A critical memory-safety flaw in the widely used cJSON library has been assigned CVE-2025-57052: a logic error in the array-index parsing code lets malformed JSON pointer strings bypass bounds checks, enabling out‑of‑bounds memory access that can crash or corrupt applications that rely on cJSON for pointer-style array lookups.
Background
cJSON is a compact, permissively licensed C library for parsing and printing JSON, popular in embedded systems, microcontrollers, desktop utilities, and server-side components where a small footprint matters. Its simplicity and permissive license have driven wide reuse across ecosystems—so a memory-safety bug in a core utility function ripples through many downstream projects and distributions. This is not the first time cJSON has appeared in vulnerability summaries; the projectf past parsing and DoS fixes, illustrating how subtle parser logic can become a systemic risk.
The new CVE-2025-57052 was publicly catalogued in early September 2025 and quickly given a high severity rating: many trackers report a CVSS v3.1 base score of 9.8 (Critical). The vulnerability affects cJSON versions 1.5.0 through 1.7.18; upstream resolved the issue in the v1.7.19 release.
What exactly is broken?
The faulty loop and how it mis-parses indices
At the heart of the issue is the function responsible for decoding an array index from a JSON pointer-style string: decode_array_index_from_pointer (implemented in cJSON_Utils.c). The function attempts to iterate over the pointer string and build a numeric index from consecutive ASCII digits. However, a logic error in the loop condition validates the first character of the pointer repeatedly rather than validating the character at the current loop position. In other words, the code checks pointer[0] where it should check pointer[position]. That subtle mistake lets non-digit characters later in the token slip through the check and be treated as if they were digits.
Concretely, an input such as the character sequence "0A" is interpreted by the vulnerable routine as index 10 rather than being rejected as malformed. That means a JSON pointer that looks syntactically plausible to the caller but contains mixed alphanumeric segments can be parsed into an index that exceeds the actual array bounds. The result is an out‑of‑bounds read (and potentially worse depending on surrounding code), which can crash the process or reveal adjacent memory.
Why this is worse than a simple crash
Out‑of‑bounds reads in C can do more than cause a transient crash. In some contexts they let attackers read memory outside of intended buffers—possibly leaking secrets. In others, repeated or crafted inputs can cause allocator/state corruption, escalate crashes into more persistent denial-of-service conditions, or in extreme cases pave the way for control-flow manipulation. The exact risk in any product depends on how the cJSON return values are used, but because parser libraries typically feed directly into application logic, the attack surface is broad. Multiple public writeups and distribution advisories emphasize the confidentiality and availability consequences.
Scope and impact
Affected versions and remediation status
- Affected: cJSON 1.5.0 through 1.7.18.
- Fixed: upstream release v1.7.19 includes an explicit fix for the incorrect pointer check in decode_array_index_from_pointer. Upstream change logs and release notes list “fix the incorrect check in decode_array_index_from_pointer” among the fixes for v1.7.19.
Multiple Linux distributions and package ecosystems quickly published advisories or package updates in response. Ubuntu, Debian, OpenSUSE and others recorded the CVE in their trackers and queued or released package updates that either backported a patch to existing cJSON packages or moved consumers to the v1.7.19 release. If you maintain systems that include native distribution packages, check your distro’s security tracker and ensure cJSON is updated to at least v1.7.19 or that the vendor-supplied patch is applied.
Who’s at risk
- Any application that consumes untrusted JSON pointer strings and relies on cJSON’s pointer-decoding utilities without additional sanitization. Examples include REST APIs, JSON-RPC endpoints, JSON pointer support in configuration or routing layers, and embedded devices that expose configuration interfaces.
- IoT and embedded devices: compact parsers like cJSON are extremely common in constrained devices; those devices often run older libraries and are slower to receive updates, increasing exposure.
- Large-scale systems and libraries that vend cJSON as a dependency: toolchains, SDKs, or redistributable libraries that bundle cJSON can propagate the vulnerability into many products.
Because the bug is in library code that performs pointer-string parsing (often used for indexing into arrays in JSON documents), the attacker model is straightforward: if an adversary can get a target to parse a crafted JSON pointer that includes alphanumeric mixing in index tokens, they can trigger out‑of‑bounds access. Several public analyses and a minimal proof‑of‑concept demonstrate how small synthetic requests generate crashes or memory reads.
