A null-pointer dereference in a compact C JSON library has quietly become a textbook reminder that tiny dependencies can create outsized operational risk: CVE-2024-31755 identifies a segmentation violation in cJSON v1.7.17 that can be triggered when the second parameter to cJSON_SetValuestring is NULL, enabling a remote or locally reachable attacker to crash processes and cause sustained availability loss in software that uses the library. (github.com)
cJSON is a minimal, MIT-licensed C library used for parsing, creating, and manipulating JSON structures in a wide range of projects — from embedded appliances and IoT firmware to server-side tooling and desktop applications. Its small API and tiny footprint make it a common choice when developers need a straightforward JSON layer in C without pulling in a heavyweight dependency.
On April 25–26, 2024 the project and public vulnerability trackers recorded a segmentation violation in version v1.7.17: the function cJSON_SetValuestring(cJSON object, const char valuestring) did not validate the incoming valuestring argument before dereferencing it, which leads to a null-pointer dereference and a crash when callers pass NULL. That defect was reported with a minimal proof-of-concept and promptly patched upstream. (github.com) (github.com)
This vulnerability has been cataloged by major vulnerability databases and distro trackers, and scored as a high-impact availability issue (CVSS ~7.5/7.6 in vendor and distro advisories), because the primary effect is denial of service through a reproducible crash. Distributors and downstream packagers have released updated packages (v1.7.18 and later) that include the fix.
The problem was discovered by fuzzing and reproduced with a compact proof-of-concept: create a cJSON string item (for example, cJSON_CreateString("apple")) then call cJSON_SetValuestring(item, NULL) — that single call can trigger the null dereference and crash the process. The issue, reported publicly, included the minimal reproduction and the affected commit tag (v1.7.17). (github.com)
Note: while some trackers present different CVSS vector strings, the consensus is clear — the primary consequence is denial of service. Where specific vendor remediation or configuration guidance differs, refer to the vendor or distro advisory for precise steps.
(Our internal tracking feeds and curated vulnerability lists aggregated the initial reports and downstream advisories while the fix circulated.)
This case also highlights positive aspects of open-source security: public fuzzing, rapid issue reporting, and a concise upstream patch led to a quick fix and clear guidance for downstream distributors. The principal operational burden is not the absence of a fix, but reaching every compiled instance in the field and applying that fix safely.
In a software ecosystem built from countless small components, small bugs can cause big outages — the right combination of SBOM-driven inventory, rapid patching, and defensive design keeps them from becoming operational crises.
Source: MSRC Security Update Guide - Microsoft Security Response Center
Background / Overview
cJSON is a minimal, MIT-licensed C library used for parsing, creating, and manipulating JSON structures in a wide range of projects — from embedded appliances and IoT firmware to server-side tooling and desktop applications. Its small API and tiny footprint make it a common choice when developers need a straightforward JSON layer in C without pulling in a heavyweight dependency.On April 25–26, 2024 the project and public vulnerability trackers recorded a segmentation violation in version v1.7.17: the function cJSON_SetValuestring(cJSON object, const char valuestring) did not validate the incoming valuestring argument before dereferencing it, which leads to a null-pointer dereference and a crash when callers pass NULL. That defect was reported with a minimal proof-of-concept and promptly patched upstream. (github.com) (github.com)
This vulnerability has been cataloged by major vulnerability databases and distro trackers, and scored as a high-impact availability issue (CVSS ~7.5/7.6 in vendor and distro advisories), because the primary effect is denial of service through a reproducible crash. Distributors and downstream packagers have released updated packages (v1.7.18 and later) that include the fix.
What the bug is — technical anatomy
How the crash happens
At the heart of CVE-2024-31755 is a simple logical oversight: the implementation checks whether the existing string pointer inside the cJSON object is NULL, but it fails to check whether the incoming valuestring parameter is NULL before calling strlen on it. In other words, the code assumes the caller’s pointer is valid and then dereferences it — when it’s NULL, that dereference becomes a segmentation fault.The problem was discovered by fuzzing and reproduced with a compact proof-of-concept: create a cJSON string item (for example, cJSON_CreateString("apple")) then call cJSON_SetValuestring(item, NULL) — that single call can trigger the null dereference and crash the process. The issue, reported publicly, included the minimal reproduction and the affected commit tag (v1.7.17). (github.com)
Why this matters in practice
A null-pointer dereference is not exotic: it simply kills the process that executes it. But the operational consequences depend on where cJSON is used:- In embedded or single-purpose appliances, a crash can cause service interruption until an operator manually restarts the device.
- In network-facing services, a crafted request that reaches code paths calling cJSON_SetValuestring with attacker-controlled input could repeatedly crash worker processes and force service restarts, creating a sustained denial-of-service (DoS) condition.
- In build or CI pipelines, a crashing utility can stall automation and impair developer productivity.
