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Changelog for the fix to mbedtls_mpi_exp_mod(A=0)

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In Mbed TLS 2.26.0, the bug was hard to trigger, since all methods for
parsing a bignum (mbedtls_mpi_read_xxx functions) constructed an mbedtls_mpi
object with at least one limb.

In the development branch, after the commit
"New internal function mbedtls_mpi_resize_clear", this bug could be
triggered by a TLS server, by passing invalid custom Diffie-Hellman
parameters with G=0 transmitted as a 0-length byte string.

Since the behavior change in mbedtls_mpi_read_binary and
mbedtls_mpi_read_binary_le (constructing 0 limbs instead of 1 when passed
empty input) turned out to have consequences despite being in principle an
internal detail, mention it in the changelog.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
This commit is contained in:
Gilles Peskine 2021-06-09 18:55:57 +02:00
parent 3da1a8ff39
commit 3008dde386
1 changed files with 7 additions and 0 deletions
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7
ChangeLog.d/mpi_exp_mod-zero.txt Normal file
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Bugfix
* Fix a null pointer dereference when mbedtls_mpi_exp_mod() was called with
A=0 represented with 0 limbs. Up to and including Mbed TLS 2.26, this bug
could not be triggered by code that constructed A with one of the
mbedtls_mpi_read_xxx functions (including in particular TLS code) since
those always built an mpi object with at least one limb.
Credit to OSS-Fuzz. Fixes #4641.