x73-5h7r](https://github.com/amazon-ion/ion-c/security/advisories/GHSA-7mgf-6x73-5h7r)
for technical details on the affected serialization path.

Detection Methods for CVE-2025-12829

Indicators of Compromise

Ion-C shared libraries or statically linked binaries reporting a version earlier than v1.1.4
Ion text output containing unexpected or anomalous \u or \x UTF-8 escape sequences that do not correspond to legitimate input characters
Application logs showing serialization of attacker-controlled payloads immediately before suspicious Ion text artifacts appear downstream

Detection Strategies

Inventory build systems and container images for the ion-c dependency and compare against v1.1.4 using software composition analysis tools
Review Software Bill of Materials (SBOM) documents for references to the amazon-ion/ion-c project at vulnerable versions
Correlate CWE-125 scanner findings against services that process untrusted data through Ion text writers

Monitoring Recommendations

Monitor file integrity on binaries that statically embed Ion-C to detect unpatched versions remaining in production
Audit outbound data flows from services using Ion text serialization for abnormal escape sequence density, which may indicate exploitation attempts
Track CI/CD dependency manifests to ensure new builds pull Ion-C v1.1.4 or later

How to Mitigate CVE-2025-12829

Immediate Actions Required

Upgrade Amazon Ion-C to v1.1.4 across all affected build artifacts and container images
Rebuild and redeploy any downstream applications that statically link Ion-C to ensure the patched version is in effect
Restrict local access to systems that invoke Ion text serialization on attacker-controlled data until patching is complete

Patch Information

Amazon published the fix in Ion-C v1.1.4. The patch is available in the GitHub Release v1.1.4 and is referenced in the AWS Security Bulletin AWS-2025-027 and GitHub Security Advisory GHSA-7mgf-6x73-5h7r. Upgrade all consumers to this release.

Workarounds

Avoid serializing untrusted or attacker-controlled values to Ion text until the upgrade to v1.1.4 is applied
Where feasible, prefer the Ion binary writer over the Ion text writer, since the vulnerability is specific to the text serialization path
Sanitize or filter serialized Ion text output to strip unexpected UTF-8 escape sequences before transmitting the data to external parties

bash

# Verify the installed Ion-C version meets the patched release
git clone https://github.com/amazon-ion/ion-c.git
cd ion-c
git checkout v1.1.4
git submodule update --init --recursive