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CVE-2026-39373 medium CVSS 5.3

Vulnerability in jwcrypto (CVE-2026-39373)

Summary

Vulnerability in jwcrypto (CVE-2026-39373). Risk of unauthorized operations or information disclosure. Exploitable via data. Mitigation: upgrade to 1.5.7 or later.

Original description

The description as published by the source.

Summary

The fix for GHSA-j857-7rvv-vj97 in v1.5.6 is weak in that it does not allow to fully control the amount of plaintext the receiver is willing to deal with and provides just a weak upper bound. The patch limits input token size to 250KB but does not validate the decompressed output size. An unauthenticated attacker can craft a JWE token under the 250KB input limit that decompresses to very large data that may exceed small devices memory availability, causing Denial of Service via memory exhaustion.

Although this is technically not unbounded I do recognize that it may be too much for devices and is something that could be surprising to developers, and we can do better than that.

NOTE: the original report was sloppy (probably AI slop) and claimed arbitrary memory consumption, but simple testing showed that while 100MB could be decompressed a 1GB output was denied because the token exceeded the 250K compressed serialization.

NOTE WELL: The proposed solution was also sloppy, proposing to first decompress the data completely in memory (therefore causing the memory exhaustion) and then checking how much memory was already used to deny the operation. I intentionally left the "details" section untouched to show how bad AI slop is and how uncritical the submitter was, even as it was obvious the "suggested fix" is actually no solution at all, as it was using the very call that he claimed was causing "arbitrary" memory exhaustion and wrapping it around an "if" ... the a…

Source: OSV.dev (Open Source Vulnerabilities) NVD GitHub Advisory OSV

Auto-generated summary

Assembled by SecPulse from the source data (CVSS, CWE, affected versions) using templates — no AI involved. Check the original description and references below for exact details.

A vulnerability tracked as CVE-2026-39373 has been found in jwcrypto.
Attackers can target a specific entry point like data over the network to misuse the product.
Risk of unauthorized operations or information disclosure. CVSS score: 5.3/10.
What to do: upgrade jwcrypto to 1.5.7 or later.
If unsure, ask your IT team or search "jwcrypto CVE-2026-39373" on the vendor's site.

CVE-2026-39373 (jwcrypto) — CWE-409 / CVSS v3 5.3
Attack vector: remote (network-reachable) / unauthenticated / no user interaction
Attack surface: data
Patched: 1.5.7 — apply immediately
Plan: 1) Audit SBOM/dependencies, 2) Stage→prod upgrade, 3) Add WAF/proxy monitoring on affected endpoints, 4) Hunt IOCs in logs.
Refs: see the GHSA / vendor advisory / patched release linked on this page.

❓ What is the problem

A vulnerability (CWE-409) exists in jwcrypto. Attackers reach the vulnerable code path via data without authentication.

📍 Affected scope

Check the vendor advisory for the affected versions of jwcrypto. Attack surface: data.

🔥 Severity

Severity: Medium (CVSS 5.3/10). Risk of unauthorized operations or information disclosure.

🔧 How to fix

Update to 1.5.7.

🛡️ Workaround

Until the patch is applied: disable the affected feature, apply WAF rules, or restrict access via network ACLs.

🔍 Detection

Search webserver/proxy logs for unusual request patterns matching this CVE's known IOCs. Run grep -r 'jwcrypto' . against your dependency files (package-lock.json, requirements.txt, go.sum) to find affected services.

Response actions

Concrete steps and command examples for SOC/SRE teams to execute in order

  1. 1
    Identify exposure identify 
    grep -r 'jwcrypto' . | grep -v node_modules

    リポジトリと本番環境の依存ファイル (package-lock.json / requirements.txt / go.sum / Gemfile.lock 等) で jwcrypto を grep し、稼働しているサービス・バージョンを把握する。

  2. 2
    Apply patch patch 
    Upgrade jwcrypto to 1.5.7

    ステージング環境で 1.5.7 に上げて回帰テスト → 本番反映。回帰テストはアプリの主要ハッピーパスを含めること。

  3. 3
    Post-deployment verification verify 
    Confirm patched version is live in production

    パッチ適用後、ステージングで PoC または同等の悪用パターンを再現して脆弱性が閉じたことを確認。本番ではアクセスログ・WAF でアラート再発が無いか継続監視。

Affected packages

PyPI jwcrypto
[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"1.5.7"}]}]
pip jwcrypto
[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"1.5.7"}]}]

Fix status by distribution

Fixed packages published by each distribution for this CVE

References

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