Technical analysis: proof-of-fix and verification
The correct change
Multiple published analyses and the upstream fix confirm the corrective action: the loop condition should check the character at the current position rather than always checking the first character of the pointer token. The vulnerable snippet looks like this in concept:
for (position = 0; (pointer[position] >= '0') && (pointer[0] <= '9'); position++)
The repaired logic replaces the repeated pointer[0] check with pointer[position]:
for (position = 0; (pointer[position] >= '0') && (pointer[position] <= '9'); position++)
This change ensures every examined character is validated as a digit before being incorporated into the accumulated index value, preventing alphanumeric sequences like "0A" from being interpreted as numeric 10. Public patch sets and the v1.7.19 release reference this exact correction.
Reproducing the issue (conceptually)
Security researchers published minimal demonstrators showing the difference between correct and incorrect inputs. In the typical demonstration, a JSON array with a small number of elements is indexed via pointer strings "0", "1", and a crafted string such as "0A". The vulnerable parser returns element 10 (or attempts to), which is out of range and triggers a segmentation fault or an out‑of‑bounds read. Researchers highlight that the same malformed index is unlikely to be rejected by naive validation that only checks the first character. A short C test harness using cJSON shows the crash conditions without requiring exotic privileges. Several advisories reproduce that exact pattern.
Practical mitigation guidance
If you manage software that uses cJSON (directly or transitively), follow these prioritized steps to reduce risk immediately.
- Inventory dependencies.
- Identify all direct cJSON consumers in your codebase and any third-party components that might bundle cJSON statically. Use build-system dependency graphs and binary scanning where necessary.
- Update immediately.
- Replace cJSON with upstream v1.7.19 or a vendor-supplied package that contains the backport fix. Many distributions and package maintainers have already published updates; apply vendor patches or upgrade packages as soon as practical.
- Add input validation and hardening.
- Where JSON pointer strings are accepted from untrusted inputs (HTTP requests, configuration uploads, device interfaces), validate pointer tokens against a strict numeric pattern before handing them to cJSON utilities. Reject or sanitize any token that contains non-digit characters when an array index is expected.
- Apply defense-in-depth at the call site.
- Treat cJSON results cautiously: check returned node pointers and bounds before dereferencing, and assert-generated indices against array sizes where possible.
- Monitor for signs of exploitation.
- Watch logs for repeated crashes, segmentation faults, or unusual memory-read behaviors in components that parse JSON pointers. Correlate crash signatures with input sources and apply mitigations or rate limits to suspicious endpoints.
- For embedded devices and long‑tail consumers: plan firmware and supply‑chain updates.
- If you manufacture devices that include cJSON, schedule firmware updates and communicate urgency clearly to downstream customers; unpatched floating devices are the highest long-term exposure.
If immediate upgrade is infeasible, the most reliable stopgap is strict input validation and rejecting any pointer tokens that deviate from an expected numeric-only pattern. However, that approach imposes application-level constraints and may not protect all call paths; updating the library is the correct, durable fix.
Attack scenarios and real-world consequences
Two high-level exploitation vectors matter in practice:
- Denial of Service (DoS): a remote attacker sends crafted JSON pointer strings that trigger repeated out‑of‑bounds reads or segmentation faults, crashing services or causing persistent unavailability. This is the easiest scenario to weaponize and requires only the ability to submit JSON to a vulnerable parsing endpoint. Multiple advisories emphasize this as the immediate operational risk.
- Information disclosure / memory exposure: in some runtimes and memory layouts, an out‑of‑bounds read can leak adjacent heap or stack contents to a requestor. If the vulnerable code returns pointer-aggregated data or logs unguarded memory, sensitive data may be exposed. The precise viability of data exfiltration depends on the host application logic and memory layout, which is why application-specific reviews are necessary.
Less likely but technically feasible in narrow circumstances is an elevation-of-impact scenario: memory corruption rippling into use-after-free or control-flow manipulation that could allow code execution. Public reporting so far documents out‑of‑bounds reads and DoS; no broadly credible RCE reports have been confirmed in the wild as of the most recent advisories, but defenders must treat the worst-case consequences seriously due to the high CVSS rating and the library’s reach.
Why did this bug slip through, and lessons for C projects
This vulnerability is a textbook example of how small logic mistakes in pointer arithmetic and loop conditions can create critical security issues in C code. A single-character indexing error—checking pointer[0] instead of pointer[position]—translates into systemic risk because:
- Parsers are central: they touch external input and feed internal state.
