Summary
serde_with: KeyValueMap serialization panics on empty sequence or map entries
The public KeyValueMap serializer assumes that each mapped element has at least one field or item to use as the map key, but it subtracts 1 from the caller-visible length before validating that assumption. An application that serializes attacker-controlled data through #[serde_as(as = "KeyValueMap<_>")] can be crashed by an empty inner sequence or map entry.
Details
The affected public surface includes:
- Serialization of
#[serde_as(as = "KeyValueMap<_>")]values throughserde_json::to_stringor any other Serde serializer` - Public
KeyValueMapconversions for sequence and map-backed entries`
The root cause is: The KeyValueMap serializer preallocating Vec::with_capacity(len - 1) or Vec::with_capacity(len.unwrap_or(17) - 1) before checking that the element actually contains the required first key field or item.
The vulnerable data/control flow is: attacker-controlled empty entry -> serde_json::to_string -> KeyValueMap<TAs>::serialize_as -> SeqAsMapSerializer::{serialize_seq,serialize_map} -> Vec::with_capacity(len - 1) or Vec::with_capacity(len.unwrap_or(17) - 1) -> panic
Relevant source locations:
serde_with/src/key_value_map.rs:590serde_with/src/key_value_map.rs:599serde_with/src/key_value_map.rs:613serde_with/src/key_value_map.rs:632serde_with/src/key_value_map.rs:648
PoC
/*
[dependencies]
serde = {version = "*", features = ["derive"]}
serde_with = "*"
serde_json = "*"
*/
use serde::Serialize;
use serde_with::{serde_as, KeyValueMap};
#[derive(Serialize)]
#[serde(transparent)]
struct Seq(Vec<String>);
#[serde_as]
#[derive(Serialize)]
#[serde(transparent)]
struct KVMap {
#[serde_as(as = "KeyValueMap<_>")]
foo: Vec<Seq>,
}
fn main() {
let value = KVMap {
foo: vec![Seq(Vec::new())],
};
let _ = serde_json::to_string(&value).unwrap();
}
Impact
A local attacker who can trigger serialization of attacker-controlled data through KeyValueMap can terminate the process, causing a denial of service.
The application does not adequately validate input before processing it, allowing unexpected values to reach sensitive code paths. Typical impact: varies by context: data corruption, logic bypass, or denial of service.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
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Frequently Asked Questions
- What is GHSA-7GCF-G7XR-8HXJ? GHSA-7GCF-G7XR-8HXJ is a medium-severity improper input validation vulnerability in serde_with (rust), affecting versions < 3.21.0. It is fixed in 3.21.0. The application does not adequately validate input before processing it, allowing unexpected values to reach sensitive code paths.
- Which versions of serde_with are affected by GHSA-7GCF-G7XR-8HXJ? serde_with (rust) versions < 3.21.0 is affected.
- Is there a fix for GHSA-7GCF-G7XR-8HXJ? Yes. GHSA-7GCF-G7XR-8HXJ is fixed in 3.21.0. Upgrade to this version or later.
- Is GHSA-7GCF-G7XR-8HXJ exploitable, and should I be worried? Whether GHSA-7GCF-G7XR-8HXJ is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether GHSA-7GCF-G7XR-8HXJ is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix GHSA-7GCF-G7XR-8HXJ? Upgrade
serde_withto 3.21.0 or later.