Timeline and fixes
- March 25, 2024 — a fuzzing report exposing the segmentation fault was opened against the cJSON repository, with a compact proof-of-concept showing that passing NULL to cJSON_SetValuestring triggers the crash. (github.com)
- April 26, 2024 — a pull request adding a NULL check for the valuestring parameter was merged into the cJSON master branch; maintainers marked the PR as addressing the issue. (github.com)
- Shortly thereafter downstream packagers and distribution maintainers produced updated packages; upstream releases labeled v1.7.18 and later include the fix along with other maintenance changes. Multiple vulnerability trackers and distro security advisories documented CVE-2024-31755 and recommended updating to v1.7.18 or vendor-packaged equivalents.
Confirmed impact & severity
Multiple independent vulnerability databases mark this as a NULL pointer dereference (CWE‑476) with a high impact on availability. Several maintainers assigned a CVSS rating in the mid‑7 range to reflect a high availability impact but generally low confidentiality and integrity impact, because the flaw results in process termination rather than arbitrary code execution. Those ratings reflect the realistic operational risk: repeated exploitation can render services unavailable or severely degraded.Note: while some trackers present different CVSS vector strings, the consensus is clear — the primary consequence is denial of service. Where specific vendor remediation or configuration guidance differs, refer to the vendor or distro advisory for precise steps.
Who is affected
- Any application, library, device firmware, or package that includes cJSON v1.7.17 and exposes code paths that might call cJSON_SetValuestring with externally influenced inputs should be considered at risk.
- Downstream products that statically link cJSON into their binaries (common in embedded images and lightweight appliances) are particularly susceptible because they cannot rely on runtime package managers to provide a quick fix.
- Runtime-only inclusion via shared libraries is easier to remediate (through package updates), but you must still verify which processes are using the library version in question.
Detection — how to find it in your environment
Finding instances of the vulnerable library and detecting potential exploitation requires two concurrent efforts: inventory and monitoring.Inventory (SBOM / dependency scanning)
- Generate or update your SBOMs for all software images, packages, and firmware. Search dependencies for cJSON and check the reported version string.
- For compiled artifacts, use binary dependency scanners (SCA tools) that can detect static-included libraries by symbol names or known file fingerprints.
- Audit embedded firmware images and vendor appliances; many embedded systems ship with static, older copies of cJSON that will not be updated automatically.
Runtime detection
- Look for repeated segmentation faults, core dumps, or worker crashes in services that parse JSON payloads or accept JSON over the network.
- Instrument process crash logs and watch for patterns where the same function name (or the library k traces or crash dumps.
- Monitor for increased process restarts, high crash rates after certain inputs, or alerts coming from fuzzing/QA harnesses. These indicators are the most direct sign of attempted exploitation.
(Our internal tracking feeds and curated vulnerability lists aggregated the initial reports and downstream advisories while the fix circulated.)
Short-term mitigations and hardening (apply immediately)
If you cannot immediately update all affected artifacts to v1.7.18 or later, apply compensating controls to reduce exposure:- Restrict inbound exposure: limit which network interfaces and ports accept JSON-bearing requests — e.g., place JSON endpoints behind API gateways or WAF rules that validate and constrain input size and form.
- Input validation: enforce server-side schema validation before unmarshalling into native structures; reject or sanitize unexpected fields, overly large strings, and malformed JSON that could reach cJSON internals.
- Sandbox JSON handling: run JSON‑parsing logic in a separate, disposable worker process with tight resource limits (ulimit, cgroups) and automatic restart policies to contain crashes and prevent entire services from failing.
- Resource caps: set RLIMIT_FSIZE, RLIMIT_AS and similar limits on processes that parse untrusted JSON so that repeated crashes do not carry a disproportionate recovery cost.
- Fallback behavior: where possible, detect and reject NULL/empty value inputs at the boundary layer before they reach lower-level JSON operations.
Permanent remediation steps (recommended)
- Inventory: compile a list of every binary, container image, firmware, and package that links against cJSON. Prioritize internet-facing components and appliances that are difficult to update.
- Update: upgrade to cJSON v1.7.18 or newer — ensure that the package you deploy includes the upstream merged fix that adds the NULL check in cJSON_SetValuestring. Verify the package’s changelog or source commit id to confirm the fix is present. (github.com)
- Rebuild and redeploy: for statically linked artifacts (common in embedded systems), rebuild the image with the fixed library and push firmware/agent updates through your normal vendor or CI pipeline.
- Test: run your regression and fuzzing suites against the updated binaries to ensure that the fix does not introduce regressions and that previously failing inputs are handled safely.
- Monitor: maintain crash and error telemetry after the rollout; track for lingering crashes which may indicate missed artifacts or secondary issues.
- Document: update your SBOM and change records to reference the patched library versions. Ensure future procurement and vendor contracts require timely vulnerability disclosure and patching for embedded dependencies.