- Library reuse amplifies risk: compact C libraries are statically linked or vendored into many projects.
- Regression testing often lacks adversarial inputs: standard unit tests validate common cases but rarely include malformed, adversarial pointer tokens that reveal off-by-one or index-misinterpretation bugs.
Lessons for maintainers and consumers:
- Prefer fuzzing and adversarial test cases for parsers—cifuzz and other CI fuzzers flagged related issues in cJSON’s CI runs. Library maintainers should run continuous fuzzing and include generated malformed inputs in regression suites.
- Treat parser logic as security-sensitive: even simple utilities deserve rigorous review and threat modeling.
- Track third-party advisories and CVE databases: library consumers should automate scanning and patching pipelines to reduce windows of exposure.
How the community and distributors responded
Within days of public disclosure, the cJSON maintainers merged a fix and cut v1.7.19 that explicitly references the corrected array-index check. Major Linux distributions and package ecosystems created advisories and prepared packages: Ubuntu, Debian, OpenSUSE and others documented the CVE in their trackers and shipped updates or backports for affected releases. Security trackers and vulnerability databases cross-referenced the issue and, in several cases, flagged associated packages that include cJSON. Operators should treat those vendor advisories as authoritative for their platforms and follow the vendor-specific remediation guidance.
Risk matrix — prioritization for responders
- High urgency (apply within hours): Internet‑facing services that accept JSON pointers from untrusted users and are critical to availability or confidentiality. These endpoints are the easiest to weaponize into a sustained DoS or data-exposure scenario.
- Medium urgency (apply within days): Internal services exposed via API gateways or multi-tenant platforms where attackers may gain access through lateral movement. Also, device management interfaces for IoT fleets.
- Lower urgency (apply within weeks): Desktop utilities and internal tools where exposure to untrusted inputs is limited, but still necessary to patch to lower supply-chain risk. Note that many embedded devices fall into this category operationally, though they can be practically high impact if devices are widely deployed and hard to update.
Verification checklist for mitigations
When you apply the upstream v1.7.19 update or a vendor patch, verify the change and test behavior:
- Confirm package version or source matches v1.7.19 or contains the pointer-position fix. Check release notes and changelog entries for the phrase “decode_array_index_from_pointer” or “incorrect check”.
- Run unit/regression tests that exercise JSON pointer parsing with malformed tokens such as “0A”, “01B”, and leading/trailing alphanumerics to ensure no out‑of‑bounds reads occur.
- Perform fuzzing of pointer parsing paths where feasible, or run existing fuzz harnesses against the updated library to confirm the fix is robust.
- Validate that distribution packages replace the vulnerable library and that downstream products rebuild or relink against patched binaries where applicable. ([bugs.debian.org]org/1114757)
Final assessment and recommended actions
CVE-2025-57052 is a high-severity memory-safety defect introduced by a deceptively small logical error in a central parsing routine. The technical fix is straightforward and upstream has released v1.7.19 that addresses the incorrect character check. Given the library’s prevalence, operators and maintainers should assume that vulnerable code exists in many places: prioritize internet-exposed JSON pointer consumers for immediate remediation, apply vendor patches, and adopt input-validation controls as interim mitigations.
Action summary (fast path):
- Update cJSON to v1.7.19 or apply vendor-supplied patches immediately for internet-facing or high-risk services.
- If patching is delayed, add strict input validation to reject non-numeric array tokens before they reach cJSON code paths.
- Add targeted tests and fuzzing to prevent regression and to raise confidence that the fix holds across downstream integrations.
This vulnerability underscores a recurring truth of systems programming: tiny logical mistakes in trusted utilities can generate outsized security consequences when those utilities are embedded widely. The combination of a clear upstream fix and rapid distribution updates means the operational path forward is straightforward—patch, validate, and harden the input surface where cJSON is used.
In closing, treat this CVE as a supply‑chain and runtime safety issue: remediating the library in place is necessary, but not sufficient. Add caller-side validation, strengthen test coverage around malformed inputs, and treat parser code with the same adversarial scrutiny you give network-facing protocol handlers. The fix exists; the harder work is ensuring every product and firmware image that includes cJSON is updated or mitigated.
Source: MSRC
Security Update Guide - Microsoft Security Response Center