Description
Heap-based buffer underflow in the modmul function in sshbn.c in PuTTY before 0.63 allows remote SSH servers to cause a denial of service (crash) and possibly trigger memory corruption or code execution via a crafted DSA signature, which is not properly handled when performing certain bit-shifting operations during modular multiplication.
Fixed in Linux distributions
Distributions backport fixes, so the version that fixes this vulnerability differs per release — upgrade to at least the version shown. From the distributions' security trackers and advisories.
Debian
Security tracker ↗| Release | Package | Fixed in | Advisory |
|---|---|---|---|
| Debian 14 | filezilla | 3.7.3-1 | — |
| Debian 14 | putty | 0.63-1 | — |
| Debian 13 | filezilla | 3.7.3-1 | — |
| Debian 13 | putty | 0.63-1 | — |
| Debian 12 | filezilla | 3.7.3-1 | — |
| Debian 12 | putty | 0.63-1 | — |
| Debian 7 | putty | 0.62-9+deb7u1 | DSA-2736-1 |
| Debian 6.0 | putty | 0.60+2010-02-20-1+squeeze2 | DSA-2736-1 |
openSUSE
Security tracker ↗| Release | Package | Fixed in | Advisory |
|---|---|---|---|
| openSUSE Tumbleweed | filezilla | 3.23.0.2-1.1 | openSUSE-SU-2024:10399-1 |
Advisories 2
- DSA-2736-1 putty - several Debian Security (DSA / DLA) · Mar 9, 2026
- openSUSE-SU-2024:10399-1 filezilla-3.23.0.2-1.1 on GA media SUSE / openSUSE Security Updates · Feb 4, 2026
References
- [oss-security] 20130806 CVE request: three additional flaws fixed in putty 0.63 mailing-list
- 54533 third-party-advisory
- http://www.chiark.greenend.org.uk/~sgtatham/putty/wishlist/vuln-modmul.html
- DSA-2736 vendor-advisory
- openSUSE-SU-2013:1347 vendor-advisory
- 54379 third-party-advisory
- http://svn.tartarus.org/sgt/putty/sshbn.c?sortby=date&r1=9977&r2=9976&pathrev=9977