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High severity7.5NVD Advisory· Published Mar 27, 2017· Updated May 13, 2026

CVE-2016-9243

CVE-2016-9243

Description

HKDF in cryptography before 1.5.2 returns an empty byte-string if used with a length less than algorithm.digest_size.

Affected packages

Versions sourced from the GitHub Security Advisory.

PackageAffected versionsPatched versions
cryptographyPyPI
< 1.5.31.5.3

Patches

1
b924696b2e87

Fixes #3211 -- fixed hkdf's output with short length (#3215)

https://github.com/pyca/cryptographyAlex GaynorNov 6, 2016via ghsa
commit
2 files changed · +12 −1
  • src/cryptography/hazmat/primitives/kdf/hkdf.py+1 −1 modified
    @@ -91,7 +91,7 @@ def _expand(self, key_material):
         output = [b""]
         counter = 1
 
-        while (self._algorithm.digest_size // 8) * len(output) < self._length:
+        while self._algorithm.digest_size * (len(output) - 1) < self._length:
             h = hmac.HMAC(key_material, self._algorithm, backend=self._backend)
             h.update(output[-1])
             h.update(self._info)
  • tests/hazmat/primitives/test_hkdf.py+11 −0 modified
    @@ -142,6 +142,17 @@ def test_unicode_typeerror(self, backend):
 
             hkdf.verify(b"foo", u"bar")
 
+    def test_derive_short_output(self, backend):
+        hkdf = HKDF(
+            hashes.SHA256(),
+            4,
+            salt=None,
+            info=None,
+            backend=backend
+        )
+
+        assert hkdf.derive(b"\x01" * 16) == b"gJ\xfb{"
+
 
 @pytest.mark.requires_backend_interface(interface=HMACBackend)
 class TestHKDFExpand(object):

Vulnerability mechanics

Generated by null/stub on May 9, 2026. Inputs: CWE entries + fix-commit diffs from this CVE's patches. Citations validated against bundle.

References

19

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