Why the fix mattered — what changed upstream
The upstream change was small and surgical: a single NULL-check for the valuestring parameter was added before any dereference, preventing the strlen call on a NULL pointer. The patch was merged into the project’s master tree and was included in release v1.7.18; downstream packaging teams across distributions and vendors incorporated the fix into their builds. The promptness of the upstream fix and the clarity of the PoC made the remediation straightforward, but the real operational difficulty lies in reaching every compiled copy of the library in the field. (github.com)Risk assessment for operators
- Availability risk (High): repeated or automated exploitation allows an attacker to cause sustained denial-of-service by crashing affected processes. This is the principal impact tracked by CVE databases and distro advisories.
- Confidentiality / Integrity risk (Low): the defect is a straightforward null dereference rather than a classic memory corruption that enables code execution; public reporting has not established reliable exploitation beyond causing crashes.
- Exploitability (Moderate): the flaw can be triggered with well-formed API calls if the calling code passes NULL values influenced by attacker-controlled inputs. The likelihood of practical remote exploitation depends on whether the vulnerable call path can be reached with attacker-controlled data over network boundaries.
- Supply-chain risk (Medium–High): static linking and embedded images mean devices and appliances can remain vulnerable long after a source-level fix is available — making discovery and coordinated remediation challenging.
Developer guidance — safe coding and testing practices
- Validate external inputs before calling library functions. Even when using widely used libraries, assume that any pointer coming from upper layers may be NULL and explicitly check it.
- Practice defensive programming: use helper wrappers that sanitize or normalize inputs, e.g., wrapper functions that coalesce NULL to empty strings or return early with controlled behavior.
- Include unit tests and fuzzing as part of CI for JSON parsing and string-manipulation functions. The original defect was found by fuzzing; integrating fuzzing into pipelines catches similar issues early.
- Prefer memory-safe parsing libraries where feasible for high‑risk, internet-facing surfaces — or at least compile with sanitizers (ASan/UBSan) during development to reveal subtle issues.
- For projects that vendor third-party libraries into source trees (common in embedded work), create a process for tracking upstream security fixes and regularly re-basing vendored code.
Incident response considerations
If you suspect exploitation:- Immediately collect process crash logs, core dumps, and any network request logs that correlate with crash timestamps.
- Quarantine affected hosts and switch to a degraded but safer configuration — for example, temporarily block external access to the affected endpoints while pushing mitigations.
- Redeploy updated artifacts (v1.7.18 or later) to a test stage and verify stability before rolling out to production.
- If you operate critical embedded devices in the field, coordinate firmware updates with device owners and provide clear instructions and urgency — static linking often requires full image replacement.
- After remediation, monitor for reoccurrence and review your vendor notification channels: remind suppliers that their images may embed vulnerable versions and ask for patch schedules.
Broader lessons: why tiny libraries matter
CVE-2024-31755 is unremarkable in its technical mechanics but significant in what it demonstrates about modern software ecosystems: a one-line oversight in a tiny library can ripple across dozens or hundreds of products because of static linking, vendor packaging, and embedded device lifecycles. Supply-chain hygiene — including SBOMs, dependency scanning, and coordinated patching — remains vital to operational resilience.This case also highlights positive aspects of open-source security: public fuzzing, rapid issue reporting, and a concise upstream patch led to a quick fix and clear guidance for downstream distributors. The principal operational burden is not the absence of a fix, but reaching every compiled instance in the field and applying that fix safely.
Quick checklist (actionable steps for ops teams)
- Identify: scan for cJSON in your SBOMs and binary images; flag any v1.7.17 occurrences.
- Isolate: temporarily restrict access to services that parse untrusted JSON if immediate updates are not possible.
- Patch: upgrade to cJSON v1.7.18 or apply vendor-updated packages; rebuild static binaries where necessary.
- Validate: run regression tests and sanity checks after the upgrade; monitor crash/error telemetry closely for 72–120 hours.
- Document: update inventory and change logs; coordinate with vendors and third-party suppliers to confirm their remediation schedules.
Final assessment and call to action
CVE-2024-31755 is a high‑availability risk caused by a simple null-pointer dereference in cJSON v1.7.17 when cJSON_SetValuestring is invoked with a NULL second argument. The flaw is straightforward to understand and to fix; upstream maintainers merged a NULL-check patch and downstream packagers have issued updates. However, because cJSON is often embedded and statically linked in various products — especially in IoT and firmware — operators and developers must actively search, patch, and in some cases rebuild and reflash devices to obtain full remediation. Treat this as a prioritized availability and supply-chain patching task: inventory your deployments, update to v1.7.18 or later as soon as practicable, and harden parsing surfaces so that a single bad pointer cannot become a site-wide outage. (github.com)In a software ecosystem built from countless small components, small bugs can cause big outages — the right combination of SBOM-driven inventory, rapid patching, and defensive design keeps them from becoming operational crises.
Source: MSRC Security Update Guide - Microsoft Security Response Center
